The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
by Charles Darwin · English · Gutenberg #4346 · 972,847 chars · public domain (US)
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Title: The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
Author: Charles Darwin
Release date: August 1, 2003 [eBook #4346]
Most recently updated: June 28, 2023
Language: English
Other information and formats: www.gutenberg.org/ebooks/4346
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*** START OF THE PROJECT GUTENBERG EBOOK THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM ***
THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM.
BY
CHARLES DARWIN, M.A., F.R.S., ETC.
CONTENTS.
CHAPTER I.
INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of
plants.--Benefits derived from cross-fertilisation.--Self-fertilisation
favourable to the propagation of the species.--Brief history of the
subject.--Object of the experiments, and the manner in which they were
tried.--Statistical value of the measurements.--The experiments carried
on during several successive generations.--Nature of the relationship of
the plants in the later generations.--Uniformity of the conditions to
which the plants were subjected.--Some apparent and some real causes of
error.--Amount of pollen employed.--Arrangement of the work.--Importance
of the conclusions.
CHAPTER II.
CONVOLVULACEAE.
Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.--Greater
constitutional vigour of the crossed plants.--The effects on the
offspring of crossing different flowers on the same plant, instead of
crossing distinct individuals.--The effects of a cross with a fresh
stock.--The descendants of the self-fertilised plant named
Hero.--Summary on the growth, vigour, and fertility of the successive
crossed and self-fertilised generations.--Small amount of pollen in the
anthers of the self-fertilised plants of the later generations, and the
sterility of their first-produced flowers.--Uniform colour of the
flowers produced by the self-fertilised plants.--The advantage from a
cross between two distinct plants depends on their differing in
constitution.
CHAPTER III.
SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.
Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.--Appearance of a
new, tall, and highly self-fertile variety.--Offspring from a cross
between self-fertilised plants.--Effects of a cross with a fresh
stock.--Effects of crossing flowers on the same plant.--Summary on
Mimulus luteus.--Digitalis purpurea, superiority of the crossed
plants.--Effects of crossing flowers on the same
plant.--Calceolaria.--Linaria vulgaris.--Verbascum thapsus.--Vandellia
nummularifolia.--Cleistogene flowers.--Gesneria pendulina.--Salvia
coccinea.--Origanum vulgare, great increase of the crossed plants by
stolons.--Thunbergia alata.
CHAPTER IV.
CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
Brassica oleracea, crossed and self-fertilised plants.--Great effect of
a cross with a fresh stock on the weight of the offspring.--Iberis
umbellata.--Papaver vagum.--Eschscholtzia californica, seedlings from a
cross with a fresh stock not more vigorous, but more fertile than the
self-fertilised seedlings.--Reseda lutea and odorata, many individuals
sterile with their own pollen.--Viola tricolor, wonderful effects of a
cross.--Adonis aestivalis.--Delphinium consolida.--Viscaria oculata,
crossed plants hardly taller, but more fertile than the
self-fertilised.--Dianthus caryophyllus, crossed and self-fertilised
plants compared for four generations.--Great effects of a cross with a
fresh stock.--Uniform colour of the flowers on the self-fertilised
plants.--Hibiscus africanus.
CHAPTER V.
GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.
Pelargonium zonale, a cross between plants propagated by cuttings does
no good.--Tropaeolum minus.--Limnanthes douglasii.--Lupinus luteus and
pilosus.--Phaseolus multiflorus and vulgaris.--Lathyrus odoratus,
varieties of, never naturally intercross in England.--Pisum sativum,
varieties of, rarely intercross, but a cross between them highly
beneficial.--Sarothamnus scoparius, wonderful effects of a
cross.--Ononis minutissima, cleistogene flowers of.--Summary on the
Leguminosae.--Clarkia elegans.--Bartonia aurea.--Passiflora
gracilis.--Apium petroselinum.--Scabiosa atropurpurea.--Lactuca
sativa.--Specularia speculum.--Lobelia ramosa, advantages of a cross
during two generations.--Lobelia fulgens.--Nemophila insignis, great
advantages of a cross.--Borago officinalis.--Nolana prostrata.
CHAPTER VI.
SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.
Petunia violacea, crossed and self-fertilised plants compared for four
generations.--Effects of a cross with a fresh stock.--Uniform colour of
the flowers on the self-fertilised plants of the fourth
generation.--Nicotiana tabacum, crossed and self-fertilised plants of
equal height.--Great effects of a cross with a distinct sub-variety on
the height, but not on the fertility, of the offspring.--Cyclamen
persicum, crossed seedlings greatly superior to the
self-fertilised.--Anagallis collina.--Primula veris.--Equal-styled
variety of Primula veris, fertility of, greatly increased by a cross
with a fresh stock.--Fagopyrum esculentum.--Beta vulgaris.--Canna
warscewiczi, crossed and self-fertilised plants of equal height.--Zea
mays.--Phalaris canariensis.
CHAPTER VII.
SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED
PLANTS.
Number of species and plants measured.--Tables given.--Preliminary
remarks on the offspring of plants crossed by a fresh stock.--Thirteen
cases specially considered.--The effects of crossing a self-fertilised
plant either by another self-fertilised plant or by an intercrossed
plant of the old stock.--Summary of the results.--Preliminary remarks on
the crossed and self-fertilised plants of the same stock.--The
twenty-six exceptional cases considered, in which the crossed plants did
not exceed greatly in height the self-fertilised.--Most of these cases
shown not to be real exceptions to the rule that cross-fertilisation is
beneficial.--Summary of results.--Relative weights of the crossed and
self-fertilised plants.
CHAPTER VIII.
DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL
VIGOUR AND IN OTHER RESPECTS.
Greater constitutional vigour of crossed plants.--The effects of great
crowding.--Competition with other kinds of plants.--Self-fertilised
plants more liable to premature death.--Crossed plants generally flower
before the self-fertilised.--Negative effects of intercrossing flowers
on the same plant.--Cases described.--Transmission of the good effects
of a cross to later generations.--Effects of crossing plants of closely
related parentage.--Uniform colour of the flowers on plants
self-fertilised during several generations and cultivated under similar
conditions.
CHAPTER IX.
THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE
PRODUCTION OF SEEDS.
Fertility of plants of crossed and self-fertilised parentage, both lots
being fertilised in the same manner.--Fertility of the parent-plants
when first crossed and self-fertilised, and of their crossed and
self-fertilised offspring when again crossed and
self-fertilised.--Comparison of the fertility of flowers fertilised with
their own pollen and with that from other flowers on the same
plant.--Self-sterile plants.--Causes of self-sterility.--The appearance
of highly self-fertile varieties.--Self-fertilisation apparently in some
respects beneficial, independently of the assured production of
seeds.--Relative weights and rates of germination of seeds from crossed
and self-fertilised flowers.
CHAPTER X.
MEANS OF FERTILISATION.
Sterility and fertility of plants when insects are excluded.--The means
by which flowers are cross-fertilised.--Structures favourable to
self-fertilisation.--Relation between the structure and conspicuousness
of flowers, the visits of insects, and the advantages of
cross-fertilisation.--The means by which flowers are fertilised with
pollen from a distinct plant.--Greater fertilising power of such
pollen.--Anemophilous species.--Conversion of anemophilous species into
entomophilous.--Origin of nectar.--Anemophilous plants generally have
their sexes separated.--Conversion of diclinous into hermaphrodite
flowers.--Trees often have their sexes separated.
CHAPTER XI.
THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.
Insects visit the flowers of the same species as long as they
can.--Cause of this habit.--Means by which bees recognise the flowers of
the same species.--Sudden secretion of nectar.--Nectar of certain
flowers unattractive to certain insects.--Industry of bees, and the
number of flowers visited within a short time.--Perforation of the
corolla by bees.--Skill shown in the operation.--Hive-bees profit by the
holes made by humble-bees.--Effects of habit.--The motive for
perforating flowers to save time.--Flowers growing in crowded masses
chiefly perforated.
CHAPTER XII.
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.--Allied species differ greatly in the means by which
cross-fertilisation is favoured and self-fertilisation avoided.--The
benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.--The evil effects not due to the
combination of morbid tendencies in the parents.--Nature of the
conditions to which plants are subjected when growing near together in a
state of nature or under culture, and the effects of such
conditions.--Theoretical considerations with respect to the interaction
of differentiated sexual elements.--Practical lessons.--Genesis of the
two sexes.--Close correspondence between the effects of
cross-fertilisation and self-fertilisation, and of the legitimate and
illegitimate unions of heterostyled plants, in comparison with hybrid
unions.
INDEX.
...
THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.
CHAPTER I.
INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of plants.
Benefits derived from cross-fertilisation.
Self-fertilisation favourable to the propagation of the species.
Brief history of the subject.
Object of the experiments, and the manner in which they were tried.
Statistical value of the measurements.
The experiments carried on during several successive generations.
Nature of the relationship of the plants in the later generations.
Uniformity of the conditions to which the plants were subjected.
Some apparent and some real causes of error.
Amount of pollen employed.
Arrangement of the work.
Importance of the conclusions.
There is weighty and abundant evidence that the flowers of most kinds of
plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe,
by a distinct plant. Cross-fertilisation is sometimes ensured by the
sexes being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised
by pollen from the same plant, but can be fertilised by pollen from any
other individual of the same species. There are also very many species
which are partially sterile with their own pollen. Lastly, there is a
large class in which the flowers present no apparent obstacle of any
kind to self-fertilisation, nevertheless these plants are frequently
intercrossed, owing to the prepotency of pollen from another individual
or variety over the plant’s own pollen.
As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone
that they derived some great advantage from the process; and it is the
object of the present work to show the nature and importance of the
benefits thus derived. There are, however, some exceptions to the rule
of plants being constructed so as to allow of or to favour
cross-fertilisation, for some few plants seem to be invariably
self-fertilised; yet even these retain traces of having been formerly
adapted for cross-fertilisation. These exceptions need not make us doubt
the truth of the above rule, any more than the existence of some few
plants which produce flowers, and yet never set seed, should make us
doubt that flowers are adapted for the production of seed and the
propagation of the species.
We should always keep in mind the obvious fact that the production of
seed is the chief end of the act of fertilisation; and that this end can
be gained by hermaphrodite plants with incomparably greater certainty by
self-fertilisation, than by the union of the sexual elements belonging
to two distinct flowers or plants. Yet it is as unmistakably plain that
innumerable flowers are adapted for cross-fertilisation, as that the
teeth and talons of a carnivorous animal are adapted for catching prey;
or that the plumes, wings, and hooks of a seed are adapted for its
dissemination. Flowers, therefore, are constructed so as to gain two
objects which are, to a certain extent, antagonistic, and this explains
many apparent anomalies in their structure. The close proximity of the
anthers to the stigma in a multitude of species favours, and often
leads, to self-fertilisation; but this end could have been gained far
more safely if the flowers had been completely closed, for then the
pollen would not have been injured by the rain or devoured by insects,
as often happens. Moreover, in this case, a very small quantity of
pollen would have been sufficient for fertilisation, instead of millions
of grains being produced. But the openness of the flower and the
production of a great and apparently wasteful amount of pollen are
necessary for cross-fertilisation. These remarks are well illustrated by
the plants called cleistogene, which bear on the same stock two kinds of
flowers. The flowers of the one kind are minute and completely closed,
so that they cannot possibly be crossed; but they are abundantly
fertile, although producing an extremely small quantity of pollen. The
flowers of the other kind produce much pollen and are open; and these
can be, and often are, cross-fertilised. Hermann Muller has also made
the remarkable discovery that there are some plants which exist under
two forms; that is, produce on distinct stocks two kinds of
hermaphrodite flowers. The one form bears small flowers constructed for
self-fertilisation; whilst the other bears larger and much more
conspicuous flowers plainly constructed for cross-fertilisation by the
aid of insects; and without their aid these produce no seed.
The adaptation of flowers for cross-fertilisation is a subject which has
interested me for the last thirty-seven years, and I have collected a
large mass of observations, but these are now rendered superfluous by
the many excellent works which have been lately published. In the year
1857 I wrote a short paper on the fertilisation of the kidney bean (1/1.
‘Gardeners’ Chronicle’ 1857 page 725 and 1858 pages 824 and 844. ‘Annals
and Magazine of Natural History’ 3rd series volume 2 1858 page 462.);
and in 1862 my work ‘On the Contrivances by which British and Foreign
Orchids are Fertilised by Insects’ appeared. It seemed to me a better
plan to work out one group of plants as carefully as I could, rather
than to publish many miscellaneous and imperfect observations. My
present work is the complement of that on Orchids, in which it was shown
how admirably these plants are constructed so as to permit of, or to
favour, or to necessitate cross-fertilisation. The adaptations for
cross-fertilisation are perhaps more obvious in the Orchideae than in
any other group of plants, but it is an error to speak of them, as some
authors have done, as an exceptional case. The lever-like action of the
stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by
which the anthers are depressed and rubbed on the backs of bees, shows
as perfect a structure as can be found in any orchid. Papilionaceous
flowers, as described by various authors--for instance, by Mr. T.H.
Farrer--offer innumerable curious adaptations for cross-fertilisation.
The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful
as that of the most wonderful orchid. The stamens, according to Fritz
Muller, are irritable, so that as soon as a moth visits a flower, the
anthers explode and cover the insect with pollen; one of the filaments
which is broader than the others then moves and closes the flower for
about twelve hours, after which time it resumes its original position.
(1/2. ‘Botanische Zeitung’ 1866 page 129.) Thus the stigma cannot be
fertilised by pollen from the same flower, but only by that brought by a
moth from some other flower. Endless other beautiful contrivances for
this same purpose could be specified.
Long before I had attended to the fertilisation of flowers, a remarkable
book appeared in 1793 in Germany, ‘Das Entdeckte Geheimniss der Natur,’
by C.K. Sprengel, in which he clearly proved by innumerable
observations, how essential a part insects play in the fertilisation of
many plants. But he was in advance of his age, and his discoveries were
for a long time neglected. Since the appearance of my book on Orchids,
many excellent works on the fertilisation of flowers, such as those by
Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter
papers, have been published. (1/3. Sir John Lubbock has given an
interesting summary of the whole subject in his ‘British Wild Flowers
considered in relation to Insects’ 1875. Hermann Muller’s work ‘Die
Befruchtung der Blumen durch Insekten’ 1873, contains an immense number
of original observations and generalisations. It is, moreover,
invaluable as a repertory with references to almost everything which has
been published on the subject. His work differs from that of all others
in specifying what kinds of insects, as far as known, visit the flowers
of each species. He likewise enters on new ground, by showing not only
that flowers are adapted for their own good to the visits of certain
insects; but that the insects themselves are excellently adapted for
procuring nectar or pollen from certain flowers. The value of H.
Muller’s work can hardly be over-estimated, and it is much to be desired
that it should be translated into English. Severin Axell’s work is
written in Swedish, so that I have not been able to read it.) A list
would occupy several pages, and this is not the proper place to give
their titles, as we are not here concerned with the means, but with the
results of cross-fertilisation. No one who feels interest in the
mechanism by which nature effects her ends, can read these books and
memoirs without the most lively interest.
From my own observations on plants, guided to a certain extent by the
experience of the breeders of animals, I became convinced many years ago
that it is a general law of nature that flowers are adapted to be
crossed, at least occasionally, by pollen from a distinct plant.
Sprengel at times foresaw this law, but only partially, for it does not
appear that he was aware that there was any difference in power between
pollen from the same plant and from a distinct plant. In the
introduction to his book (page 4) he says, as the sexes are separated in
so many flowers, and as so many other flowers are dichogamous, “it
appears that nature has not willed that any one flower should be
fertilised by its own pollen.” Nevertheless, he was far from keeping
this conclusion always before his mind, or he did not see its full
importance, as may be perceived by anyone who will read his observations
carefully; and he consequently mistook the meaning of various
structures. But his discoveries are so numerous and his work so
excellent, that he can well afford to bear a small amount of blame. A
most capable judge, H. Muller, likewise says: “It is remarkable in how
very many cases Sprengel rightly perceived that pollen is necessarily
transported to the stigmas of other flowers of the same species by the
insects which visit them, and yet did not imagine that this
transportation was of any service to the plants themselves.” (1/4. ‘Die
Befruchtung der Blumen’ 1873 page 4. His words are: “Es ist merkwurdig,
in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die
Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben
anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung
zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur
die Pflanzen selbst gesucht werden musse.”)
Andrew Knight saw the truth much more clearly, for he remarks, “Nature
intended that a sexual intercourse should take place between
neighbouring plants of the same species.” (1/5. ‘Philosophical
Transactions’ 1799 page 202.) After alluding to the various means by
which pollen is transported from flower to flower, as far as was then
imperfectly known, he adds, “Nature has something more in view than that
its own proper males would fecundate each blossom.” In 1811 Kolreuter
plainly hinted at the same law, as did afterwards another famous
hybridiser of plants, Herbert. (1/6. Kolreuter ‘Mem. de l’Acad. de St.
Petersbourg’ tome 3 1809 published 1811 page 197. After showing how well
the Malvaceae are adapted for cross-fertilisation, he asks, “An id
aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo
pulvere, sed semper eo aliarum suae speciei impregnentur, merito
quaeritur? Certe natura nil facit frustra.” Herbert ‘Amaryllidaceae,
with a Treatise on Cross-bred Vegetables’ 1837.) But none of these
distinguished observers appear to have been sufficiently impressed with
the truth and generality of the law, so as to insist on it and impress
their beliefs on others.
In 1862 I summed up my observations on Orchids by saying that nature
“abhors perpetual self-fertilisation.” If the word perpetual had been
omitted, the aphorism would have been false. As it stands, I believe
that it is true, though perhaps rather too strongly expressed; and I
should have added the self-evident proposition that the propagation of
the species, whether by self-fertilisation or by cross-fertilisation, or
asexually by buds, stolons, etc. is of paramount importance. Hermann
Muller has done excellent service by insisting repeatedly on this latter
point.
It often occurred to me that it would be advisable to try whether
seedlings from cross-fertilised flowers were in any way superior to
those from self-fertilised flowers. But as no instance was known with
animals of any evil appearing in a single generation from the closest
possible interbreeding, that is between brothers and sisters, I thought
that the same rule would hold good with plants; and that it would be
necessary at the sacrifice of too much time to self-fertilise and
intercross plants during several successive generations, in order to
arrive at any result. I ought to have reflected that such elaborate
provisions favouring cross-fertilisation, as we see in innumerable
plants, would not have been acquired for the sake of gaining a distant
and slight advantage, or of avoiding a distant and slight evil.
Moreover, the fertilisation of a flower by its own pollen corresponds to
a closer form of interbreeding than is possible with ordinary bi-sexual
animals; so that an earlier result might have been expected.
I was at last led to make the experiments recorded in the present volume
from the following circumstance. For the sake of determining certain
points with respect to inheritance, and without any thought of the
effects of close interbreeding, I raised close together two large beds
of self-fertilised and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully grown were
plainly taller and more vigorous than the self-fertilised ones. Bees
incessantly visit the flowers of this Linaria and carry pollen from one
to the other; and if insects are excluded, the flowers produce extremely
few seeds; so that the wild plants from which my seedlings were raised
must have been intercrossed during all previous generations. It seemed
therefore quite incredible that the difference between the two beds of
seedlings could have been due to a single act of self-fertilisation; and
I attributed the result to the self-fertilised seeds not having been
well ripened, improbable as it was that all should have been in this
state, or to some other accidental and inexplicable cause. During the
next year, I raised for the same purpose as before two large beds close
together of self-fertilised and crossed seedlings from the carnation,
Dianthus caryophyllus. This plant, like the Linaria, is almost sterile
if insects are excluded; and we may draw the same inference as before,
namely, that the parent-plants must have been intercrossed during every
or almost every previous generation. Nevertheless, the self-fertilised
seedlings were plainly inferior in height and vigour to the crossed.
My attention was now thoroughly aroused, for I could hardly doubt that
the difference between the two beds was due to the one set being the
offspring of crossed, and the other of self-fertilised flowers.
Accordingly I selected almost by hazard two other plants, which happened
to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea
purpurea, both of which, unlike the Linaria and Dianthus, are highly
self-fertile if insects are excluded. Some flowers on a single plant of
both species were fertilised with their own pollen, and others were
crossed with pollen from a distinct individual; both plants being
protected by a net from insects. The crossed and self-fertilised seeds
thus produced were sown on opposite sides of the same pots, and treated
in all respects alike; and the plants when fully grown were measured and
compared. With both species, as in the cases of the Linaria and
Dianthus, the crossed seedlings were conspicuously superior in height
and in other ways to the self-fertilised. I therefore determined to
begin a long series of experiments with various plants, and these were
continued for the following eleven years; and we shall see that in a
large majority of cases the crossed beat the self-fertilised plants.
Several of the exceptional cases, moreover, in which the crossed plants
were not victorious, can be explained.
It should be observed that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds,
seedlings, or plants; these terms implying that they are the product of
crossed or self-fertilised flowers. Cross-fertilisation always means a
cross between distinct plants which were raised from seeds and not from
cuttings or buds. Self-fertilisation always implies that the flowers in
question were impregnated with their own pollen.
My experiments were tried in the following manner. A single plant, if it
produced a sufficiency of flowers, or two or three plants were placed
under a net stretched on a frame, and large enough to cover the plant
(together with the pot, when one was used) without touching it. This
latter point is important, for if the flowers touch the net they may be
cross-fertilised by bees, as I have known to happen; and when the net is
wet the pollen may be injured. I used at first “white cotton net,” with
very fine meshes, but afterwards a kind of net with meshes one-tenth of
an inch in diameter; and this I found by experience effectually excluded
all insects excepting Thrips, which no net will exclude. On the plants
thus protected several flowers were marked, and were fertilised with
their own pollen; and an equal number on the same plants, marked in a
different manner, were at the same time crossed with pollen from a
distinct plant. The crossed flowers were never castrated, in order to
make the experiments as like as possible to what occurs under nature
with plants fertilised by the aid of insects. Therefore, some of the
flowers which were crossed may have failed to be thus fertilised, and
afterwards have been self-fertilised. But this and some other sources of
error will presently be discussed. In some few cases of spontaneously
self-fertile species, the flowers were allowed to fertilise themselves
under the net; and in still fewer cases uncovered plants were allowed to
be freely crossed by the insects which incessantly visited them. There
are some great advantages and some disadvantages in my having
occasionally varied my method of proceeding; but when there was any
difference in the treatment, it is always so stated under the head of
each species.
Care was taken that the seeds were thoroughly ripened before being
gathered. Afterwards the crossed and self-fertilised seeds were in most
cases placed on damp sand on opposite sides of a glass tumbler covered
by a glass plate, with a partition between the two lots; and the glass
was placed on the chimney-piece in a warm room. I could thus observe the
germination of the seeds. Sometimes a few would germinate on one side
before any on the other, and these were thrown away. But as often as a
pair germinated at the same time, they were planted on opposite sides of
a pot, with a superficial partition between the two; and I thus
proceeded until from half-a-dozen to a score or more seedlings of
exactly the same age were planted on the opposite sides of several pots.
If one of the young seedlings became sickly or was in any way injured,
it was pulled up and thrown away, as well as its antagonist on the
opposite side of the same pot.
As a large number of seeds were placed on the sand to germinate, many
remained after the pairs had been selected, some of which were in a
state of germination and others not so; and these were sown crowded
together on the opposite sides of one or two rather larger pots, or
sometimes in two long rows out of doors. In these cases there was the
most severe struggle for life among the crossed seedlings on one side of
the pot, and the self-fertilised seedlings on the other side, and
between the two lots which grew in competition in the same pot. A vast
number soon perished, and the tallest of the survivors on both sides
when fully grown were measured. Plants treated in this manner, were
subjected to nearly the same conditions as those growing in a state of
nature, which have to struggle to maturity in the midst of a host of
competitors.
On other occasions, from the want of time, the seeds, instead of being
allowed to germinate on damp sand, were sown on the opposite sides of
pots, and the fully grown plants measured. But this plan is less
accurate, as the seeds sometimes germinated more quickly on one side
than on the other. It was however necessary to act in this manner with
some few species, as certain kinds of seeds would not germinate well
when exposed to the light; though the glasses containing them were kept
on the chimney-piece on one side of a room, and some way from the two
windows which faced the north-east. (1/7. This occurred in the plainest
manner with the seeds of Papaver vagum and Delphinium consolida, and
less plainly with those of Adonis aestivalis and Ononis minutissima.
Rarely more than one or two of the seeds of these four species
germinated on the bare sand, though left there for some weeks; but when
these same seeds were placed on earth in pots, and covered with a thin
layer of sand, they germinated immediately in large numbers.)
The soil in the pots in which the seedlings were planted, or the seeds
sown, was well mixed, so as to be uniform in composition. The plants on
the two sides were always watered at the same time and as equally as
possible; and even if this had not been done, the water would have
spread almost equally to both sides, as the pots were not large. The
crossed and self-fertilised plants were separated by a superficial
partition, which was always kept directed towards the chief source of
the light, so that the plants on both sides were equally illuminated. I
do not believe it possible that two sets of plants could have been
subjected to more closely similar conditions, than were my crossed and
self-fertilised seedlings, as grown in the above described manner.
In comparing the two sets, the eye alone was never trusted. Generally
the height of every plant on both sides was carefully measured, often
more than once, namely, whilst young, sometimes again when older, and
finally when fully or almost fully grown. But in some cases, which are
always specified, owing to the want of time, only one or two of the
tallest plants on each side were measured. This plan, which is not a
good one, was never followed (except with the crowded plants raised from
the seeds remaining after the pairs had been planted) unless the tallest
plants on each side seemed fairly to represent the average difference
between those on both sides. It has, however, some great advantages, as
sickly or accidentally injured plants, or the offspring of ill-ripened
seeds, are thus eliminated. When the tallest plants alone on each side
were measured, their average height of course exceeds that of all the
plants on the same side taken together. But in the case of the much
crowded plants raised from the remaining seeds, the average height of
the tallest plants was less than that of the plants in pairs, owing to
the unfavourable conditions to which they were subjected from being
greatly crowded. For our purpose, however, of the comparison of the
crossed and self-fertilised plants, their absolute height signifies
little.
As the plants were measured by an ordinary English standard divided into
inches and eighths of an inch, I have not thought it worth while to
change the fractions into decimals. The average or mean heights were
calculated in the ordinary rough method by adding up the measurements of
all, and dividing the product by the number of plants measured; the
result being here given in inches and decimals. As the different species
grow to various heights, I have always for the sake of easy comparison
given in addition the average height of the crossed plants of each
species taken as 100, and have calculated the average height of the
self-fertilised plant in relation to this standard. With respect to the
crowded plants raised from the seeds remaining after the pairs had been
planted, and of which only some of the tallest on each side were
measured, I have not thought it worth while to complicate the results by
giving separate averages for them and for the pairs, but have added up
all their heights, and thus obtained a single average.
I long doubted whether it was worth while to give the measurements of
each separate plant, but have decided to do so, in order that it may be
seen that the superiority of the crossed plants over the
self-fertilised, does not commonly depend on the presence of two or
three extra fine plants on the one side, or of a few very poor plants on
the other side. Although several observers have insisted in general
terms on the offspring from intercrossed varieties being superior to
either parent-form, no precise measurements have been given (1/8. A
summary of these statements, with references, may be found in my
‘Variation of Animals and Plants under Domestication’ chapter 17 2nd
edition 1875 volume 2 page 109.); and I have met with no observations on
the effects of crossing and self-fertilising the individuals of the same
variety. Moreover, experiments of this kind require so much time--mine
having been continued during eleven years--that they are not likely soon
to be repeated.
As only a moderate number of crossed and self-fertilised plants were
measured, it was of great importance to me to learn how far the averages
were trustworthy. I therefore asked Mr. Galton, who has had much
experience in statistical researches, to examine some of my tables of
measurements, seven in number, namely, those of Ipomoea, Digitalis,
Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if
we took by chance a dozen or score of men belonging to two nations and
measured them, it would I presume be very rash to form any judgment from
such small numbers on their average heights. But the case is somewhat
different with my crossed and self-fertilised plants, as they were of
exactly the same age, were subjected from first to last to the same
conditions, and were descended from the same parents. When only from two
to six pairs of plants were measured, the results are manifestly of
little or no value, except in so far as they confirm and are confirmed
by experiments made on a larger scale with other species. I will now
give the report on the seven tables of measurements, which Mr. Galton
has had the great kindness to draw up for me.
[“I have examined the measurements of the plants with care, and by many
statistical methods, to find out how far the means of the several sets
represent constant realities, such as would come out the same so long as
the general conditions of growth remained unaltered. The principal
methods that were adopted are easily explained by selecting one of the
shorter series of plants, say of Zea mays, for an example.”
TABLE 1/1. Zea mays (young plants). (Mr. Galton.)
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed, as recorded by Mr. Darwin.
Column 3: Self-fertilised, as recorded by Mr. Darwin.
Column 4: Crossed, in Separate Pots, arranged in order of magnitude.
Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.
Column 6: Crossed, in a Single Series, arranged in order of magnitude.
Column 7: Self-fertilised, in a Single Series, arranged in order of
magnitude.
Column 8: Difference, in a Single Series, arranged in order of magnitude.
Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8.
Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8.
Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3.
Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22 : 18 3/8 : -3 5/8.
Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18 : -3 5/8.
Pot 2 : - : - :: - : - :: 21 4/8 : 18 : -3 4/8.
Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21 : 18 : -3.
Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18 :: 21 : 17 3/8 : -3 5/8.
Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8.
Pot 3 : 21 5/8 : 18 :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8.
Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8.
Pot 3 : - : - :: - : - :: 12 : 15 2/8 : +3 2/8.
Pot 3 : 21 : 18 :: 23 : 18 :: 12 : 12 6/8 : +0 6/8.
Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18.
Pot 4 : 23 : 15 4/8 :: 21 : 15 4/8.
Pot 4 : 12 : 18 :: 12 : 12 6/8.
“The observations as I received them are shown in Table 1/1, Columns 2
and 3, where they certainly have no prima facie appearance of
regularity. But as soon as we arrange them the in order of their
magnitudes, as in columns 4 and 5, the case is materially altered. We
now see, with few exceptions, that the largest plant on the crossed side
in each pot exceeds the largest plant on the self-fertilised side, that
the second exceeds the second, the third the third, and so on. Out of
the fifteen cases in the table, there are only two exceptions to this
rule. We may therefore confidently affirm that a crossed series will
always be found to exceed a self-fertilised series, within the range of
the conditions under which the present experiment has been made.”
TABLE 1/2.
Column 1: Number (Name) of Pot.
Column 2: Crossed.
Column 3: Self-fertilised.
Column 4: Difference.
Pot 1 : 18 7/8 : 19 2/8 : +0 3/8.
Pot 2 : 20 7/8 : 19 : -1 7/8.
Pot 3 : 21 1/8 : 16 7/8 : -4 2/8.
Pot 4 : 19 6/8 : 16 : -3 6/8.
“Next as regards the numerical estimate of this excess. The mean values
of the several groups are so discordant, as is shown in Table 1/2, that
a fairly precise numerical estimate seems impossible. But the
consideration arises, whether the difference between pot and pot may not
be of much the same order of importance as that of the other conditions
upon which the growth of the plants has been modified. If so, and only
on that condition, it would follow that when all the measurements,
either of the crossed or the self-fertilised plants, were combined into
a single series, that series would be statistically regular. The
experiment is tried in Table 1/1, columns 7 and 8, where the regularity
is abundantly clear, and justifies us in considering its mean as
perfectly reliable. I have protracted these measurements, and revised
them in the usual way, by drawing a curve through them with a free hand,
but the revision barely modifies the means derived from the original
observations. In the present, and in nearly all the other cases, the
difference between the original and revised means is under 2 per cent of
their value. It is a very remarkable coincidence that in the seven kinds
of plants, whose measurements I have examined, the ratio between the
heights of the crossed and of the self-fertilised ranges in five cases
within very narrow limits. In Zea mays it is as 100 to 84, and in the
others it ranges between 100 to 76 and 100 to 86.”
“The determination of the variability (measured by what is technically
called the ‘probable error’) is a problem of more delicacy than that of
determining the means, and I doubt, after making many trials, whether it
is possible to derive useful conclusions from these few observations. We
ought to have measurements of at least fifty plants in each case, in
order to be in a position to deduce fair results. One fact, however,
bearing on variability, is very evident in most cases, though not in Zea
mays, namely, that the self-fertilised plants include the larger number
of exceptionally small specimens, while the crossed are more generally
full grown.”
“Those groups of cases in which measurements have been made of a few of
the tallest plants that grew in rows, each of which contained a
multitude of plants, show very clearly that the crossed plants exceed
the self-fertilised in height, but they do not tell by inference
anything about their respective mean values. If it should happen that a
series is known to follow the law of error or any other law, and if the
number of individuals in the series is known, it would be always
possible to reconstruct the whole series when a fragment of it has been
given. But I find no such method to be applicable in the present case.
The doubt as to the number of plants in each row is of minor importance;
the real difficulty lies in our ignorance of the precise law followed by
the series. The experience of the plants in pots does not help us to
determine that law, because the observations of such plants are too few
to enable us to lay down more than the middle terms of the series to
which they belong with any sort of accuracy, whereas the cases we are
now considering refer to one of its extremities. There are other special
difficulties which need not be gone into, as the one already mentioned
is a complete bar.”]
Mr. Galton sent me at the same time graphical representations which he
had made of the measurements, and they evidently form fairly regular
curves. He appends the words “very good” to those of Zea and Limnanthes.
He also calculated the average height of the crossed and self-fertilised
plants in the seven tables by a more correct method than that followed
by me, namely, by including the heights, as estimated in accordance with
statistical rules, of a few plants which died before they were measured;
whereas I merely added up the heights of the survivors, and divided the
sum by their number. The difference in our results is in one way highly
satisfactory, for the average heights of the self-fertilised plants, as
deduced by Mr. Galton, is less than mine in all the cases excepting one,
in which our averages are the same; and this shows that I have by no
means exaggerated the superiority of the crossed over the
self-fertilised plants.
After the heights of the crossed and self-fertilised plants had been
taken, they were sometimes cut down close to the ground, and an equal
number of both weighed. This method of comparison gives very striking
results, and I wish that it had been oftener followed. Finally a record
was often kept of any marked difference in the rate of germination of
the crossed and self-fertilised seeds,--of the relative periods of
flowering of the plants raised from them,--and of their productiveness,
that is, of the number of seed-capsules which they produced and of the
average number of seeds which each capsule contained.
When I began my experiments I did not intend to raise crossed and
self-fertilised plants for more than a single generation; but as soon as
the plants of the first generation were in flower I thought that I would
raise one more generation, and acted in the following manner. Several
flowers on one or more of the self-fertilised plants were again
self-fertilised; and several flowers on one or more of the crossed
plants were fertilised with pollen from another crossed plant of the
same lot. Having thus once begun, the same method was followed for as
many as ten successive generations with some of the species. The seeds
and seedlings were always treated in exactly the same manner as already
described. The self-fertilised plants, whether originally descended from
one or two mother-plants, were thus in each generation as closely
interbred as was possible; and I could not have improved on my plan. But
instead of crossing one of the crossed plants with another crossed
plant, I ought to have crossed the self-fertilised plants of each
generation with pollen taken from a non-related plant--that is, one
belonging to a distinct family or stock of the same species and variety.
This was done in several cases as an additional experiment, and gave
very striking results. But the plan usually followed was to put into
competition and compare intercrossed plants, which were almost always
the offspring of more or less closely related plants, with the
self-fertilised plants of each succeeding generation;--all having been
grown under closely similar conditions. I have, however, learnt more by
this method of proceeding, which was begun by an oversight and then
necessarily followed, than if I had always crossed the self-fertilised
plants of each succeeding generation with pollen from a fresh stock.
I have said that the crossed plants of the successive generations were
almost always inter-related. When the flowers on an hermaphrodite plant
are crossed with pollen taken from a distinct plant, the seedlings thus
raised may be considered as hermaphrodite brothers or sisters; those
raised from the same capsule being as close as twins or animals of the
same litter. But in one sense the flowers on the same plant are distinct
individuals, and as several flowers on the mother-plant were crossed by
pollen taken from several flowers on the father-plant, such seedlings
would be in one sense half-brothers or sisters, but more closely related
than are the half-brothers and sisters of ordinary animals. The flowers
on the mother-plant were, however, commonly crossed by pollen taken from
two or more distinct plants; and in these cases the seedlings might be
called with more truth half-brothers or sisters. When two or three
mother-plants were crossed, as often happened, by pollen taken from two
or three father-plants (the seeds being all intermingled), some of the
seedlings of the first generation would be in no way related, whilst
many others would be whole or half-brothers and sisters. In the second
generation a large number of the seedlings would be what may be called
whole or half first-cousins, mingled with whole and half-brothers and
sisters, and with some plants not at all related. So it would be in the
succeeding generations, but there would also be many cousins of the
second and more remote degrees. The relationship will thus have become
more and more inextricably complex in the later generations; with most
of the plants in some degree and many of them closely related.
I have only one other point to notice, but this is one of the highest
importance; namely, that the crossed and self-fertilised plants were
subjected in the same generation to as nearly similar and uniform
conditions as was possible. In the successive generations they were
exposed to slightly different conditions as the seasons varied, and they
were raised at different periods. But in other respects all were treated
alike, being grown in pots in the same artificially prepared soil, being
watered at the same time, and kept close together in the same greenhouse
or hothouse. They were therefore not exposed during successive years to
such great vicissitudes of climate as are plants growing out of doors.
ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS.
It has been objected to such experiments as mine, that covering plants
with a net, although only for a short time whilst in flower, may affect
their health and fertility. I have seen no such effect except in one
instance with a Myosotis, and the covering may not then have been the
real cause of injury. But even if the net were slightly injurious, and
certainly it was not so in any high degree, as I could judge by the
appearance of the plants and by comparing their fertility with that of
neighbouring uncovered plants, it would not have vitiated my
experiments; for in all the more important cases the flowers were
crossed as well as self-fertilised under a net, so that they were
treated in this respect exactly alike.
As it is impossible to exclude such minute pollen-carrying insects as
Thrips, flowers which it was intended to fertilise with their own pollen
may sometimes have been afterwards crossed with pollen brought by these
insects from another flower on the same plant; but as we shall hereafter
see, a cross of this kind does not produce any effect, or at most only a
slight one. When two or more plants were placed near one another under
the same net, as was often done, there is some real though not great
danger of the flowers which were believed to be self-fertilised being
afterwards crossed with pollen brought by Thrips from a distinct plant.
I have said that the danger is not great because I have often found that
plants which are self-sterile, unless aided by insects, remained sterile
when several plants of the same species were placed under the same net.
If, however, the flowers which had been presumably self-fertilised by me
were in any case afterwards crossed by Thrips with pollen brought from a
distinct plant, crossed seedlings would have been included amongst the
self-fertilised; but it should be especially observed that this
occurrence would tend to diminish and not to increase any superiority in
average height, fertility, etc., of the crossed over the self-fertilised
plants.
As the flowers which were crossed were never castrated, it is probable
or even almost certain that I sometimes failed to cross-fertilise them
effectually, and that they were afterwards spontaneously
self-fertilised. This would have been most likely to occur with
dichogamous species, for without much care it is not easy to perceive
whether their stigmas are ready to be fertilised when the anthers open.
But in all cases, as the flowers were protected from wind, rain, and the
access of insects, any pollen placed by me on the stigmatic surface
whilst it was immature, would generally have remained there until the
stigma was mature; and the flowers would then have been crossed as was
intended. Nevertheless, it is highly probable that self-fertilised
seedlings have sometimes by this means got included amongst the crossed
seedlings. The effect would be, as in the former case, not to exaggerate
but to diminish any average superiority of the crossed over the
self-fertilised plants.
Errors arising from the two causes just named, and from others,--such as
some of the seeds not having been thoroughly ripened, though care was
taken to avoid this error--the sickness or unperceived injury of any of
the plants,--will have been to a large extent eliminated, in those cases
in which many crossed and self-fertilised plants were measured and an
average struck. Some of these causes of error will also have been
eliminated by the seeds having been allowed to germinate on bare damp
sand, and being planted in pairs; for it is not likely that ill-matured
and well-matured, or diseased and healthy seeds, would germinate at
exactly the same time. The same result will have been gained in the
several cases in which only a few of the tallest, finest, and healthiest
plants on each side of the pots were measured.
Kolreuter and Gartner have proved that with some plants several, even as
many as from fifty to sixty, pollen-grains are necessary for the
fertilisation of all the ovules in the ovarium. (1/9. ‘Kentniss der
Befruchtung’ 1844 page 345. Naudin ‘Nouvelles Archives du Museum’ tome 1
page 27.) Naudin also found in the case of Mirabilis that if only one or
two of its very large pollen-grains were placed on the stigma, the
plants raised from such seeds were dwarfed. I was therefore careful to
give an amply sufficient supply of pollen, and generally covered the
stigma with it; but I did not take any special pains to place exactly
the same amount on the stigmas of the self-fertilised and crossed
flowers. After having acted in this manner during two seasons, I
remembered that Gartner thought, though without any direct evidence,
that an excess of pollen was perhaps injurious; and it has been proved
by Spallanzani, Quatrefages, and Newport, that with various animals an
excess of the seminal fluid entirely prevents fertilisation. (1/10.
‘Transactions of the Philosophical Society’ 1853 pages 253-258.) It was
therefore necessary to ascertain whether the fertility of the flowers
was affected by applying a rather small and an extremely large quantity
of pollen to the stigma. Accordingly a very small mass of pollen-grains
was placed on one side of the large stigma in sixty-four flowers of
Ipomoea purpurea, and a great mass of pollen over the whole surface of
the stigma in sixty-four other flowers. In order to vary the experiment,
half the flowers of both lots were on plants produced from
self-fertilised seeds, and the other half on plants from crossed seeds.
The sixty-four flowers with an excess of pollen yielded sixty-one
capsules; and excluding four capsules, each of which contained only a
single poor seed, the remainder contained on an average 5.07 seeds per
capsule. The sixty-four flowers with only a little pollen placed on one
side of the stigma yielded sixty-three capsules, and excluding one from
the same cause as before, the remainder contained on an average 5.129
seeds. So that the flowers fertilised with little pollen yielded rather
more capsules and seeds than did those fertilised with an excess; but
the difference is too slight to be of any significance. On the other
hand, the seeds produced by the flowers with an excess of pollen were a
little heavier of the two; for 170 of them weighed 79.67 grains, whilst
170 seeds from the flowers with very little pollen weighed 79.20 grains.
Both lots of seeds having been placed on damp sand presented no
difference in their rate of germination. We may therefore conclude that
my experiments were not affected by any slight difference in the amount
of pollen used; a sufficiency having been employed in all cases.
The order in which our subject will be treated in the present volume is
as follows. A long series of experiments will first be given in Chapters
2 to 6. Tables will afterwards be appended, showing in a condensed form
the relative heights, weights, and fertility of the offspring of the
various crossed and self-fertilised species. Another table exhibits the
striking results from fertilising plants, which during several
generations had either been self-fertilised or had been crossed with
plants kept all the time under closely similar conditions, with pollen
taken from plants of a distinct stock and which had been exposed to
different conditions. In the concluding chapters various related points
and questions of general interest will be discussed.
Anyone not specially interested in the subject need not attempt to read
all the details (marked []); though they possess, I think, some value,
and cannot be all summarised. But I would suggest to the reader to take
as an example the experiments on Ipomoea in Chapter 2; to which may be
added those on Digitalis, Origanum, Viola, or the common cabbage, as in
all these cases the crossed plants are superior to the self-fertilised
in a marked degree, but not in quite the same manner. As instances of
self-fertilised plants being equal or superior to the crossed, the
experiments on Bartonia, Canna, and the common pea ought to be read; but
in the last case, and probably in that of Canna, the want of any
superiority in the crossed plants can be explained.
Species were selected for experiment belonging to widely distinct
families, inhabiting various countries. In some few cases several genera
belonging to the same family were tried, and these are grouped together;
but the families themselves have been arranged not in any natural order,
but in that which was the most convenient for my purpose. The
experiments have been fully given, as the results appear to me of
sufficient value to justify the details. Plants bearing hermaphrodite
flowers can be interbred more closely than is possible with bisexual
animals, and are therefore well-fitted to throw light on the nature and
extent of the good effects of crossing, and on the evil effects of close
interbreeding or self-fertilisation. The most important conclusion at
which I have arrived is that the mere act of crossing by itself does no
good. The good depends on the individuals which are crossed differing
slightly in constitution, owing to their progenitors having been
subjected during several generations to slightly different conditions,
or to what we call in our ignorance spontaneous variation. This
conclusion, as we shall hereafter see, is closely connected with various
important physiological problems, such as the benefit derived from
slight changes in the conditions of life, and this stands in the closest
connection with life itself. It throws light on the origin of the two
sexes and on their separation or union in the same individual, and
lastly on the whole subject of hybridism, which is one of the greatest
obstacles to the general acceptance and progress of the great principle
of evolution.
In order to avoid misapprehension, I beg leave to repeat that throughout
this volume a crossed plant, seedling, or seed, means one of crossed
PARENTAGE, that is, one derived from a flower fertilised with pollen
from a distinct plant of the same species. And that a self-fertilised
plant, seedling, or seed, means one of self-fertilised PARENTAGE, that
is, one derived from a flower fertilised with pollen from the same
flower, or sometimes, when thus stated, from another flower on the same
plant.
CHAPTER II.
CONVOLVULACEAE.
Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.
Greater constitutional vigour of the crossed plants.
The effects on the offspring of crossing different flowers on the same
plant, instead of crossing distinct individuals.
The effects of a cross with a fresh stock.
The descendants of the self-fertilised plant named Hero.
Summary on the growth, vigour, and fertility of the successive crossed
and self-fertilised generations.
Small amount of pollen in the anthers of the self-fertilised plants of
the later generations, and the sterility of their first-produced
flowers.
Uniform colour of the flowers produced by the self-fertilised plants.
The advantage from a cross between two distinct plants depends on their
differing in constitution.
A plant of Ipomoea purpurea, or as it is often called in England the
convolvulus major, a native of South America, grew in my greenhouse. Ten
flowers on this plant were fertilised with pollen from the same flower;
and ten other flowers on the same plant were crossed with pollen from a
distinct plant. The fertilisation of the flowers with their own pollen
was superfluous, as this convolvulus is highly self-fertile; but I acted
in this manner to make the experiments correspond in all respects.
Whilst the flowers are young the stigma projects beyond the anthers; and
it might have been thought that it could not be fertilised without the
aid of humble-bees, which often visit the flowers; but as the flower
grows older the stamens increase in length, and their anthers brush
against the stigma, which thus receives some pollen. The number of seeds
produced by the crossed and self-fertilised flowers differed very
little.
[Crossed and self-fertilised seeds obtained in the above manner were
allowed to germinate on damp sand, and as often as pairs germinated at
the same time they were planted in the manner described in the
Introduction (Chapter 1), on the opposite sides of two pots. Five pairs
were thus planted; and all the remaining seeds, whether or not in a
state of germination, were planted on the opposite sides of a third pot,
so that the young plants on both sides were here greatly crowded and
exposed to very severe competition. Rods of iron or wood of equal
diameter were given to all the plants to twine up; and as soon as one of
each pair reached the summit both were measured. A single rod was placed
on each side of the crowded pot, Number 3, and only the tallest plant on
each side was measured.
TABLE 2/1. Ipomoea purpurea (First Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Seedlings from Crossed Plants.
Column 3: Seedlings from Self-fertilised Plants.
Pot 1 : 87 4/8 : 69.
Pot 1 : 87 4/8 : 66.
Pot 1 : 89 : 73.
Pot 2 : 88 : 68 4/8.
Pot 2 : 87 : 60 4/8.
Pot 3 : 77 : 57.
Plants crowded; the tallest one measured on each side.
Total : 516 : 394.
The average height of the six crossed plants is here 86 inches, whilst
that of the six self-fertilised plants is only 65.66 inches, so that the
crossed plants are to the self-fertilised in height as 100 to 76. It
should be observed that this difference is not due to a few of the
crossed plants being extremely tall, or to a few of the self-fertilised
being extremely short, but to all the crossed plants attaining a greater
height than their antagonists. The three pairs in Pot 1 were measured at
two earlier periods, and the difference was sometimes greater and
sometimes less than that at the final measuring. But it is an
interesting fact, of which I have seen several other instances, that one
of the self-fertilised plants, when nearly a foot in height, was half an
inch taller than the crossed plant; and again, when two feet high, it
was 1 3/8 of an inch taller, but during the ten subsequent days the
crossed plant began to gain on its antagonist, and ever afterward
asserted its supremacy, until it exceeded its self-fertilised opponent
by 16 inches.
The five crossed plants in Pots 1 and 2 were covered with a net, and
produced 121 capsules; the five self-fertilised plants produced
eighty-four capsules, so that the numbers of capsules were as 100 to 69.
Of the 121 capsules on the crossed plants sixty-five were the product of
flowers crossed with pollen from a distinct plant, and these contained
on an average 5.23 seeds per capsule; the remaining fifty-six capsules
were spontaneously self-fertilised. Of the eighty-four capsules on the
self-fertilised plants, all the product of renewed self-fertilisation,
fifty-five (which were alone examined) contained on an average 4.85
seeds per capsule. Therefore the cross-fertilised capsules, compared
with the self-fertilised capsules, yielded seeds in the proportion of
100 to 93. The crossed seeds were relatively heavier than the
self-fertilised seeds. Combining the above data (i.e., number of
capsules and average number of contained seeds), the crossed plants,
compared with the self-fertilised, yielded seeds in the ratio of 100 to
64.
These crossed plants produced, as already stated, fifty-six
spontaneously self-fertilised capsules, and the self-fertilised plants
produced twenty-nine such capsules. The former contained on an average,
in comparison with the latter, seeds in the proportion of 100 to 99.
In Pot 3, on the opposite sides of which a large number of crossed and
self-fertilised seeds had been sown and the seedlings allowed to
struggle together, the crossed plants had at first no great advantage.
At one time the tallest crossed was 25 1/8 inches high, and the tallest
self-fertilised plants 21 3/8. But the difference afterwards became much
greater. The plants on both sides, from being so crowded, were poor
specimens. The flowers were allowed to fertilise themselves
spontaneously under a net; the crossed plants produced thirty-seven
capsules, the self-fertilised plants only eighteen, or as 100 to 47. The
former contained on an average 3.62 seeds per capsule; and the latter
3.38 seeds, or as 100 to 93. Combining these data (i.e., number of
capsules and average number of seeds), the crowded crossed plants
produced seeds compared with the self-fertilised as 100 to 45. These
latter seeds, however, were decidedly heavier, a hundred weighing 41.64
grains, than those from the capsules on the crossed plants, of which a
hundred weighed 36.79 grains; and this probably was due to the fewer
capsules borne by the self-fertilised plants having been better
nourished. We thus see that the crossed plants in this the first
generation, when grown under favourable conditions, and when grown under
unfavourable conditions from being much crowded, greatly exceeded in
height, and in the number of capsules produced, and slightly in the
number of seeds per capsule, the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Flowers on the crossed plants of the last generation (Table 2/1) were
crossed by pollen from distinct plants of the same generation; and
flowers on the self-fertilised plants were fertilised by pollen from the
same flower. The seeds thus produced were treated in every respect as
before, and we have in Table 2/2 the result.
TABLE 2/2. Ipomoea purpurea (Second Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 87 : 67 4/8.
Pot 1 : 83 : 68 4/8.
Pot 1 : 83 : 80 4/8.
Pot 2 : 85 4/8 : 61 4/8.
Pot 2 : 89 : 79.
Pot 2 : 77 4/8 : 41.
Total : 505 : 398.
Here again every single crossed plant is taller than its antagonist. The
self-fertilised plant in Pot 1, which ultimately reached the unusual
height of 80 4/8 inches, was for a long time taller than the opposed
crossed plant, though at last beaten by it. The average height of the
six crossed plants is 84.16 inches, whilst that of the six
self-fertilised plants is 66.33 inches, or as 100 to 79.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Seeds from the crossed plants of the last generation (Table 2/2) again
crossed, and from the self-fertilised plants again self-fertilised, were
treated in all respects exactly as before, with the following result:--
TABLE 2/3. Ipomoea purpurea (Third Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 74 : 56 4/8.
Pot 1 : 72 : 51 4/8.
Pot 1 : 73 4/8 : 54.
Pot 2 : 82 : 59.
Pot 2 : 81 : 30.
Pot 2 : 82 : 66.
Total : 464.5 : 317.
Again all the crossed plants are higher than their antagonists: their
average height is 77.41 inches, whereas that of the self-fertilised is
52.83 inches, or as 100 to 68.
I attended closely to the fertility of the plants of this third
generation. Thirty flowers on the crossed plants were crossed with
pollen from other crossed plants of the same generation, and the
twenty-six capsules thus produced contained, on an average, 4.73 seeds;
whilst thirty flowers on the self-fertilised plants, fertilised with the
pollen from the same flower, produced twenty-three capsules, each
containing 4.43 seeds. Thus the average number of seeds in the crossed
capsules was to that in the self-fertilised capsules as 100 to 94. A
hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of
the self-fertilised seeds weighed only 37.63 grains. Many of these
lighter self-fertilised seeds placed on damp sand germinated before the
crossed; thus thirty-six of the former germinated whilst only thirteen
of the latter or crossed seeds germinated. In Pot 1 the three crossed
plants produced spontaneously under the net (besides the twenty-six
artificially cross-fertilised capsules) seventy-seven self-fertilised
capsules containing on an average 4.41 seeds; whilst the three
self-fertilised plants produced spontaneously (besides the twenty-three
artificially self-fertilised capsules) only twenty-nine self-fertilised
capsules, containing on an average 4.14 seeds. Therefore the average
number of seeds in the two lots of spontaneously self-fertilised
capsules was as 100 to 94. Taking into consideration the number of
capsules together with the average number of seeds, the crossed plants
(spontaneously self-fertilised) produced seeds in comparison with the
self-fertilised plants (spontaneously self-fertilised) in the proportion
of 100 to 35. By whatever method the fertility of these plants is
compared, the crossed are more fertile than the self-fertilised plants.
I tried in several ways the comparative vigour and powers of growth of
the crossed and self-fertilised plants of this third generation. Thus,
four self-fertilised seeds which had just germinated were planted on one
side of a pot, and after an interval of forty-eight hours, four crossed
seeds in the same state of germination were planted on the opposite
side; and the pot was kept in the hothouse. I thought that the advantage
thus given to the self-fertilised seedlings would have been so great
that they would never have been beaten by the crossed ones. They were
not beaten until all had grown to a height of 18 inches; and the degree
to which they were finally beaten is shown in Table 2/4. We here see
that the average height of the four crossed plants is 76.62, and of the
four self-fertilised plants 65.87 inches, or as 100 to 86; therefore
less than when both sides started fair.
TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised
plants having had a start of forty-eight hours).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 3 : 78 4/8 : 73 4/8.
Pot 3 : 77 4/8 : 53.
Pot 3 : 73 : 61 4/8.
Pot 3 : 77 4/8 : 75 4/8.
Total : 306.5 : 263.5.
Crossed and self-fertilised seeds of the third generation were also sown
out of doors late in the summer, and therefore under unfavourable
conditions, and a single stick was given to each lot of plants to twine
up. The two lots were sufficiently separate so as not to interfere with
each other’s growth, and the ground was clear of weeds. As soon as they
were killed by the first frost (and there was no difference in their
hardiness), the two tallest crossed plants were found to be 24.5 and
22.5 inches, whilst the two tallest self-fertilised plants were only 15
and 12.5 inches in height, or as 100 to 59.
I likewise sowed at the same time two lots of the same seeds in a part
of the garden which was shady and covered with weeds. The crossed
seedlings from the first looked the most healthy, but they twined up a
stick only to a height of 7 1/4 inches; whilst the self-fertilised were
not able to twine at all; and the tallest of them was only 3 1/2 inches
in height.
Lastly, two lots of the same seeds were sown in the midst of a bed of
candy-tuft (Iberis) growing vigorously. The seedlings came up, but all
the self-fertilised ones soon died excepting one, which never twined and
grew to a height of only 4 inches. Many of the crossed seedlings, on the
other hand, survived; and some twined up the stems of the Iberis to the
height of 11 inches. These cases prove that the crossed seedlings have
an immense advantage over the self-fertilised, both when growing
isolated under very unfavourable conditions, and when put into
competition with each other or with other plants, as would happen in a
state of nature.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.
Seedlings raised as before from the crossed and self-fertilised plants
of the third generation in Table 2/3, gave results as follows:--
TABLE 2/5. Ipomoea purpurea (Fourth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 : 80.
Pot 1 : 47 : 44 1/2.
Pot 2 : 83 : 73 1/2.
Pot 2 : 59 : 51 1/2.
Pot 3 : 82 : 56 1/2.
Pot 3 : 65 1/2 : 63.
Pot 3 : 68 : 52.
Total : 488.5 : 421.0.
Here the average height of the seven crossed plants is 69.78 inches, and
that of the seven self-fertilised plants 60.14; or as 100 to 86. This
smaller difference relatively to that in the former generations, may be
attributed to the plants having been raised during the depth of winter,
and consequently to their not having grown vigorously, as was shown by
their general appearance and from several of them never reaching the
summits of the rods. In Pot 2, one of the self-fertilised plants was for
a long time taller by two inches than its opponent, but was ultimately
beaten by it, so that all the crossed plants exceeded their opponents in
height. Of twenty-eight capsules produced by the crossed plants
fertilised by pollen from a distinct plant, each contained on an average
4.75 seeds; of twenty-seven self-fertilised capsules on the
self-fertilised plants, each contained on an average 4.47 seeds; so that
the proportion of seeds in the crossed and self-fertilised capsules was
as 100 to 94.
Some of the same seeds, from which the plants in Table 2/5 had been
raised, were planted, after they had germinated on damp sand, in a
square tub, in which a large Brugmansia had long been growing. The soil
was extremely poor and full of roots; six crossed seeds were planted in
one corner, and six self-fertilised seeds in the opposite corner. All
the seedlings from the latter soon died excepting one, and this grew to
the height of only 1 1/2 inches. Of the crossed plants three survived,
and they grew to the height of 2 1/2 inches, but were not able to twine
round a stick; nevertheless, to my surprise, they produced some small
miserable flowers. The crossed plants thus had a decided advantage over
the self-fertilised plants under this extremity of bad conditions.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.
These were raised in the same manner as before, and when measured gave
the following results:--
TABLE 2/6. Ipomoea purpurea (Fifth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 96 : 73.
Pot 1 : 86 : 78.
Pot 1 : 69 : 29.
Pot 2 : 84 : 51.
Pot 2 : 84 : 84.
Pot 2 : 76 1/4 : 59.
Total : 495.25 : 374.00.
The average height of the six crossed plants is 82.54 inches, and that
of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every
crossed plant exceeded its antagonist in height. In Pot 1 the middle
plant on the crossed side was slightly injured whilst young by a blow,
and was for a time beaten by its opponent, but ultimately recovered the
usual superiority. The crossed plants produced spontaneously a vast
number more capsules than did the self-fertilised plants; and the
capsules of the former contained on an average 3.37 seeds, whilst those
of the latter contained only 3.0 per capsule, or as 100 to 89. But
looking only to the artificially fertilised capsules, those on the
crossed plants again crossed contained on an average 4.46 seeds, whilst
those on the self-fertilised plants again self-fertilised contained 4.77
seeds; so that the self-fertilised capsules were the more fertile of the
two, and of this unusual fact I can offer no explanation.
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
These were raised in the usual manner, with the following result. I
should state that there were originally eight plants on each side; but
as two of the self-fertilised became extremely unhealthy and never grew
to near their full height, these as well as their opponents have been
struck out of the list. If they had been retained, they would have made
the average height of the crossed plants unfairly greater than that of
the self-fertilised. I have acted in the same manner in a few other
instances, when one of a pair plainly became very unhealthy.
TABLE 2/7. Ipomoea purpurea (Sixth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 93 : 50 1/2.
Pot 1 : 91 : 65.
Pot 2 : 79 : 50.
Pot 2 : 86 1/2 : 87.
Pot 2 : 88 : 62.
Pot 3 : 87 1/2 : 64 1/2.
Total : 525 : 379.
The average height of the six crossed plants is here 87.5, and of the
six self-fertilised plants 63.16, or as 100 to 72. This large difference
was chiefly due to most of the plants, especially the self-fertilised
ones, having become unhealthy towards the close of their growth, and
they were severely attacked by aphides. From this cause nothing can be
inferred with respect to their relative fertility. In this generation we
have the first instance of a self-fertilised plant in Pot 2 exceeding
(though only by half an inch) its crossed opponent. This victory was
fairly won after a long struggle. At first the self-fertilised plant was
several inches taller than its opponent, but when the latter was 4 1/2
feet high it had grown equal; it then grew a little taller than the
self-fertilised plant, but was ultimately beaten by it to the extent of
half an inch, as shown in Table 2/7. I was so much surprised at this
case that I saved the self-fertilised seeds of this plant, which I will
call the “Hero,” and experimented on its descendants, as will hereafter
be described.
Besides the plants included in Table 2/7, nine crossed and nine
self-fertilised plants of the same lot were raised in two other pots, 4
and 5. These pots had been kept in the hothouse, but from want of room
were, whilst the plants were young, suddenly moved during very cold
weather into the coldest part of the greenhouse. They all suffered
greatly, and never quite recovered. After a fortnight only two of the
nine self-fertilised seedlings were alive, whilst seven of the crossed
survived. The tallest of these latter plants when measured was 47 inches
in height, whilst the tallest of the two surviving self-fertilised
plants was only 32 inches. Here again we see how much more vigorous the
crossed plants are than the self-fertilised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.
These were raised as heretofore with the following result:--
TABLE 2/8. Ipomoea purpurea (Seventh Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 4/8 : 74 6/8.
Pot 1 : 84 6/8 : 84.
Pot 1 : 76 2/8 : 55 4/8.
Pot 2 : 84 4/8 : 65.
Pot 2 : 90 : 51 2/8.
Pot 2 : 82 2/8 : 80 4/8.
Pot 3 : 83 : 67 6/8.
Pot 3 : 86 : 60 2/8.
Pot 4 : 84 2/8 : 75 2/8.
Total : 755.50 : 614.25.
Each of these nine crossed plants is higher than its opponent, though in
one case only by three-quarters of an inch. Their average height is
83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to
81. These plants, after growing to their full height, became very
unhealthy and infested with aphides, just when the seeds were setting,
so that many of the capsules failed, and nothing can be said on their
relative fertility.
CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.
As just stated, the plants of the last generation, from which the
present ones were raised, were very unhealthy and their seeds of
unusually small size; and this probably accounts for the two lots
behaving differently to what they did in any of the previous or
succeeding generations. Many of the self-fertilised seeds germinated
before the crossed ones, and these were of course rejected. When the
crossed seedlings in Table 2/9 had grown to a height of between 1 and 2
feet, they were all, or almost all, shorter than their self-fertilised
opponents, but were not then measured. When they had acquired an average
height of 32.28 inches, that of the self-fertilised plants was 40.68, or
as 100 to 122. Moreover, every one of the self-fertilised plants, with a
single exception, exceeded its crossed opponent. When, however, the
crossed plants had grown to an average height of 77.56 inches, they just
exceeded (namely, by .7 of an inch) the average height of the
self-fertilised plants; but two of the latter were still taller than
their crossed opponents. I was so much astonished at this whole case,
that I tied string to the summits of the rods; the plants being thus
allowed to continue climbing upwards. When their growth was complete
they were untwined, stretched straight, and measured. The crossed plants
had now almost regained their accustomed superiority, as may be seen in
Table 2/9.
The average height of the eight crossed plants is here 113.25 inches,
and that of the self-fertilised plants 96.65, or as 100 to 85.
Nevertheless two of the self-fertilised plants, as may be seen in Table
2/9, were still higher than their crossed opponents. The latter
manifestly had much thicker stems and many more lateral branches, and
looked altogether more vigorous than the self-fertilised plants, and
generally flowered before them. The earlier flowers produced by these
self-fertilised plants did not set any capsules, and their anthers
contained only a small amount of pollen; but to this subject I shall
return. Nevertheless capsules produced by two other self-fertilised
plants of the same lot, not included in Table 2/9, which had been highly
favoured by being grown in separate pots, contained the large average
number of 5.1 seeds per capsule.
TABLE 2/9. Ipomoea purpurea (Eighth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 111 6/8 : 96.
Pot 1 : 127 : 54.
Pot 1 : 130 6/8 : 93 4/8.
Pot 2 : 97 2/8 : 94.
Pot 2 : 89 4/8 : 125 6/8.
Pot 3 : 103 6/8 : 115 4/8.
Pot 3 : 100 6/8 : 84 6/8.
Pot 3 : 147 4/8 : 109 6/8.
Total : 908.25 : 773.25.
CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.
The plants of this generation were raised in the same manner as before,
with the result shown in Table 2/10.
The fourteen crossed plants average in height 81.39 inches and the
fourteen self-fertilised plants 64.07, or as 100 to 79. One
self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in
height, its opponent. The self-fertilised plants showed no sign of
inheriting the precocious growth of their parents; this having been due,
as it would appear, to the abnormal state of the seeds from the
unhealthiness of their parents. The fourteen self-fertilised plants
yielded only forty spontaneously self-fertilised capsules, to which must
be added seven, the product of ten flowers artificially self-fertilised.
On the other hand, the fourteen crossed plants yielded 152 spontaneously
self-fertilised capsules; but thirty-six flowers on these plants were
crossed (yielding thirty-three capsules), and these flowers would
probably have produced about thirty spontaneously self-fertilised
capsules. Therefore an equal number of the crossed and self-fertilised
plants would have produced capsules in the proportion of about 182 to
47, or as 100 to 26. Another phenomenon was well pronounced in this
generation, but I believe had occurred previously to a slight extent;
namely, that most of the flowers on the self-fertilised plants were
somewhat monstrous. The monstrosity consisted in the corolla being
irregularly split so that it did not open properly, with one or two of
the stamens slightly foliaceous, coloured, and firmly coherent to the
corolla. I observed this monstrosity in only one flower on the crossed
plants. The self-fertilised plants, if well nourished, would almost
certainly, in a few more generations, have produced double flowers, for
they had already become in some degree sterile. (2/1. See on this
subject ‘Variation of Animals and Plants under Domestication’ chapter 18
2nd edition volume 2 page 152.)
TABLE 2/10. Ipomoea purpurea (Ninth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 83 4/8 : 57.
Pot 1 : 85 4/8 : 71.
Pot 1 : 83 4/8 : 48 3/8.
Pot 2 : 83 2/8 : 45.
Pot 2 : 64 2/8 : 43 6/8.
Pot 2 : 64 3/8 : 38 4/8.
Pot 3 : 79 : 63.
Pot 3 : 88 1/8 : 71.
Pot 3 : 61 : 89 4/8.
Pot 4 : 82 4/8 : 82 4/8.
Pot 4 : 90 : 76 1/8.
Pot 5 : 89 4/8 : 67.
Pot 5 : 92 4/8 : 74 2/8.
Pot 5 : 92 4/8 : 70.
Crowded plants.
Total : 1139.5 : 897.0.
CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.
Six plants were raised in the usual manner from the crossed plants of
the last generation (Table 2/10) again intercrossed, and from the
self-fertilised again self-fertilised. As one of the crossed plants in
Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and
producing hardly any capsules, it and its opponent have been struck out
of the table.
TABLE 2/11. Ipomoea purpurea (Tenth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 92 3/8 : 47 2/8.
Pot 1 : 94 4/8 : 34 6/8.
Pot 2 : 87 : 54 4/8.
Pot 2 : 89 5/8 : 49 2/8.
Pot 2 : 105 : 66 2/8.
Total : 468.5 : 252.0.
The five crossed plants average 93.7 inches, and the five
self-fertilised only 50.4, or as 100 to 54. This difference, however, is
so great that it must be looked at as in part accidental. The six
crossed plants (the diseased one here included) yielded spontaneously
101 capsules, and the six self-fertilised plants 88, the latter being
chiefly produced by one of the plants. But as the diseased plant, which
yielded hardly any seed, is here included, the ratio of 101 to 88 does
not fairly give the relative fertility of the two lots. The stems of the
six crossed plants looked so much finer than those of the six
self-fertilised plants, that after the capsules had been gathered and
most of the leaves had fallen off, they were weighed. Those of the
crossed plants weighed 2,693 grains, whilst those of the self-fertilised
plants weighed only 1,173 grains, or as 100 to 44; but as the diseased
and dwarfed crossed plant is here included, the superiority of the
former in weight was really greater.]
THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME
PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments, seedlings from flowers crossed by
pollen from a distinct plant (though in the later generations more or
less closely related) were put into competition with, and almost
invariably proved markedly superior in height to the offspring from
self-fertilised flowers. I wished, therefore, to ascertain whether a
cross between two flowers on the same plant would give to the offspring
any superiority over the offspring from flowers fertilised with their
own pollen. I procured some fresh seed and raised two plants, which were
covered with a net; and several of their flowers were crossed with
pollen from a distinct flower on the same plant. Twenty-nine capsules
thus produced contained on an average 4.86 seeds per capsule; and 100 of
these seeds weighed 36.77 grains. Several other flowers were fertilised
with their own pollen, and twenty-six capsules thus produced contained
on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains.
So that a cross of this kind appears to have increased slightly the
number of seeds per capsule, in the ratio of 100 to 91; but these
crossed seeds were lighter than the self-fertilised in the ratio of 86
to 100. I doubt, however, from other observations, whether these results
are fully trustworthy. The two lots of seeds, after germinating on sand,
were planted in pairs on the opposite sides of nine pots, and were
treated in every respect like the plants in the previous experiments.
The remaining seeds, some in a state of germination and some not so,
were sown on the opposite sides of a large pot (Number 10); and the four
tallest plants on each side of this pot were measured. The result is
shown in Table 2/12.
TABLE 2/12. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 82 : 77 4/8.
Pot 1 : 75 : 87.
Pot 1 : 65 : 64.
Pot 1 : 76 : 87 2/8.
Pot 2 : 78 4/8 : 84.
Pot 2 : 43 : 86 4/8.
Pot 2 : 65 4/8 : 90 4/8.
Pot 3 : 61 2/8 : 86.
Pot 3 : 85 : 69 4/8.
Pot 3 : 89 : 87 4/8.
Pot 4 : 83 : 80 4/8.
Pot 4 : 73 4/8 : 88 4/8.
Pot 4 : 67 : 84 4/8.
Pot 5 : 78 : 66 4/8.
Pot 5 : 76 6/8 : 77 4/8.
Pot 5 : 57 : 81 4/8.
Pot 6 : 70 4/8 : 80.
Pot 6 : 79 : 82 4/8.
Pot 6 : 79 6/8 : 55 4/8.
Pot 7 : 76 : 77.
Pot 7 : 84 4/8 : 83 4/8.
Pot 7 : 79 : 73 4/8.
Pot 8 : 73 : 76 4/8.
Pot 8 : 67 : 82.
Pot 8 : 83 : 80 4/8.
Pot 9 : 73 2/8 : 78 4/8.
Pot 9 : 78 : 67 4/8.
Pot 10 : 34 : 82 4/8.
Pot 10 : 82 : 36 6/8.
Pot 10 : 84 6/8 : 69 4/8.
Pot 10 : 71 : 75 2/8.
Crowded plants.
Total : 2270.25 : 2399.75.
The average height of the thirty-one crossed plants is 73.23 inches, and
that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to
106. Looking to each pair, it may be seen that only thirteen of the
crossed plants, whilst eighteen of the self-fertilised plants exceed
their opponents. A record was kept with respect to the plant which
flowered first in each pot; and only two of the crossed flowered before
one of the self-fertilised in the same pot; whilst eight of the
self-fertilised flowered first. It thus appears that the crossed plants
are slightly inferior in height and in earliness of flowering to the
self-fertilised. But the inferiority in height is so small, namely as
100 to 106, that I should have felt very doubtful on this head, had I
not cut down all the plants (except those in the crowded pot Number 10)
close to the ground and weighed them. The twenty-seven crossed plants
weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20
1/2 ounces; and this gives a ratio of 100 to 124.
A self-fertilised plant of the same parentage as those in Table 2/12 had
been raised in a separate pot for a distinct purpose; and it proved
partially sterile, the anthers containing very little pollen. Several
flowers on this plant were crossed with the little pollen which could be
obtained from the other flowers on the same plant; and other flowers
were self-fertilised. From the seeds thus produced four crossed and four
self-fertilised plants were raised, which were planted in the usual
manner on the opposite sides of two pots. All these four crossed plants
were inferior in height to their opponents; they averaged 78.18 inches,
whilst the four self-fertilised plants averaged 84.8 inches; or as 100
to 108. (2/2. From one of these self-fertilised plants, spontaneously
self-fertilised, I gathered twenty-four capsules, and they contained on
an average only 3.2 seeds per capsule; so that this plant had apparently
inherited some of the sterility of its parent.) This case, therefore,
confirms the last. Taking all the evidence together, we must conclude
that these strictly self-fertilised plants grew a little taller, were
heavier, and generally flowered before those derived from a cross
between two flowers on the same plant. These latter plants thus present
a wonderful contrast with those derived from a cross between two
distinct individuals.
THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK
BELONGING TO THE SAME VARIETY.
From the two foregoing series of experiments we see, firstly, the good
effects during several successive generations of a cross between
distinct plants, although these were in some degree inter-related and
had been grown under nearly the same conditions; and, secondly, the
absence of all such good effects from a cross between flowers on the
same plant; the comparison in both cases being made with the offspring
of flowers fertilised with their own pollen. The experiments now to be
given show how powerfully and beneficially plants, which have been
intercrossed during many successive generations, having been kept all
the time under nearly uniform conditions, are affected by a cross with
another plant belonging to the same variety, but to a distinct family or
stock, which had grown under different conditions.
[Several flowers on the crossed plants of the ninth generation in Table
2/10, were crossed with pollen from another crossed plant of the same
lot. The seedlings thus raised formed the tenth intercrossed generation,
and I will call them the “INTERCROSSED PLANTS.” Several other flowers on
the same crossed plants of the ninth generation were fertilised (not
having been castrated) with pollen taken from plants of the same
variety, but belonging to a distinct family, which had been grown in a
distant garden at Colchester, and therefore under somewhat different
conditions. The capsules produced by this cross contained, to my
surprise, fewer and lighter seeds than did the capsules of the
intercrossed plants; but this, I think, must have been accidental. The
seedlings raised from them I will call the “COLCHESTER-CROSSED.” The two
lots of seeds, after germinating on sand, were planted in the usual
manner on the opposite sides of five pots, and the remaining seeds,
whether or not in a state of germination, were thickly sown on the
opposite sides of a very large pot, Number 6 in Table 2/13. In three of
the six pots, after the young plants had twined a short way up their
sticks, one of the Colchester-crossed plants was much taller than any
one of the intercrossed plants on the opposite side of the same pot; and
in the three other pots somewhat taller. I should state that two of the
Colchester-crossed plants in Pot 4, when about two-thirds grown, became
much diseased, and were, together with their intercrossed opponents,
rejected. The remaining nineteen plants, when almost fully grown, were
measured, with the following result:
TABLE 2/13. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Colchester-Crossed Plants.
Column 3: Intercrossed Plants of the Tenth Generation.
Pot 1 : 87 : 78.
Pot 1 : 87 4/8 : 68 4/8.
Pot 1 : 85 1/8 : 94 4/8.
Pot 2 : 93 6/8 : 60.
Pot 2 : 85 4/8 : 87 2/8.
Pot 2 : 90 5/8 : 45 4/8.
Pot 3 : 84 2/8 : 70 1/8.
Pot 3 : 92 4/8 : 81 6/8.
Pot 3 : 85 : 86 2/8.
Pot 4 : 95 6/8 : 65 1/8.
Pot 5 : 90 4/8 : 85 6/8.
Pot 5 : 86 6/8 : 63.
Pot 5 : 84 : 62 6/8.
Pot 6 : 90 4/8 : 43 4/8.
Pot 6 : 75 : 39 6/8.
Pot 6 : 71 : 30 2/8.
Pot 6 : 83 6/8 : 86.
Pot 6 : 63 : 53.
Pot 6 : 65 : 48 6/8.
Crowded plants in a very large pot.
Total : 1596.50 : 1249.75.
In sixteen out of these nineteen pairs, the Colchester-crossed plant
exceeded in height its intercrossed opponent. The average height of the
Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78
inches; or as 100 to 78. With respect to the fertility of the two lots,
it was too troublesome to collect and count the capsules on all the
plants; so I selected two of the best pots, 5 and 6, and in these the
Colchester-crossed produced 269 mature and half-mature capsules, whilst
an equal number of the intercrossed plants produced only 154 capsules;
or as 100 to 57. By weight the capsules from the Colchester-crossed
plants were to those from the intercrossed plants as 100 to 51; so that
the former probably contained a somewhat larger average number of
seeds.]
We learn from this important experiment that plants in some degree
related, which had been intercrossed during the nine previous
generations, when they were fertilised with pollen from a fresh stock,
yielded seedlings as superior to the seedlings of the tenth intercrossed
generation, as these latter were to the self-fertilised plants of the
corresponding generation. For if we look to the plants of the ninth
generation in Table 2/10 (and these offer in most respects the fairest
standard of comparison) we find that the intercrossed plants were in
height to the self-fertilised as 100 to 79, and in fertility as 100 to
26; whilst the Colchester-crossed plants are in height to the
intercrossed as 100 to 78, and in fertility as 100 to 51.
[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH
APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.
In the five generations before the sixth, the crossed plant of each pair
was taller than its self-fertilised opponent; but in the sixth
generation (Table 2/7, Pot 2) the Hero appeared, which after a long and
dubious struggle conquered its crossed opponent, though by only half an
inch. I was so much surprised at this fact, that I resolved to ascertain
whether this plant would transmit its powers of growth to its seedlings.
Several flowers on Hero were therefore fertilised with their own pollen,
and the seedlings thus raised were put into competition with
self-fertilised and intercrossed plants of the corresponding generation.
The three lots of seedlings thus all belong to the seventh generation.
Their relative heights are shown in Tables 2/14 and 2/15.
TABLE 2/14. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 74 : 89 4/8.
Pot 1 : 60 : 61.
Pot 1 : 55 2/8 : 49.
Pot 2 : 92 : 82.
Pot 2 : 91 6/8 : 56.
Pot 2 : 74 2/8 : 38.
Total : 447.25 : 375.50.
The average height of the six self-fertilised children of Hero is 74.54
inches, whilst that of the ordinary self-fertilised plants of the
corresponding generation is only 62.58 inches, or as 100 to 84.
TABLE 2/15. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Intercrossed Plants of the Seventh Generation.
Pot 3 : 92 : 76 6/8.
Pot 4 : 87 : 89.
Pot 4 : 87 6/8 : 86 6/8.
Total : 266.75 : 252.50.
Here the average height of the three self-fertilised children of Hero is
88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100
to 95. We thus see that the self-fertilised children of Hero certainly
inherit the powers of growth of their parents; for they greatly exceed
in height the self-fertilised offspring of the other self-fertilised
plants, and even exceed by a trifle the intercrossed plants,--all of the
corresponding generation.
Several flowers on the self-fertilised children of Hero in Table 2/14
were fertilised with pollen from the same flower; and from the seeds
thus produced, self-fertilised plants of the eighth generation
(grandchildren of Hero) were raised. Several other flowers on the same
plants were crossed with pollen from the other children of Hero. The
seedlings raised from this cross may be considered as the offspring of
the union of brothers and sisters. The result of the competition between
these two sets of seedlings (namely self-fertilised and the offspring of
brothers and sisters) is given in Table 2/16.
TABLE 2/16. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Grandchildren of Hero, from the
Self-fertilised Children. Eighth Generation.
Column 3: Grandchildren from a cross between the self-fertilised
children of Hero. Eighth Generation.
Pot 1 : 86 6/8 : 95 6/8.
Pot 1 : 90 3/8 : 95 3/8.
Pot 2 : 96 : 85.
Pot 2 : 77 2/8 : 93.
Pot 3 : 73 : 86 2/8.
Pot 3 : 66 : 82 2/8.
Pot 3 : 84 4/8 : 70 6/8.
Pot 4 : 88 1/8 : 66 3/8.
Pot 4 : 84 : 15 4/8.
Pot 4 : 36 2/8 : 38.
Pot 4 : 74 : 78 3/8.
Pot 5 : 90 1/8 : 82 6/8.
Pot 5 : 90 5/8 : 83 6/8.
Total : 1037.00 : 973.16.
The average height of the thirteen self-fertilised grandchildren of Hero
is 79.76 inches, and that of the grandchildren from a cross between the
self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of
the crossed plants grew only to a height of 15 1/2 inches; and if this
plant and its opponent are struck out, as would be the fairest plan, the
average height of the crossed plants exceeds only by a fraction of an
inch that of the self-fertilised plants. It is therefore clear that a
cross between the self-fertilised children of Hero did not produce any
beneficial effect worth notice; and it is very doubtful whether this
negative result can be attributed merely to the fact of brothers and
sisters having been united, for the ordinary intercrossed plants of the
several successive generations must often have been derived from the
union of brothers and sisters (as shown in Chapter 1), and yet all of
them were greatly superior to the self-fertilised plants. We are
therefore driven to the suspicion, which we shall soon see strengthened,
that Hero transmitted to its offspring a peculiar constitution adapted
for self-fertilisation.
It would appear that the self-fertilised descendants of Hero have not
only inherited from Hero a power of growth equal to that of the ordinary
intercrossed plants, but have become more fertile when self-fertilised
than is usual with the plants of the present species. The flowers on the
self-fertilised grandchildren of Hero in Table 2.16 (the eighth
generation of self-fertilised plants) were fertilised with their own
pollen and produced plenty of capsules, ten of which (though this is too
few a number for a safe average) contained 5.2 seeds per capsule,--a
higher average than was observed in any other case with the
self-fertilised plants. The anthers produced by these self-fertilised
grandchildren were also as well developed and contained as much pollen
as those on the intercrossed plants of the corresponding generation;
whereas this was not the case with the ordinary self-fertilised plants
of the later generations. Nevertheless some few of the flowers produced
by the grandchildren of Hero were slightly monstrous, like those of the
ordinary self-fertilised plants of the later generations. In order not
to recur to the subject of fertility, I may add that twenty-one
self-fertilised capsules, spontaneously produced by the
great-grandchildren of Hero (forming the ninth generation of
self-fertilised plants), contained on an average 4.47 seeds; and this is
as high an average as the self-fertilised flowers of any generation
usually yielded.
Several flowers on the self-fertilised grandchildren of Hero in Table
2/16 were fertilised with pollen from the same flower; and the seedlings
raised from them (great-grandchildren of Hero) formed the ninth
self-fertilised generation. Several other flowers were crossed with
pollen from another grandchild, so that they may be considered as the
offspring of brothers and sisters, and the seedlings thus raised may be
called the INTERCROSSED great-grandchildren. And lastly, other flowers
were fertilised with pollen from a distinct stock, and the seedlings
thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In
my anxiety to see what the result would be, I unfortunately planted the
three lots of seeds (after they had germinated on sand) in the hothouse
in the middle of winter, and in consequence of this the seedlings
(twenty in number of each kind) became very unhealthy, some growing only
a few inches in height, and very few to their proper height. The result,
therefore, cannot be fully trusted; and it would be useless to give the
measurements in detail. In order to strike as fair an average as
possible, I first excluded all the plants under 50 inches in height,
thus rejecting all the most unhealthy plants. The six self-fertilised
thus left were on an average 66.86 inches high; the eight intercrossed
plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that
there was not much difference between the three sets, the
self-fertilised plants having a slight advantage. Nor was there any
great difference when only the plants under 36 inches in height were
excluded. Nor again when all the plants, however much dwarfed and
unhealthy, were included. In this latter case the Colchester-crossed
gave the lowest average of all; and if these plants had been in any
marked manner superior to the other two lots, as from my former
experience I fully expected they would have been, I cannot but think
that some vestige of such superiority would have been evident,
notwithstanding the very unhealthy condition of most of the plants. No
advantage, as far as we can judge, was derived from intercrossing two of
the grandchildren of Hero, any more than when two of the children were
crossed. It appears therefore that Hero and its descendants have varied
from the common type, not only in acquiring great power of growth, and
increased fertility when subjected to self-fertilisation, but in not
profiting from a cross with a distinct stock; and this latter fact, if
trustworthy, is a unique case, as far as I have observed in all my
experiments.]
SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE
GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea
purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.
In Table 2/17, we see the average or mean heights of the ten successive
generations of the intercrossed and self-fertilised plants, grown in
competition with each other; and in the right hand column we have the
ratios of the one to the other, the height of the intercrossed plants
being taken at 100. In the bottom line the mean height of the
seventy-three intercrossed plants is shown to be 85.84 inches, and that
of the seventy-three self-fertilised plants 66.02 inches, or as 100 to
77.
TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten
generations.
Heights of Plants in inches:
Column 1: Name of Generation.
Column 2: Number of Crossed Plants.
Column 3: Average Height of Crossed Plants.
Column 4: Number of Self-fertilised Plants.
Column 5: Average Height of Self-fertilised Plants.
Column 6: n in Ratio between Average Heights of Crossed and
Self-fertilised Plants, expressed as 100 to n.
First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76.
Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79.
Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68.
Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86.
Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75.
Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72.
Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81.
Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85.
Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79.
Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54.
All ten generations together : 73 : 85.84 : 73 : 66.02 : 77.
(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and
self-fertilised plants of Ipomoea purpurea in the ten generations; the
mean height of the crossed plants being taken as 100. On the right hand,
the mean heights of the crossed and self-fertilised plants of all the
generations taken together are shown (as eleven pairs of unequal
vertical lines.))
The mean height of the self-fertilised plants in each of the ten
generations is also shown in the diagram 2/1, that of the intercrossed
plants being taken at 100, and on the right side we see the relative
heights of the seventy-three intercrossed plants, and of the
seventy-three self-fertilised plants. The difference in height between
the crossed and self-fertilised plants will perhaps be best appreciated
by an illustration: If all the men in a country were on an average 6
feet high, and there were some families which had been long and closely
interbred, these would be almost dwarfs, their average height during ten
generations being only 4 feet 8 1/4 inches.
It should be especially observed that the average difference between the
crossed and self-fertilised plants is not due to a few of the former
having grown to an extraordinary height, or to a few of the
self-fertilised being extremely short, but to all the crossed plants
having surpassed their self-fertilised opponents, with the few following
exceptions. The first occurred in the sixth generation, in which the
plant named “Hero” appeared; two in the eighth generation, but the
self-fertilised plants in this generation were in an anomalous
condition, as they grew at first at an unusual rate and conquered for a
time the opposed crossed plants; and two exceptions in the ninth
generation, though one of these plants only equalled its crossed
opponent. Therefore, of the seventy-three crossed plants, sixty-eight
grew to a greater height than the self-fertilised plants, to which they
were opposed.
In the right-hand column of figures, the difference in height between
the crossed and self-fertilised plants in the successive generations is
seen to fluctuate much, as might indeed have been expected from the
small number of plants measured in each generation being insufficient to
give a fair average. It should be remembered that the absolute height of
the plants goes for nothing, as each pair was measured as soon as one of
them had twined up to the summit of its rod. The great difference in the
tenth generation, namely, 100 to 54, no doubt was partly accidental,
though, when these plants were weighed, the difference was even greater,
namely, 100 to 44. The smallest amount of difference occurred in the
fourth and the eighth generations, and this was apparently due to both
the crossed and self-fertilised plants having become unhealthy, which
prevented the former attaining their usual degree of superiority. This
was an unfortunate circumstance, but my experiments were not thus
vitiated, as both lots of plants were exposed to the same conditions,
whether favourable or unfavourable.
There is reason to believe that the flowers of this Ipomoea, when
growing out of doors, are habitually crossed by insects, so that the
first seedlings which I raised from purchased seeds were probably the
offspring of a cross. I infer that this is the case, firstly from
humble-bees often visiting the flowers, and from the quantity of pollen
left by them on the stigmas of such flowers; and, secondly, from the
plants raised from the same lot of seed varying greatly in the colour of
their flowers, for as we shall hereafter see, this indicates much
intercrossing. (2/3. Verlot says ‘Sur la Production des Variétés’ 1865
page 66, that certain varieties of a closely allied plant, the
Convolvulus tricolor, cannot be kept pure unless grown at a distance
from all other varieties.) It is, therefore, remarkable that the plants
raised by me from flowers which were, in all probability,
self-fertilised for the first time after many generations of crossing,
should have been so markedly inferior in height to the intercrossed
plants as they were, namely, as 76 to 100. As the plants which were
self-fertilised in each succeeding generation necessarily became much
more closely interbred in the later than in the earlier generations, it
might have been expected that the difference in height between them and
the crossed plants would have gone on increasing; but, so far is this
from being the case, that the difference between the two sets of plants
in the seventh, eighth, and ninth generations taken together is less
than in the first and second generations together. When, however, we
remember that the self-fertilised and crossed plants are all descended
from the same mother-plant, that many of the crossed plants in each
generation were related, often closely related, and that all were
exposed to the same conditions, which, as we shall hereafter find, is a
very important circumstance, it is not at all surprising that the
difference between them should have somewhat decreased in the later
generations. It is, on the contrary, an astonishing fact, that the
crossed plants should have been victorious, even to a slight degree,
over the self-fertilised plants of the later generations.
The much greater constitutional vigour of the crossed than of the
self-fertilised plants, was proved on five occasions in various ways;
namely, by exposing them, while young, to a low temperature or to a
sudden change of temperature, or by growing them, under very
unfavourable conditions, in competition with full-grown plants of other
kinds.
With respect to the productiveness of the crossed and self-fertilised
plants of the successive generations, my observations unfortunately were
not made on any uniform plan, partly from the want of time, and partly
from not having at first intended to observe more than a single
generation. A summary of the results is here given in a tabulated form,
the fertility of the crossed plants being taken as 100.
TABLE 2/18. Ratio of productiveness of crossed and self-fertilised
plants. Ipomoea purpurea.
FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN
COMPETITION WITH ONE ANOTHER.
Sixty-five capsules produced from flowers on five crossed plants
fertilised by pollen from a distinct plant, and fifty-five capsules
produced from flowers on five self-fertilised plants fertilised by their
own pollen, contained seeds in the proportion of : 100 to 93.
Fifty-six spontaneously self-fertilised capsules on the above five
crossed plants, and twenty-five spontaneously self-fertilised capsules
on the above five self-fertilised plants, yielded seeds in the
proportion of : 100 to 99.
Combining the total number of capsules produced by these plants, and the
average number of seeds in each, the above crossed and self-fertilised
plants yielded seeds in the proportion of : 100 to 64.
Other plants of this first generation grown under unfavourable
conditions and spontaneously self-fertilised, yielded seeds in the
proportion of : 100 to 45.
THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Crossed capsules compared with self-fertilised capsules contained seeds
in the ratio of : 100 to 94.
An equal number of crossed and self-fertilised plants, both
spontaneously self-fertilised, produced capsules in the ratio of : 100
to 38.
And these capsules contained seeds in the ratio of : 100 to 94.
Combining these data, the productiveness of the crossed to the
self-fertilised plants, both spontaneously self-fertilised, was as : 100
to 35.
FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Capsules from flowers on the crossed plants fertilised by pollen from
another plant, and capsules from flowers on the self-fertilised plants
fertilised with their own pollen, contained seeds in the proportion of :
100 to 94.
FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
The crossed plants produced spontaneously a vast number more pods (not
actually counted) than the self-fertilised, and these contained seeds in
the proportion of : 100 to 89.
NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Fourteen crossed plants, spontaneously self-fertilised, and fourteen
self-fertilised plants spontaneously self-fertilised, yielded capsules
(the average number of seeds per capsule not having been ascertained) in
the proportion of : 100 to 26.
PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH
INTERCROSSED PLANTS.
The offspring of intercrossed plants of the ninth generation, crossed by
a fresh stock, compared with plants of the same stock intercrossed
during ten generations, both sets of plants left uncovered and naturally
fertilised, produced capsules by weight as : 100 to 51.
We see in this table that the crossed plants are always in some degree
more productive than the self-fertilised plants, by whatever standard
they are compared. The degree differs greatly; but this depends chiefly
on whether an average was taken of the seeds alone, or of the capsules
alone, or of both combined. The relative superiority of the crossed
plants is chiefly due to their producing a much greater number of
capsules, and not to each capsule containing a larger average number of
seeds. For instance, in the third generation the crossed and
self-fertilised plants produced capsules in the ratio of 100 to 38,
whilst the seeds in the capsules on the crossed plants were to those on
the self-fertilised plants only as 100 to 94. In the eighth generation
the capsules on two self-fertilised plants (not included in table 2/18),
grown in separate pots and thus not subjected to any competition,
yielded the large average of 5.1 seeds. The smaller number of capsules
produced by the self-fertilised plants may be in part, but not
altogether, attributed to their lessened size or height; this being
chiefly due to their lessened constitutional vigour, so that they were
not able to compete with the crossed plants growing in the same pots.
The seeds produced by the crossed flowers on the crossed plants were not
always heavier than the self-fertilised seeds on the self-fertilised
plants. The lighter seeds, whether produced from crossed or
self-fertilised flowers, generally germinated before the heavier seeds.
I may add that the crossed plants, with very few exceptions, flowered
before their self-fertilised opponents, as might have been expected from
their greater height and vigour.
The impaired fertility of the self-fertilised plants was shown in
another way, namely, by their anthers being smaller than those in the
flowers on the crossed plants. This was first observed in the seventh
generation, but may have occurred earlier. Several anthers from flowers
on the crossed and self-fertilised plants of the eighth generation were
compared under the microscope; and those from the former were generally
longer and plainly broader than the anthers of the self-fertilised
plants. The quantity of pollen contained in one of the latter was, as
far as could be judged by the eye, about half of that contained in one
from a crossed plant. The impaired fertility of the self-fertilised
plants of the eighth generation was also shown in another manner, which
may often be observed in hybrids--namely, by the first-formed flowers
being sterile. For instance, the fifteen first flowers on a
self-fertilised plant of one of the later generations were carefully
fertilised with their own pollen, and eight of them dropped off; at the
same time fifteen flowers on a crossed plant growing in the same pot
were self-fertilised, and only one dropped off. On two other crossed
plants of the same generation, several of the earliest flowers were
observed to fertilise themselves and to produce capsules. In the plants
of the ninth, and I believe of some previous generations, very many of
the flowers, as already stated, were slightly monstrous; and this
probably was connected with their lessened fertility.
All the self-fertilised plants of the seventh generation, and I believe
of one or two previous generations, produced flowers of exactly the same
tint, namely, of a rich dark purple. So did all the plants, without any
exception, in the three succeeding generations of self-fertilised
plants; and very many were raised on account of other experiments in
progress not here recorded. My attention was first called to this fact
by my gardener remarking that there was no occasion to label the
self-fertilised plants, as they could always be known by their colour.
The flowers were as uniform in tint as those of a wild species growing
in a state of nature; whether the same tint occurred, as is probable, in
the earlier generations, neither my gardener nor self could recollect.
The flowers on the plants which were first raised from purchased seed,
as well as during the first few generations, varied much in the depth of
the purple tint; many were more or less pink, and occasionally a white
variety appeared. The crossed plants continued to the tenth generation
to vary in the same manner as before, but to a much less degree, owing,
probably, to their having become more or less closely inter-related. We
must therefore attribute the extraordinary uniformity of colour in the
flowers on the plants of the seventh and succeeding self-fertilised
generations, to inheritance not having been interfered with by crosses
during several preceding generations, in combination with the conditions
of life having been very uniform.
A plant appeared in the sixth self-fertilised generation, named the
Hero, which exceeded by a little in height its crossed antagonist, and
which transmitted its powers of growth and increased self-fertility to
its children and grandchildren. A cross between the children of Hero did
not give to the grandchildren any advantage over the self-fertilised
grandchildren raised from the self-fertilised children. And as far as my
observations can be trusted, which were made on very unhealthy plants,
the great-grandchildren raised from intercrossing the grandchildren had
no advantage over the seedlings from the grandchildren the product of
continued self-fertilisation; and what is far more remarkable, the
great-grandchildren raised by crossing the grandchildren with a fresh
stock, had no advantage over either the intercrossed or self-fertilised
great-grandchildren. It thus appears that Hero and its descendants
differed in constitution in an extraordinary manner from ordinary plants
of the present species.
Although the plants raised during ten successive generations from
crosses between distinct yet inter-related plants almost invariably
exceeded in height, constitutional vigour, and fertility their
self-fertilised opponents, it has been proved that seedlings raised by
intercrossing flowers on the same plant are by no means superior, on the
contrary are somewhat inferior in height and weight, to seedlings raised
from flowers fertilised with their own pollen. This is a remarkable
fact, which seems to indicate that self-fertilisation is in some manner
more advantageous than crossing, unless the cross brings with it, as is
generally the case, some decided and preponderant advantage; but to this
subject I shall recur in a future chapter.
The benefits which so generally follow from a cross between two plants
apparently depend on the two differing somewhat in constitution or
character. This is shown by the seedlings from the intercrossed plants
of the ninth generation, when crossed with pollen from a fresh stock,
being as superior in height and almost as superior in fertility to the
again intercrossed plants, as these latter were to seedlings from
self-fertilised plants of the corresponding generation. We thus learn
the important fact that the mere act of crossing two distinct plants,
which are in some degree inter-related and which have been long
subjected to nearly the same conditions, does little good as compared
with that from a cross between plants belonging to different stocks or
families, and which have been subjected to somewhat different
conditions. We may attribute the good derived from the crossing of the
intercrossed plants during the ten successive generations to their still
differing somewhat in constitution or character, as was indeed proved by
their flowers still differing somewhat in colour. But the several
conclusions which may be deduced from the experiments on Ipomoea will be
more fully considered in the final chapters, after all my other
observations have been given.
CHAPTER III.
SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.
Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.
Appearance of a new, tall, and highly self-fertile variety.
Offspring from a cross between self-fertilised plants.
Effects of a cross with a fresh stock.
Effects of crossing flowers on the same plant.
Summary on Mimulus luteus.
Digitalis purpurea, superiority of the crossed plants.
Effects of crossing flowers on the same plant.
Calceolaria.
Linaria vulgaris.
Verbascum thapsus.
Vandellia nummularifolia.
Cleistogene flowers.
Gesneria pendulina.
Salvia coccinea.
Origanum vulgare, great increase of the crossed plants by stolons.
Thunbergia alata.
In the family of the Scrophulariaceae I experimented on species in the
six following genera: Mimulus, Digitalis, Calceolaria, Linaria,
Verbascum, and Vandellia.
[3/2. SCROPHULARIACEAE.--Mimulus luteus.
The plants which I raised from purchased seed varied greatly in the
colour of their flowers, so that hardly two individuals were quite
alike; the corolla being of all shades of yellow, with the most
diversified blotches of purple, crimson, orange, and coppery brown. But
these plants differed in no other respect. (3/1. I sent several
specimens with variously coloured flowers to Kew, and Dr. Hooker informs
me that they all consisted of Mimulus luteus. The flowers with much red
have been named by horticulturists as var. Youngiana.) The flowers are
evidently well adapted for fertilisation by the agency of insects; and
in the case of a closely allied species, Mimulus rosea, I have watched
bees entering the flowers, thus getting their backs well dusted with
pollen; and when they entered another flower the pollen was licked off
their backs by the two-lipped stigma, the lips of which are irritable
and close like a forceps on the pollen-grains. If no pollen is enclosed
between the lips, these open again after a time. Mr. Kitchener has
ingeniously explained the use of these movements, namely, to prevent the
self-fertilisation of the flower. (3/2. ‘A Year’s Botany’ 1874 page
118.) If a bee with no pollen on its back enters a flower it touches the
stigma, which quickly closes, and when the bee retires dusted with
pollen, it can leave none on the stigma of the same flower. But as soon
as it enters any other flower, plenty of pollen is left on the stigma,
which will be thus cross-fertilised. Nevertheless, if insects are
excluded, the flowers fertilise themselves perfectly and produce plenty
of seed; but I did not ascertain whether this is effected by the stamens
increasing in length with advancing age, or by the bending down of the
pistil. The chief interest in my experiments on the present species,
lies in the appearance in the fourth self-fertilised generation of a
variety which bore large peculiarly-coloured flowers, and grew to a
greater height than the other varieties; it likewise became more highly
self-fertile, so that this variety resembles the plant named Hero, which
appeared in the sixth self-fertilised generation of Ipomoea.
Some flowers on one of the plants raised from the purchased seeds were
fertilised with their own pollen; and others on the same plant were
crossed with pollen from a distinct plant. The seeds from twelve
capsules thus produced were placed in separate watch-glasses for
comparison; and those from the six crossed capsules appeared to the eye
hardly more numerous than those from the six self-fertilised capsules.
But when the seeds were weighed, those from the crossed capsules
amounted to 1.02 grain, whilst those from the self-fertilised capsules
were only .81 grain; so that the former were either heavier or more
numerous than the latter, in the ratio of 100 to 79.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.
Having ascertained, by leaving crossed and self-fertilised seed on damp
sand, that they germinated simultaneously, both kinds were thickly sown
on opposite sides of a broad and rather shallow pan; so that the two
sets of seedlings, which came up at the same time, were subjected to the
same unfavourable conditions. This was a bad method of treatment, but
this species was one of the first on which I experimented. When the
crossed seedlings were on an average half an inch high, the
self-fertilised ones were only a quarter of an inch high. When grown to
their full height under the above unfavourable conditions, the four
tallest crossed plants averaged 7.62, and the four tallest
self-fertilised 5.87 inches in height; or as 100 to 77. Ten flowers on
the crossed plants were fully expanded before one on the self-fertilised
plants. A few of these plants of both lots were transplanted into a
large pot with plenty of good earth, and the self-fertilised plants, not
now being subjected to severe competition, grew during the following
year as tall as the crossed plants; but from a case which follows it is
doubtful whether they would have long continued equal. Some flowers on
the crossed plants were crossed with pollen from another plant, and the
capsules thus produced contained a rather greater weight of seed than
those on the self-fertilised plants again self-fertilised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Seeds from the foregoing plants, fertilised in the manner just stated,
were sown on the opposite sides of a small pot (1) and came up crowded.
The four tallest crossed seedlings, at the time of flowering, averaged 8
inches in height, whilst the four tallest self-fertilised plants
averaged only 4 inches. Crossed seeds were sown by themselves in a
second small pot, and self-fertilised seeds were sown by themselves in a
third small pot so that there was no competition whatever between these
two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher
on an average than the self-fertilised. Both lots looked equally
vigorous, but the crossed plants flowered earlier and more profusely
than the self-fertilised. In Pot 1, in which the two lots competed with
each other, the crossed plants flowered first and produced a large
number of capsules, whilst the self-fertilised produced only nineteen.
The contents of twelve capsules from the crossed flowers on the crossed
plants, and of twelve capsules from self-fertilised flowers on the
self-fertilised plants, were placed in separate watch-glasses for
comparison; and the crossed seeds seemed more numerous by half than the
self-fertilised.
The plants on both sides of Pot 1, after they had seeded, were cut down
and transplanted into a large pot with plenty of good earth, and on the
following spring, when they had grown to a height of between 5 and 6
inches, the two lots were equal, as occurred in a similar experiment in
the last generation. But after some weeks the crossed plants exceeded
the self-fertilised ones on the opposite side of the same pot, though
not nearly to so great a degree as before, when they were subjected to
very severe competition.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Crossed seeds from the crossed plants, and self-fertilised seeds from
the self-fertilised plants of the last generation, were sown thickly on
opposite sides of a small pot, Number 1. The two tallest plants on each
side were measured after they had flowered, and the two crossed ones
were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2
inches in height; that is, in the ratio of 100 to 69. Twenty flowers on
the crossed plants were again crossed and produced twenty capsules; ten
of which contained 1.33 grain weight of seeds. Thirty flowers on the
self-fertilised plants were again self-fertilised and produced
twenty-six capsules; ten of the best of which (many being very poor)
contained only .87 grain weight of seeds; that is, in the ratio of 100
to 65 by weight.
The superiority of the crossed over the self-fertilised plants was
proved in various ways. Self-fertilised seeds were sown on one side of a
pot, and two days afterwards crossed seeds on the opposite side. The two
lots of seedlings were equal until they were above half an inch high;
but when fully grown the two tallest crossed plants attained a height of
12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants
were only 8 and 5 1/2 inches high.
In a third pot, crossed seeds were sown four days after the
self-fertilised, and the seedlings from the latter had at first, as
might have been expected, an advantage; but when the two lots were
between 5 and 6 inches in height, they were equal, and ultimately the
three tallest crossed plants were 11, 10, and 8 inches, whilst the three
tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So
that there was not much difference between them, the crossed plants
having an average advantage of only the third of an inch. The plants
were cut down, and without being disturbed were transplanted into a
larger pot. Thus the two lots started fair on the following spring, and
now the crossed plants showed their inherent superiority, for the two
tallest were 13 inches, whilst the two tallest self-fertilised plants
were only 11 and 8 1/2 inches in height; or as 100 to 75. The two lots
were allowed to fertilise themselves spontaneously: the crossed plants
produced a large number of capsules, whilst the self-fertilised produced
very few and poor ones. The seeds from eight of the capsules on the
crossed plants weighed .65 grain, whilst those from eight of the
capsules on the self-fertilised plants weighed only .22 grain; or as 100
to 34.
The crossed plants in the above three pots, as in almost all the
previous experiments, flowered before the self-fertilised. This occurred
even in the third pot in which the crossed seeds were sown four days
after the self-fertilised seeds.
Lastly, seeds of both lots were sown on opposite sides of a large pot in
which a Fuchsia had long been growing, so that the earth was full of
roots. Both lots grew miserably; but the crossed seedlings had an
advantage at all times, and ultimately attained to a height of 3 1/2
inches, whilst the self-fertilised seedlings never exceeded 1 inch. The
several foregoing experiments prove in a decisive manner the superiority
in constitutional vigour of the crossed over the self-fertilised plants.
In the three generations now described and taken together, the average
height of the ten tallest crossed plants was 8.19 inches, and that of
the ten tallest self-fertilised plants 5.29 inches (the plants having
been grown in small pots), or as 100 to 65.
In the next or fourth self-fertilised generation, several plants of a
new and tall variety appeared, which increased in the later
self-fertilised generations, owing to its great self-fertility, to the
complete exclusion of the original kinds. The same variety also appeared
amongst the crossed plants, but as it was not at first regarded with any
particular attention, I know not how far it was used for raising the
intercrossed plants; and in the later crossed generations it was rarely
present. Owing to the appearance of this tall variety, the comparison of
the crossed and self-fertilised plants of the fifth and succeeding
generations was rendered unfair, as all the self-fertilised and only a
few or none of the crossed plants consisted of it. Nevertheless, the
results of the later experiments are in some respects well worth giving.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.
Seeds of the two kinds, produced in the usual way from the two sets of
plants of the third generation, were sown on opposite sides of two pots
(1 and 2); but the seedlings were not thinned enough and did not grow
well. Many of the self-fertilised plants, especially in one of the pots,
consisted of the new and tall variety above referred to, which bore
large and almost white flowers marked with crimson blotches. I will call
it the WHITE VARIETY. I believe that it first appeared amongst both the
crossed and self-fertilised plants of the last generation; but neither
my gardener nor myself could remember any such variety in the seedlings
raised from the purchased seed. It must therefore have arisen either
through ordinary variation, or, judging from its appearance amongst both
the crossed and self-fertilised plants, more probably through reversion
to a formerly existing variety.
In Pot 1 the tallest crossed plant was 8 1/2 inches, and the tallest
self-fertilised 5 inches in height. In Pot 2, the tallest crossed plant
was 6 1/2 inches, and the tallest self-fertilised plant, which consisted
of the white variety, 7 inches in height; and this was the first
instance in my experiments on Mimulus in which the tallest
self-fertilised plant exceeded the tallest crossed. Nevertheless, the
two tallest crossed plants taken together were to the two tallest
self-fertilised plants in height as 100 to 80. As yet the crossed plants
were superior to the self-fertilised in fertility; for twelve flowers on
the crossed plants were crossed and yielded ten capsules, the seeds of
which weighed 1.71 grain. Twenty flowers on the self-fertilised plants
were self-fertilised, and produced fifteen capsules, all appearing poor;
and the seeds from ten of them weighed only .68 grain, so that from an
equal number of capsules the crossed seeds were to the self-fertilised
in weight as 100 to 40.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.
Seeds from both lots of the fourth generation, fertilised in the usual
manner, were sown on opposite sides of three pots. When the seedlings
flowered, most of the self-fertilised plants were found to consist of
the tall white variety. Several of the crossed plants in Pot 1 likewise
belonged to this variety, as did a very few in Pots 2 and 3. The tallest
crossed plant in Pot 1 was 7 inches, and the tallest self-fertilised
plant on the opposite side 8 inches; in Pots 2 and 3 the tallest crossed
were 4 1/2 and 5 1/2, and the tallest self-fertilised 7 and 6 1/2 inches
in height; so that the average height of the tallest plants in the two
lots was as 100 for the crossed to 126 for the self-fertilised; and thus
we have a complete reversal of what occurred in the four previous
generations. Nevertheless, in all three pots the crossed plants retained
their habit of flowering before the self-fertilised. The plants were
unhealthy from being crowded and from the extreme heat of the season,
and were in consequence more or less sterile; but the crossed plants
were somewhat less sterile than the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
Seeds from plants of the fifth generation crossed and self-fertilised in
the usual manner were sown on opposite sides of several pots. On the
self-fertilised side every single plant belonged to the tall white
variety. On the crossed side some plants belonged to this variety, but
the greater number approached in character to the old and shorter kinds
with smaller yellowish flowers blotched with coppery brown. When the
plants on both sides were from 2 to 3 inches in height they were equal,
but when fully grown the self-fertilised were decidedly the tallest and
finest plants, but, from want of time, they were not actually measured.
In half the pots the first plant which flowered was a self-fertilised
one, and in the other half a crossed one. And now another remarkable
change was clearly perceived, namely, that the self-fertilised plants
had become more self-fertile than the crossed. The pots were all put
under a net to exclude insects, and the crossed plants produced
spontaneously only fifty-five capsules, whilst the self-fertilised
plants produced eighty-one capsules, or as 100 to 147. The seeds from
nine capsules of both lots were placed in separate watch-glasses for
comparison, and the self-fertilised appeared rather the more numerous.
Besides these spontaneously self-fertilised capsules, twenty flowers on
the crossed plants again crossed yielded sixteen capsules; twenty-five
flowers on the self-fertilised plants again self-fertilised yielded
seventeen capsules, and this is a larger proportional number of capsules
than was produced by the self-fertilised flowers on the self-fertilised
plants in the previous generations. The contents of ten capsules of both
these lots were compared in separate watch-glasses, and the seeds from
the self-fertilised appeared decidedly more numerous than those from the
crossed plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.
Crossed and self-fertilised seeds from the crossed and self-fertilised
plants of the sixth generation were sown in the usual manner on opposite
sides of three pots, and the seedlings were well and equally thinned.
Every one of the self-fertilised plants (and many were raised) in this,
as well as in the eighth and ninth generations, belonged to the tall
white variety. Their uniformity of character, in comparison with the
seedlings first raised from the purchased seed, was quite remarkable. On
the other hand, the crossed plants differed much in the tints of their
flowers, but not, I think, to so great a degree as those first raised. I
determined this time to measure the plants on both sides carefully. The
self-fertilised seedlings came up rather before the crossed, but both
lots were for a time of equal height. When first measured, the average
height of the six tallest crossed plants in the three pots was 7.02, and
that of the six tallest self-fertilised plants 8.97 inches, or as 100 to
128. When fully grown the same plants were again measured, with the
result shown in Table 3/18.
TABLE 3/18. Mimulus luteus (Seventh Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 11 2/8 : 19 1/8.
Pot 1 : 11 7/8 : 18.
Pot 2 : 12 6/8 : 18 2/8.
Pot 2 : 11 2/8 : 14 6/8.
Pot 3 : 9 6/8 : 12 6/8.
Pot 3 : 11 6/8 : 11.
Total : 68.63 : 93.88.
The average height of the six crossed is here 11.43, and that of the six
self-fertilised 15.64, or as 100 to 137.
As it is now evident that the tall white variety transmitted its
characters faithfully, and as the self-fertilised plants consisted
exclusively of this variety, it was manifest that they would always
exceed in height the crossed plants which belonged chiefly to the
original shorter varieties. This line of experiment was therefore
discontinued, and I tried whether intercrossing two self-fertilised
plants of the sixth generation, growing in distinct pots, would give
their offspring any advantage over the offspring of flowers on one of
the same plants fertilised with their own pollen. These latter seedlings
formed the seventh generation of self-fertilised plants, like those in
the right hand column in Table 3/18; the crossed plants were the product
of six previous self-fertilised generations with an intercross in the
last generation. The seeds were allowed to germinate on sand, and were
planted in pairs on opposite sides of four pots, all the remaining seeds
being sown crowded on opposite sides of Pot 5 in Table 3/19; the three
tallest on each side in this latter pot being alone measured. All the
plants were twice measured--the first time whilst young, and the average
height of the crossed plants to that of the self-fertilised was then as
100 to 122. When fully grown they were again measured, as in Table 3/19.
TABLE 3/19. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth
Generation.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 12 6/8 : 15 2/8.
Pot 1 : 10 4/8 : 11 5/8.
Pot 1 : 10 : 11.
Pot 1 : 14 5/8 : 11.
Pot 2 : 10 2/8 : 11 3/8.
Pot 2 : 7 6/8 : 11 4/8.
Pot 2 : 12 1/8 : 8 5/8.
Pot 2 : 7 : 14 3/8.
Pot 3 : 13 5/8 : 10 3/8.
Pot 3 : 12 2/8 : 11 6/8.
Pot 4 : 7 1/8 : 14 6/8.
Pot 4 : 8 2/8 : 7.
Pot 4 : 7 2/8 : 8.
Pot 5 : 8 5/8 : 10 2/8
Pot 5 : 9 : 9 3/8.
Pot 5 : 8 2/8 : 9 2/8.
Crowded.
Total : 159.38 : 175.50.
The average height of the sixteen intercrossed plants is here 9.96
inches, and that of the sixteen self-fertilised plants 10.96, or as 100
to 110; so that the intercrossed plants, the progenitors of which had
been self-fertilised for the six previous generations, and had been
exposed during the whole time to remarkably uniform conditions, were
somewhat inferior in height to the plants of the seventh self-fertilised
generation. But as we shall presently see that a similar experiment made
after two additional generations of self-fertilisation gave a different
result, I know not how far to trust the present one. In three of the
five pots in Table 3/19 a self-fertilised plant flowered first, and in
the other two a crossed plant. These self-fertilised plants were
remarkably fertile, for twenty flowers fertilised with their own pollen
produced no less than nineteen very fine capsules!
THE EFFECTS OF A CROSS WITH A DISTINCT STOCK.
Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were
fertilised with their own pollen, and plants of the eighth
self-fertilised generation were thus raised, merely to serve as parents
in the following experiment. Several flowers on these plants were
allowed to fertilise themselves spontaneously (insects being of course
excluded), and the plants raised from these seeds formed the ninth
self-fertilised generation; they consisted wholly of the tall white
variety with crimson blotches. Other flowers on the same plants of the
eighth self-fertilised generation were crossed with pollen taken from
another plant of the same lot; so that the seedlings thus raised were
the offspring of eight previous generations of self-fertilisation with
an intercross in the last generation; these I will call the INTERCROSSED
PLANTS. Lastly, other flowers on the same plants of the eighth
self-fertilised generation were crossed with pollen taken from plants
which had been raised from seed procured from a garden at Chelsea. The
Chelsea plants bore yellow flowers blotched with red, but differed in no
other respect. They had been grown out of doors, whilst mine had been
cultivated in pots in the greenhouse for the last eight generations, and
in a different kind of soil. The seedlings raised from this cross with a
wholly different stock may be called the CHELSEA-CROSSED. The three lots
of seeds thus obtained were allowed to germinate on bare sand; and
whenever a seed in all three lots, or in only two, germinated at the
same time, they were planted in pots superficially divided into three or
two compartments. The remaining seeds, whether or not in a state of
germination, were thickly sown in three divisions in a large pot, 10, in
Table 3/20. When the plants had grown to their full height they were
measured, as shown in Table 3/20; but only the three tallest plants in
each of the three divisions in Pot 10 were measured.
TABLE 3/20. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants from Self-fertilised Plants of the Eighth Generation
crossed by Chelsea Plants.
Column 3: Plants from an intercross between the Plants of the Eighth
Self-fertilised Generation.
Column 4: Self-fertilised Plants of the Ninth Generation from Plants of
the Eighth Self-fertilised Generation.
Pot 1 : 30 7/8 : 14 : 9 4/8.
Pot 1 : 28 3/8 : 13 6/8 : 10 5/8.
Pot 1 : -- : 13 7/8 : 10.
Pot 2 : 20 6/8 : 11 4/8 : 11 6/8.
Pot 2 : 22 2/8 : 12 : 12 3/8.
Pot 2 : -- : 9 1/8 : --.
Pot 3 : 23 6/8 : 12 2/8 : 8 5/8.
Pot 3 : 24 1/8 : -- : 11 4/8.
Pot 3 : 25 6/8 : -- : 6 7/8.
Pot 4 : 22 5/8 : 9 2/8 : 4.
Pot 4 : 22 : 8 1/8 : 13 3/8.
Pot 4 : 17 : -- : 11.
Pot 5 : 22 3/8 : 9 : 4 4/8.
Pot 5 : 19 5/8 : 11 : 13.
Pot 5 : 23 4/8 : -- : 13 4/8.
Pot 6 : 28 2/8 : 18 6/8 : 12.
Pot 6 : 22 : 7 : 16 1/8.
Pot 6 : -- : 12 4/8 : --.
Pot 7 : 12 4/8 : 15 : --.
Pot 7 : 24 3/8 : 12 3/8 : --.
Pot 7 : 20 4/8 : 11 2/8 : --.
Pot 7 : 26 4/8 : 15 2/8 : --.
Pot 8 : 17 2/8 : 13 3/8 : --.
Pot 8 : 22 6/8 : 14 5/8 : --.
Pot 8 : 27 : 14 3/8 : --.
Pot 9 : 22 6/8 : 11 6/8 : --.
Pot 9 : 6 : 17 : --.
Pot 9 : 20 2/8 : 14 7/8 : --.
Pot 10 : 18 1/8 : 9 2/8 : 10 3/8.
Pot 10 : 16 5/8 : 8 2/8 : 8 1/8.
Pot 10 : 17 4/8 : 10 : 11 2/8.
Crowded plants.
Total : 605.38 : 329.50 : 198.50.
In this table the average height of the twenty-eight Chelsea-crossed
plants is 21.62 inches; that of the twenty-seven intercrossed plants
12.2; and that of the nineteen self-fertilised 10.44. But with respect
to the latter it will be the fairest plan to strike out two dwarfed ones
(only 4 inches in height), so as not to exaggerate the inferiority of
the self-fertilised plants; and this will raise the average height of
the seventeen remaining self-fertilised plants to 11.2 inches. Therefore
the Chelsea-crossed are to the intercrossed in height as 100 to 56; the
Chelsea-crossed to the self-fertilised as 100 to 52; and the
intercrossed to the self-fertilised as 100 to 92. We thus see how
immensely superior in height the Chelsea-crossed are to the intercrossed
and to the self-fertilised plants. They began to show their superiority
when only one inch high. They were also, when fully grown, much more
branched with larger leaves and somewhat larger flowers than the plants
of the other two lots, so that if they had been weighed, the ratio would
certainly have been much higher than that of 100 to 56 and 52.
The intercrossed plants are here to the self-fertilised in height as 100
to 92; whereas in the analogous experiment given in Table 3/19 the
intercrossed plants from the self-fertilised plants of the sixth
generation were inferior in height to the self-fertilised plants in the
ratio of 100 to 110. I doubt whether this discordance in the results of
the two experiments can be explained by the self-fertilised plants in
the present case having been raised from spontaneously self-fertilised
seeds, whereas in the former case they were raised from artificially
self-fertilised seeds; nor by the present plants having been
self-fertilised during two additional generations, though this is a more
probable explanation.
With respect to fertility, the twenty-eight Chelsea-crossed plants
produced 272 capsules; the twenty-seven intercrossed plants produced 24;
and the seventeen self-fertilised plants 17 capsules. All the plants
were left uncovered so as to be naturally fertilised, and empty capsules
were rejected.
Therefore 20 Chelsea-crossed plants would have produced 194.29 capsules.
Therefore 20 Intercrossed plants would have produced 17.77 capsules.
Therefore 20 Self-fertilised plants would have produced 20.00 capsules.
The seeds contained in 8 capsules from the Chelsea-crossed plants
weighed 1.1 grains.
The seeds contained in 8 capsules from the Intercrossed plants weighed
0.51 grains.
The seeds contained in 8 capsules from the Self-fertilised plants
weighed 0.33 grains.
If we combine the number of capsules produced together with the average
weight of contained seeds, we get the following extraordinary ratios:
Weight of seed produced by the same number of Chelsea-crossed and
intercrossed plants as 100 to 4.
Weight of seed produced by the same number of Chelsea-crossed and
self-fertilised plants as 100 to 3.
Weight of seeds produced by the same number of intercrossed and
self-fertilised plants as 100 to 73.
It is also a remarkable fact that the Chelsea-crossed plants exceeded
the two other lots in hardiness, as greatly as they did in height,
luxuriance, and fertility. In the early autumn most of the pots were
bedded out in the open ground; and this always injures plants which have
been long kept in a warm greenhouse. All three lots consequently
suffered greatly, but the Chelsea-crossed plants much less than the
other two lots. On the 3rd of October the Chelsea-crossed plants began
to flower again, and continued to do so for some time; whilst not a
single flower was produced by the plants of the other two lots, the
stems of which were cut almost down to the ground and seemed half dead.
Early in December there was a sharp frost, and the stems of
Chelsea-crossed were now cut down; but on the 23rd of December they
began to shoot up again from the roots, whilst all the plants of the
other two lots were quite dead.
Although several of the self-fertilised seeds, from which the plants in
the right hand column in Table 3/20 were raised, germinated (and were of
course rejected) before any of those of the other two lots, yet in only
one of the ten pots did a self-fertilised plant flower before the
Chelsea-crossed or the intercrossed plants growing in the same pots. The
plants of these two latter lots flowered at the same time, though the
Chelsea-crossed grew so much taller and more vigorously than the
intercrossed.
As already stated, the flowers of the plants originally raised from the
Chelsea seeds were yellow; and it deserves notice that every one of the
twenty-eight seedlings raised from the tall white variety fertilised,
without being castrated, with pollen from the Chelsea plants, produced
yellow flowers; and this shows how prepotent this colour, which is the
natural one of the species, is over the white colour.
THE EFFECTS ON THE OFFSPRING OF INTERCROSSING FLOWERS ON THE SAME PLANT,
INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments the crossed plants were the product of
a cross between distinct plants. I now selected a very vigorous plant in
Table 3/20, raised by fertilising a plant of the eighth self-fertilised
generation with pollen from the Chelsea stock. Several flowers on this
plant were crossed with pollen from other flowers on the same plant, and
several other flowers were fertilised with their own pollen. The seed
thus produced was allowed to germinate on bare sand; and the seedlings
were planted in the usual manner on the opposite sides of six pots. All
the remaining seeds, whether or not in a state of germination, were sown
thickly in Pot 7; the three tallest plants on each side of this latter
pot being alone measured. As I was in a hurry to learn the result, some
of these seeds were sown late in the autumn, but the plants grew so
irregularly during the winter, that one crossed plant was 28 1/2 inches,
and two others only 4, or less than 4 inches in height, as may be seen
in Table 3/21. Under such circumstances, as I have observed in many
other cases, the result is not in the least trustworthy; nevertheless I
feel bound to give the measurements.
TABLE 3/21. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 17 : 17.
Pot 1 : 9 : 3 1/8.
Pot 2 : 28 2/8 : 19 1/8.
Pot 2 : 16 4/8 : 6.
Pot 2 : 13 5/8 : 2.
Pot 3 : 4 : 15 6/8.
Pot 3 : 2 2/8 : 10.
Pot 4 : 23 4/8 : 6 2/8.
Pot 4 : 15 4/8 : 7 1/8.
Pot 5 : 7 : 13 4/8.
Pot 6 : 18 3/8 : 1 4/8.
Pot 6 : 11 : 2.
Pot 7 : 21 : 15 1/8.
Pot 7 : 11 6/8 : 11.
Pot 7 : 12 1/8 : 11 2/8.
Crowded.
Total : 210.88 : 140.75.
The fifteen crossed plants here average 14.05, and the fifteen
self-fertilised plants 9.38 in height, or as 100 to 67. But if all the
plants under ten inches in height are struck out, the ratio of the
eleven crossed plants to the eight self-fertilised plants is as 100 to
82.
On the following spring, some remaining seeds of the two lots were
treated in exactly the same manner; and the measurements of the
seedlings are given in Table 3/22.
TABLE 3/22. Mimulus luteus.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 15 1/8 : 19 1/8.
Pot 1 : 12 : 20 5/8.
Pot 1 : 10 1/8 : 12 6/8.
Pot 2 : 16 2/8 : 11 2/8.
Pot 2 : 13 5/8 : 19 3/8.
Pot 2 : 20 1/8 : 17 4/8.
Pot 3 : 18 7/8 : 12 6/8.
Pot 3 : 15 : 15 6/8.
Pot 3 : 13 7/8 : 17.
Pot 4 : 19 2/8 : 16 2/8.
Pot 4 : 19 6/8 : 21 5/8.
Pot 5 : 25 3/8 : 22 5/8.
Pot 6 : 15 : 19 5/8.
Pot 6 : 20 2/8 : 16 2/8.
Pot 6 : 27 2/8 : 19 5/8.
Pot 7 : 7 6/8 : 7 6/8.
Pot 7 : 14 : 8.
Pot 7 : 13 4/8 : 7.
Pot 8 : 18 2/8 : 20 3/8.
Pot 8 : 18 6/8 : 17 6/8.
Pot 8 : 18 3/8 : 15 4/8.
Pot 8 : 18 3/8 : 15 1/8.
Crowded.
Total : 370.88 : 353.63.
Here the average height of the twenty-two crossed plants is 16.85, and
that of the twenty-two self-fertilised plants 16.07; or as 100 to 95.
But if four of the plants in Pot 7, which are much shorter than any of
the others, are struck out (and this would be the fairest plan), the
twenty-one crossed are to the nineteen self-fertilised plants in height
as 100 to 100.6--that is, are equal. All the plants, except the crowded
ones in Pot 8, after being measured were cut down, and the eighteen
crossed plants weighed 10 ounces, whilst the same number of
self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if
the dwarfed plants in Pot 7 had been excluded, the self-fertilised would
have exceeded the crossed in weight in a higher ratio. In all the
previous experiments in which seedlings were raised from a cross between
distinct plants, and were put into competition with self-fertilised
plants, the former generally flowered first; but in the present case, in
seven out of the eight pots a self-fertilised plant flowered before a
crossed one on the opposite side. Considering all the evidence with
respect to the plants in Table3/ 22, a cross between two flowers on the
same plant seems to give no advantage to the offspring thus produced,
the self-fertilised plants being in weight superior. But this conclusion
cannot be absolutely trusted, owing to the measurements given in Table
3/21, though these latter, from the cause already assigned, are very
much less trustworthy than the present ones.]
SUMMARY OF OBSERVATIONS ON Mimulus luteus.
In the three first generations of crossed and self-fertilised plants,
the tallest plants alone on each side of the several pots were measured;
and the average height of the ten crossed to that of the ten
self-fertilised plants was as 100 to 64. The crossed were also much more
fertile than the self-fertilised, and so much more vigorous that they
exceeded them in height, even when sown on the opposite side of the same
pot after an interval of four days. The same superiority was likewise
shown in a remarkable manner when both kinds of seeds were sown on the
opposite sides of a pot with very poor earth full of the roots of
another plant. In one instance crossed and self-fertilised seedlings,
grown in rich soil and not put into competition with each other,
attained to an equal height. When we come to the fourth generation the
two tallest crossed plants taken together exceeded by only a little the
two tallest self-fertilised plants, and one of the latter beat its
crossed opponent,--a circumstance which had not occurred in the previous
generations. This victorious self-fertilised plant consisted of a new
white-flowered variety, which grew taller than the old yellowish
varieties. From the first it seemed to be rather more fertile, when
self-fertilised, than the old varieties, and in the succeeding
self-fertilised generations became more and more self-fertile. In the
sixth generation the self-fertilised plants of this variety compared
with the crossed plants produced capsules in the proportion of 147 to
100, both lots being allowed to fertilise themselves spontaneously. In
the seventh generation twenty flowers on one of these plants
artificially self-fertilised yielded no less than nineteen very fine
capsules!
This variety transmitted its characters so faithfully to all the
succeeding self-fertilised generations, up to the last or ninth, that
all the many plants which were raised presented a complete uniformity of
character; thus offering a remarkable contrast with the seedlings raised
from the purchased seeds. Yet this variety retained to the last a latent
tendency to produce yellow flowers; for when a plant of the eighth
self-fertilised generation was crossed with pollen from a
yellow-flowered plant of the Chelsea stock, every single seedling bore
yellow flowers. A similar variety, at least in the colour of its
flowers, also appeared amongst the crossed plants of the third
generation. No attention was at first paid to it, and I know not how far
it was at first used either for crossing or self-fertilisation. In the
fifth generation most of the self-fertilised plants, and in the sixth
and all the succeeding generations every single plant consisted of this
variety; and this no doubt was partly due to its great and increasing
self-fertility. On the other hand, it disappeared from amongst the
crossed plants in the later generations; and this was probably due to
the continued intercrossing of the several plants. From the tallness of
this variety, the self-fertilised plants exceeded the crossed plants in
height in all the generations from the fifth to the seventh inclusive;
and no doubt would have done so in the later generations, had they been
grown in competition with one another. In the fifth generation the
crossed plants were in height to the self-fertilised, as 100 to 126; in
the sixth, as 100 to 147; and in the seventh generation, as 100 to 137.
This excess of height may be attributed not only to this variety
naturally growing taller than the other plants, but to its possessing a
peculiar constitution, so that it did not suffer from continued
self-fertilisation.
This variety presents a strikingly analogous case to that of the plant
called the Hero, which appeared in the sixth self-fertilised generation
of Ipomoea. If the seeds produced by Hero had been as greatly in excess
of those produced by the other plants, as was the case with Mimulus, and
if all the seeds had been mingled together, the offspring of Hero would
have increased to the entire exclusion of the ordinary plants in the
later self-fertilised generations, and from naturally growing taller
would have exceeded the crossed plants in height in each succeeding
generation.
Some of the self-fertilised plants of the sixth generation were
intercrossed, as were some in the eighth generation; and the seedlings
from these crosses were grown in competition with self-fertilised plants
of the two corresponding generations. In the first trial the
intercrossed plants were less fertile than the self-fertilised, and less
tall in the ratio of 100 to 110. In the second trial, the intercrossed
plants were more fertile than the self-fertilised in the ratio of 100 to
73, and taller in the ratio of 100 to 92. Notwithstanding that the
self-fertilised plants in the second trial were the product of two
additional generations of self-fertilisation, I cannot understand this
discordance in the results of the two analogous experiments.
The most important of all the experiments on Mimulus are those in which
flowers on plants of the eighth self-fertilised generation were again
self-fertilised; other flowers on distinct plants of the same lot were
intercrossed; and others were crossed with a new stock of plants from
Chelsea. The Chelsea-crossed seedlings were to the intercrossed in
height as 100 to 56, and in fertility as 100 to 4; and they were to the
self-fertilised plants, in height as 100 to 52, and in fertility as 100
to 3. These Chelsea-crossed plants were also much more hardy than the
plants of the other two lots; so that altogether the gain from the cross
with a fresh stock was wonderfully great.
Lastly, seedlings raised from a cross between flowers on the same plant
were not superior to those from flowers fertilised with their own
pollen; but this result cannot be absolutely trusted, owing to some
previous observations, which, however, were made under very unfavourable
circumstances.
[Digitalis purpurea.
The flowers of the common Foxglove are proterandrous; that is, the
pollen is mature and mostly shed before the stigma of the same flower is
ready for fertilisation. This is effected by the larger humble-bees,
which, whilst in search of nectar, carry pollen from flower to flower.
The two upper and longer stamens shed their pollen before the two lower
and shorter ones. The meaning of this fact probably is, as Dr. Ogle
remarks, that the anthers of the longer stamens stand near to the
stigma, so that they would be the most likely to fertilise it (3/3.
‘Popular Science Review’ January 1870 page 50.); and as it is an
advantage to avoid self-fertilisation, they shed their pollen first,
thus lessening the chance. There is, however, but little danger of
self-fertilisation until the bifid stigma opens; for Hildebrand found
that pollen placed on the stigma before it had opened produced no
effect. (3/4. ‘Geschlechter-Vertheilung bei den Pflanzen’ 1867 page 20.)
The anthers, which are large, stand at first transversely with respect
to the tubular corolla, and if they were to dehisce in this position
they would, as Dr. Ogle also remarks, smear with pollen the whole back
and sides of an entering humble-bee in a useless manner; but the anthers
twist round and place themselves longitudinally before they dehisce. The
lower and inner side of the mouth of the corolla is thickly clothed with
hairs, and these collect so much of the fallen pollen that I have seen
the under surface of a humble-bee thickly dusted with it; but this can
never be applied to the stigma, as the bees in retreating do not turn
their under surfaces upwards. I was therefore puzzled whether these
hairs were of any use; but Mr. Belt has, I think, explained their use:
the smaller kinds of bees are not fitted to fertilise the flowers, and
if they were allowed to enter easily they would steal much nectar, and
fewer large bees would haunt the flowers. Humble-bees can crawl into the
dependent flowers with the greatest ease, using the “hairs as footholds
while sucking the honey; but the smaller bees are impeded by them, and
when, having at length struggled through them, they reach the slippery
precipice above, they are completely baffled.” Mr. Belt says that he
watched many flowers during a whole season in North Wales, and “only
once saw a small bee reach the nectary, though many were seen trying in
vain to do so.” (3/5. ‘The Naturalist in Nicaragua’ 1874 page 132. But
it appears from H. Muller ‘Die Befruchtung der Blumen’ 1873 page 285,
that small insects sometimes succeed in entering the flowers.)
I covered a plant growing in its native soil in North Wales with a net,
and fertilised six flowers each with its own pollen, and six others with
pollen from a distinct plant growing within the distance of a few feet.
The covered plant was occasionally shaken with violence, so as to
imitate the effects of a gale of wind, and thus to facilitate as far as
possible self-fertilisation. It bore ninety-two flowers (besides the
dozen artificially fertilised), and of these only twenty-four produced
capsules; whereas almost all the flowers on the surrounding uncovered
plants were fruitful. Of the twenty-four spontaneously self-fertilised
capsules, only two contained their full complement of seed; six
contained a moderate supply; and the remaining sixteen extremely few
seeds. A little pollen adhering to the anthers after they had dehisced,
and accidentally falling on the stigma when mature, must have been the
means by which the above twenty-four flowers were partially
self-fertilised; for the margins of the corolla in withering do not curl
inwards, nor do the flowers in dropping off turn round on their axes, so
as to bring the pollen-covered hairs, with which the lower surface is
clothed, into contact with the stigma--by either of which means
self-fertilisation might be effected.
Seeds from the above crossed and self-fertilised capsules, after
germinating on bare sand, were planted in pairs on the opposite sides of
five moderately-sized pots, which were kept in the greenhouse. The
plants after a time appeared starved, and were therefore, without being
disturbed, turned out of their pots, and planted in the open ground in
two close parallel rows. They were thus subjected to tolerably severe
competition with one another; but not nearly so severe as if they had
been left in the pots. At the time when they were turned out, their
leaves were between 5 and 8 inches in length, and the longest leaf on
the finest plant on each side of each pot was measured, with the result
that the leaves of the crossed plants exceeded, on an average, those of
the self-fertilised plants by .4 of an inch.
In the following summer the tallest flower-stem on each plant, when
fully grown, was measured. There were seventeen crossed plants; but one
did not produce a flower-stem. There were also, originally, seventeen
self-fertilised plants, but these had such poor constitutions that no
less than nine died in the course of the winter and spring, leaving only
eight to be measured, as in Table 3/23.
TABLE 3/23. Digitalis purpurea.
The tallest Flower-stem on each Plant measured in inches: 0 means that
the Plant died before a Flower-stem was produced.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 53 6/8 : 27 4/8.
Pot 1 : 57 4/8 : 55 6/8.
Pot 1 : 57 6/8 : 0.
Pot 1 : 65 : 0.
Pot 2 : 34 4/8 : 39.
Pot 2 : 52 4/8 : 32.
Pot 2 : 63 6/8 : 21.
Pot 3 : 57 4/8 : 53 4/8.
Pot 3 : 53 4/8 : 0.
Pot 3 : 50 6/8 : 0.
Pot 3 : 37 2/8 : 0.
Pot 4 : 64 4/8 : 34 4/8.
Pot 4 : 37 4/8 : 23 6/8.
Pot 4 : -- : 0.
Pot 5 : 53 : 0.
Pot 5 : 47 6/8 : 0.
Pot 5 : 34 6/8 : 0.
Total : 821.25 : 287.00.
The average height of the flower-stems of the sixteen crossed plants is
here 51.33 inches; and that of the eight self-fertilised plants, 35.87;
or as 100 to 70. But this difference in height does not give at all a
fair idea of the vast superiority of the crossed plants. These latter
produced altogether sixty-four flower-stems, each plant producing, on an
average, exactly four flower-stems, whereas the eight self-fertilised
plants produced only fifteen flower-stems, each producing an average
only of 1.87 stems, and these had a less luxuriant appearance. We may
put the result in another way: the number of flower-stems on the crossed
plants was to those on an equal number of self-fertilised plants as 100
to 48.
Three crossed seeds in a state of germination were also planted in three
separate pots; and three self-fertilised seeds in the same state in
three other pots. These plants were therefore at first exposed to no
competition with one another, and when turned out of their pots into the
open ground they were planted at a moderate distance apart, so that they
were exposed to much less severe competition than in the last case. The
longest leaves on the three crossed plants, when turned out, exceeded
those on the self-fertilised plants by a mere trifle, namely, on an
average by .17 of an inch. When fully grown the three crossed plants
produced twenty-six flower-stems; the two tallest of which on each plant
were on an average 54.04 inches in height. The three self-fertilised
plants produced twenty-three flower-stems, the two tallest of which on
each plant had an average height of 46.18 inches. So that the difference
between these two lots, which hardly competed together, is much less
than in the last case when there was moderately severe competition,
namely, as 100 to 85, instead of as 100 to 70.
THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE
SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
A fine plant growing in my garden (one of the foregoing seedlings) was
covered with a net, and six flowers were crossed with pollen from
another flower on the same plant, and six others were fertilised with
their own pollen. All produced good capsules. The seeds from each were
placed in separate watch-glasses, and no difference could be perceived
by the eye between the two lots of seeds; and when they were weighed
there was no difference of any significance, as the seeds from the
self-fertilised capsules weighed 7.65 grains, whilst those from the
crossed capsules weighed 7.7 grains. Therefore the sterility of the
present species, when insects are excluded, is not due to the impotence
of pollen on the stigma of the same flower. Both lots of seeds and
seedlings were treated in exactly the same manner as in Table 3/23,
excepting that after the pairs of germinating seeds had been planted on
the opposite sides of eight pots, all the remaining seeds were thickly
sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young
plants during the following spring were turned out of their pots,
without being disturbed, and planted in the open ground in two rows, not
very close together, so that they were subjected to only moderately
severe competition with one another. Very differently to what occurred
in the first experiment, when the plants were subjected to somewhat
severe mutual competition, an equal number on each side either died or
did not produce flower-stems. The tallest flower-stems on the surviving
plants were measured, as shown in Table 3/24.
TABLE 3/24. Digitalis purpurea.
The tallest Flower-stem on each Plant measured in inches: 0 signifies
that the Plant died, or did not produce a Flower-stem.
Column 1: Number (Name) of Pot.
Column 2: Plants raised from a Cross between different Flowers on the
same Plant.
Column 3: Plants raised from Flowers fertilised with their own Pollen.
Pot 1 : 49 4/8 : 45 5/8.
Pot 1 : 46 7/8 : 52.
Pot 1 : 43 6/8 : 0.
Pot 2 : 38 4/8 : 54 4/8.
Pot 2 : 47 4/8 : 47 4/8.
Pot 2 : 0 : 32 5/8.
Pot 3 : 54 7/8 : 46 5/8.
Pot 4 : 32 1/8 : 41 3/8.
Pot 4 : 0 : 29 7/8.
Pot 4 : 43 7/8 : 37 1/8.
Pot 5 : 46 6/8 : 42 1/8.
Pot 5 : 40 4/8 : 42 1/8.
Pot 5 : 43 : 0.
Pot 6 : 48 2/8 : 47 7/8.
Pot 6 : 46 2/8 : 48 3/8.
Pot 7 : 48 5/8 : 25.
Pot 7 : 42 : 40 5/8.
Pot 8 : 46 7/8 : 39 1/8.
Pot 9 : 49 : 30 3/8.
Pot 9 : 50 3/8 : 15.
Pot 9 : 46 3/8 : 36 7/8.
Pot 9 : 47 6/8 : 44 1/8.
Pot 9 : 0 : 31 6/8.
Crowded Plants.
Pot 10 : 46 4/8 : 47 7/8.
Pot 10 : 35 2/8 : 0.
Pot 10 : 24 5/8 : 34 7/8.
Pot 10 : 41 4/8 : 40 7/8.
Pot 10 : 17 3/8 : 41 1/8.
Crowded Plants.
Total : 1078.00 : 995.38.
The average height of the flower-stems on the twenty-five crossed plants
in all the pots taken together is 43.12 inches, and that of the
twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to
test this result, the plants planted in pairs in Pots 1 and 8 were
considered by themselves, and the average height of the sixteen crossed
plants is here 44.9, and that of the sixteen self-fertilised plants
42.03, or as 100 to 94. Again, the plants raised from the thickly sown
seed in Pots 9 and 10, which were subjected to very severe mutual
competition, were taken by themselves, and the average height of the
nine crossed plants is 39.86, and that of the nine self-fertilised
plants 35.88, or as 100 to 90. The plants in these two latter pots (9
and 10), after being measured, were cut down close to the ground and
weighed: the nine crossed plants weighed 57.66 ounces, and the nine
self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we
may conclude, especially from the evidence of weight, that seedlings
from a cross between flowers on the same plant have a decided, though
not great, advantage over those from flowers fertilised with their own
pollen, more especially in the case of the plants subjected to severe
mutual competition. But the advantage is much less than that exhibited
by the crossed offspring of distinct plants, for these exceeded the
self-fertilised plants in height as 100 to 70, and in the number of
flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and
almost certainly from Mimulus, as with these two species a cross between
flowers on the same plant did no good.
CALCEOLARIA.
A BUSHY GREENHOUSE VARIETY, WITH YELLOW FLOWERS BLOTCHED WITH PURPLE.
The flowers in this genus are constructed so as to favour or almost
ensure cross-fertilisation (3/6. Hildebrand as quoted by H. Muller ‘Die
Befruchtung der Blumen’ 1873 page 277.); and Mr. Anderson remarks that
extreme care is necessary to exclude insects in order to preserve any
kind true. (3/7. ‘Gardeners’ Chronicle’ 1853 page 534.) He adds the
interesting statement, that when the corolla is cut quite away, insects,
as far as he has seen, never discover or visit the flowers. This plant
is, however, self-fertile if insects are excluded. So few experiments
were made by me, that they are hardly worth giving. Crossed and
self-fertilised seeds were sown on opposite sides of a pot, and after a
time the crossed seedlings slightly exceeded the self-fertilised in
height. When a little further grown, the longest leaves on the former
were very nearly 3 inches in length, whilst those on the self-fertilised
plants were only 2 inches. Owing to an accident, and to the pot being
too small, only one plant on each side grew up and flowered; the crossed
plant was 19 1/2 inches in height, and the self-fertilised one 15
inches; or as 100 to 77.
Linaria vulgaris.
It has been mentioned in the introductory chapter that two large beds of
this plant were raised by me many years ago from crossed and
self-fertilised seeds, and that there was a conspicuous difference in
height and general appearance between the two lots. The trial was
afterwards repeated with more care; but as this was one of the first
plants experimented on, my usual method was not followed. Seeds were
taken from wild plants growing in this neighbourhood and sown in poor
soil in my garden. Five plants were covered with a net, the others being
left exposed to the bees, which incessantly visit the flowers of this
species, and which, according to H. Muller, are the exclusive
fertilisers. This excellent observer remarks that, as the stigma lies
between the anthers and is mature at the same time with them,
self-fertilisation is possible. (3/8. ‘Die Befruchtung’ etc. page 279.)
But so few seeds are produced by protected plants, that the pollen and
stigma of the same flower seem to have little power of mutual
interaction. The exposed plants bore numerous capsules forming solid
spikes. Five of these capsules were examined and appeared to contain an
equal number of seeds; and these being counted in one capsule, were
found to be 166. The five protected plants produced altogether only
twenty-five capsules, of which five were much finer than all the others,
and these contained an average of 23.6 seeds, with a maximum in one
capsule of fifty-five. So that the number of seeds in the capsules on
the exposed plants to the average number in the finest capsules on the
protected plants was as 100 to 14.
Some of the spontaneously self-fertilised seeds from under the net, and
some seeds from the uncovered plants naturally fertilised and almost
certainly intercrossed by the bees, were sown separately in two large
pots of the same size; so that the two lots of seedlings were not
subjected to any mutual competition. Three of the crossed plants when in
full flower were measured, but no care was taken to select the tallest
plants; their heights were 7 4/8, 7 2/8, and 6 4/8 inches; averaging
7.08 in height. The three tallest of all the self-fertilised plants were
then carefully selected, and their heights were 6 3/8, 5 5/8, and 5 2/8,
averaging 5.75 in height. So that the naturally crossed plants were to
the spontaneously self-fertilised plants in height, at least as much as
100 to 81.
Verbascum thapsus.
The flowers of this plant are frequented by various insects, chiefly by
bees, for the sake of the pollen. Hermann Muller, however, has shown
(‘Die Befruchtung’ etc. page 277) that V. nigrum secretes minute drops
of nectar. The arrangement of the reproductive organs, though not at all
complex, favours cross-fertilisation; and even distinct species are
often crossed, for a greater number of naturally produced hybrids have
been observed in this genus than in almost any other. (3/9. I have given
a striking case of a large number of such hybrids between Verbascum
thapsus and lychnitis found growing wild: ‘Journal of Linnean Society
Botany’ volume 10 page 451.) Nevertheless the present species is
perfectly self-fertile, if insects are excluded; for a plant protected
by a net was as thickly loaded with fine capsules as the surrounding
uncovered plants. Verbascum lychnitis is rather less self-fertile, for
some protected plants did not yield quite so many capsules as the
adjoining uncovered plants.
Plants of Verbascum thapsus had been raised for a distinct purpose from
self-fertilised seeds; and some flowers on these plants were again
self-fertilised, yielding seed of the second self-fertilised generation;
and other flowers were crossed with pollen from a distinct plant. The
seeds thus produced were sown on the opposite sides of four large pots.
They germinated, however, so irregularly (the crossed seedlings
generally coming up first) that I was able to save only six pairs of
equal age. These when in full flower were measured, as in Table 3/25.
TABLE 3/25. Verbascum thapsus.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants of the Second Generation.
Pot 1 : 76 : 53 4/8.
Pot 2 : 54 : 66.
Pot 3 : 62 : 75.
Pot 3 : 60 5/8 : 30 4/8.
Pot 4 : 73 : 62.
Pot 4 : 66 4/8 : 52.
Total : 392.13 : 339.00.
We here see that two of the self-fertilised plants exceed in height
their crossed opponents. Nevertheless the average height of the six
crossed plants is 65.34 inches, and that of the six self-fertilised
plants 56.5 inches; or as 100 to 86.
Vandellia nummularifolia.
Seeds were sent to me by Mr. J. Scott from Calcutta of this small Indian
weed, which bears perfect and cleistogene flowers. (3/10. The convenient
term of CLEISTOGENE was proposed by Kuhn in an article on the present
genus in ‘Bot. Zeitung’ 1867 page 65.) The latter are extremely small,
imperfectly developed, and never expand, yet yield plenty of seeds. The
perfect and open flowers are also small, of a white colour with purple
marks; they generally produce seed, although the contrary has been
asserted; and they do so even if protected from insects. They have a
rather complicated structure, and appear to be adapted for
cross-fertilisation, but were not carefully examined by me. They are not
easy to fertilise artificially, and it is possible that some of the
flowers which I thought that I had succeeded in crossing were afterwards
spontaneously self-fertilised under the net. Sixteen capsules from the
crossed perfect flowers contained on an average ninety-three seeds (with
a maximum in one capsule of 137), and thirteen capsules from the
self-fertilised perfect flowers contained sixty-two seeds (with a
maximum in one capsule of 135); or as 100 to 67. But I suspect that this
considerable excess was accidental, as on one occasion nine crossed
capsules were compared with seven self-fertilised capsules (both
included in the above number), and they contained almost exactly the
same average number of seed. I may add that fifteen capsules from
self-fertilised cleistogene flowers contained on an average sixty-four
seeds, with a maximum in one of eighty-seven.
Crossed and self-fertilised seeds from the perfect flowers, and other
seeds from the self-fertilised cleistogene flowers, were sown in five
pots, each divided superficially into three compartments. The seedlings
were thinned at an early age, so that twenty plants were left in each of
the three divisions. The crossed plants when in full flower averaged 4.3
inches, and the self-fertilised plants from the perfect flowers 4.27
inches in height; or as 100 to 99. The self-fertilised plants from the
cleistogene flowers averaged 4.06 inches in height; so that the crossed
were in height to these latter plants as 100 to 94.
I determined to compare again the growth of plants raised from crossed
and self-fertilised perfect flowers, and obtained two fresh lots of
seeds. These were sown on opposite sides of five pots, but they were not
sufficiently thinned, so that they grew rather crowded. When fully
grown, all those above 2 inches in height were selected, all below this
standard being rejected; the former consisted of forty-seven crossed and
forty-one self-fertilised plants; thus a greater number of the crossed
than of the self-fertilised plants grew to a height of above 2 inches.
Of the crossed plants, the twenty-four tallest were on an average 3.6
inches in height; whilst the twenty-four tallest self-fertilised plants
were 3.38 inches in average height; or as 100 to 94. All these plants
were then cut down close to the ground, and the forty-seven crossed
plants weighed 1090.3 grains, and the forty-one self-fertilised plants
weighed 887.4 grains. Therefore an equal number of crossed and
self-fertilised would have been to each other in weight as 100 to 97.
From these several facts we may conclude that the crossed plants had
some real, though very slight, advantage in height and weight over the
self-fertilised plants, when grown in competition with one another.
The crossed plants were, however, inferior in fertility to the
self-fertilised. Six of the finest plants were selected out of the
forty-seven crossed plants, and six out of the forty-one self-fertilised
plants; and the former produced 598 capsules, whilst the latter or
self-fertilised plants produced 752 capsules. All these capsules were
the product of cleistogene flowers, for the plants did not bear during
the whole of this season any perfect flowers. The seeds were counted in
ten cleistogene capsules produced by crossed plants, and their average
number was 46.4 per capsule; whilst the number in ten cleistogene
capsules produced by the self-fertilised plants was 49.4; or as 100 to
106.
3. GESNERIACEAE.--Gesneria pendulina.
In Gesneria the several parts of the flower are arranged on nearly the
same plan as in Digitalis, and most or all of the species are
dichogamous. (3/11. Dr. Ogle ‘Popular Science Review’ January 1870 page
51.) Plants were raised from seed sent me by Fritz Muller from South
Brazil. Seven flowers were crossed with pollen from a distinct plant,
and produced seven capsules containing by weight 3.01 grains of seeds.
Seven flowers on the same plants were fertilised with their own pollen,
and their seven capsules contained exactly the same weight of seeds.
Germinating seeds were planted on opposite sides of four pots, and when
fully grown measured to the tips of their leaves.
TABLE 3/26. Gesneria pendulina.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 42 2/8 : 39.
Pot 1 : 24 4/8 : 27 3/8.
Pot 2 : 33 : 30 6/8.
Pot 2 : 27 : 19 2/8.
Pot 3 : 33 4/8 : 31 7/8.
Pot 3 : 29 4/8 : 28 6/8.
Pot 4 : 30 6/8 : 29 6/8.
Pot 4 : 36 : 26 3/8.
Total : 256.50 : 233.13.
The average height of the eight crossed plants is 32.06 inches, and that
of the eight self-fertilised plants 29.14; or as 100 to 90.
4. LABIATAE.--Salvia coccinea. (3/12. The admirable mechanical
adaptations in this genus for favouring or ensuring cross-fertilisation,
have been fully described by Sprengel, Hildebrand, Delpino, H. Muller,
Ogle, and others, in their several works.)
This species, unlike most of the others in the same genus, yields a good
many seeds when insects are excluded. I gathered ninety-eight capsules
produced by flowers spontaneously self-fertilised under a net, and they
contained on an average 1.45 seeds, whilst flowers artificially
fertilised with their own pollen, in which case the stigma will have
received plenty of pollen, yielded on an average 3.3 seeds, or more than
twice as many. Twenty flowers were crossed with pollen from a distinct
plant, and twenty-six were self-fertilised. There was no great
difference in the proportional number of flowers which produced capsules
by these two processes, or in the number of the contained seeds, or in
the weight of an equal number of seeds.
Seeds of both kinds were sown rather thickly on opposite sides of three
pots. When the seedlings were about 3 inches in height, the crossed
showed a slight advantage over the self-fertilised. When two-thirds
grown, the two tallest plants on each side of each pot were measured;
the crossed averaged 16.37 inches, and the self-fertilised 11.75 in
height; or as 100 to 71. When the plants were fully grown and had done
flowering, the two tallest plants on each side were again measured, with
the results shown in Table 3/27.
TABLE 3/27. Salvia coccinea.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 32 6/8 : 25.
Pot 1 : 20 : 18 6/8.
Pot 2 : 32 3/8 : 20 6/8.
Pot 2 : 24 4/8 : 19 4/8.
Pot 3 : 29 4/8 : 25.
Pot 3 : 28 : 18.
Total : 167.13 : 127.00.
It may be here seen that each of the six tallest crossed plants exceeds
in height its self-fertilised opponent; the former averaged 27.85
inches, whilst the six tallest self-fertilised plants averaged 21.16
inches; or as 100 to 76. In all three pots the first plant which
flowered was a crossed one. All the crossed plants together produced 409
flowers, whilst all the self-fertilised together produced only 232
flowers; or as 100 to 57. So that the crossed plants in this respect
were far more productive than the self-fertilised.
Origanum vulgare.
This plant exists, according to H. Muller, under two forms; one
hermaphrodite and strongly proterandrous, so that it is almost certain
to be fertilised by pollen from another flower; the other form is
exclusively female, has a smaller corolla, and must of course be
fertilised by pollen from a distinct plant in order to yield any seeds.
The plants on which I experimented were hermaphrodites; they had been
cultivated for a long period as a pot-herb in my kitchen garden, and
were, like so many long-cultivated plants, extremely sterile. As I felt
doubtful about the specific name I sent specimens to Kew, and was
assured that the species was Origanum vulgare. My plants formed one
great clump, and had evidently spread from a single root by stolons. In
a strict sense, therefore, they all belonged to the same individual. My
object in experimenting on them was, firstly, to ascertain whether
crossing flowers borne by plants having distinct roots, but all derived
asexually from the same individual, would be in any respect more
advantageous than self-fertilisation; and, secondly, to raise for future
trial seedlings which would constitute really distinct individuals.
Several plants in the above clump were covered by a net, and about two
dozen seeds (many of which, however, were small and withered) were
obtained from the flowers thus spontaneously self-fertilised. The
remainder of the plants were left uncovered and were incessantly visited
by bees, so that they were doubtless crossed by them. These exposed
plants yielded rather more and finer seed (but still very few) than did
the covered plants. The two lots of seeds thus obtained were sown on
opposite sides of two pots; the seedlings were carefully observed from
their first growth to maturity, but they did not differ at any period in
height or in vigour, the importance of which latter observation we shall
presently see. When fully grown, the tallest crossed plant in one pot
was a very little taller than the tallest self-fertilised plant on the
opposite side, and in the other pot exactly the reverse occurred. So
that the two lots were in fact equal; and a cross of this kind did no
more good than crossing two flowers on the same plant of Ipomoea or
Mimulus.
The plants were turned out of the two pots without being disturbed and
planted in the open ground, in order that they might grow more
vigorously. In the following summer all the self-fertilised and some of
the quasi-crossed plants were covered by a net. Many flowers on the
latter were crossed by me with pollen from a distinct plant, and others
were left to be crossed by the bees. These quasi-crossed plants produced
rather more seed than did the original ones in the great clump when left
to the action of the bees. Many flowers on the self-fertilised plants
were artificially self-fertilised, and others were allowed to fertilise
themselves spontaneously under the net, but they yielded altogether very
few seeds. These two lots of seeds--the product of a cross between
distinct seedlings, instead of as in the last case between plants
multiplied by stolons, and the product of self-fertilised flowers--were
allowed to germinate on bare sand, and several equal pairs were planted
on opposite sides of two LARGE pots. At a very early age the crossed
plants showed some superiority over the self-fertilised, which was ever
afterwards retained. When the plants were fully grown, the two tallest
crossed and the two tallest self-fertilised plants in each pot were
measured, as shown in Table 3/28. I regret that from want of time I did
not measure all the pairs; but the tallest on each side seemed fairly to
represent the average difference between the two lots.
TABLE 3/28. Origanum vulgare.
Heights of Plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants (two tallest in each pot).
Column 3: Self-fertilised Plants (two tallest in each pot).
Pot 1 : 26 : 24.
Pot 1 : 21 : 21.
Pot 2 : 17 : 12.
Pot 2 : 16 : 11 4/8.
Total : 80.0 : 68.5.
The average height of the crossed plants is here 20 inches, and that of
the self-fertilised 17.12; or as 100 to 86. But this excess of height by
no means gives a fair idea of the vast superiority in vigour of the
crossed over the self-fertilised plants. The crossed flowered first and
produced thirty flower-stems, whilst the self-fertilised produced only
fifteen, or half the number. The pots were then bedded out, and the
roots probably came out of the holes at the bottom and thus aided their
growth. Early in the following summer the superiority of the crossed
plants, owing to their increase by stolons, over the self-fertilised
plants was truly wonderful. In Pot 1, and it should be remembered that
very large pots had been used, the oval clump of crossed plants was 10
by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2
inches in height; whilst the clump of self-fertilised plants, on the
opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across,
with the tallest young stem 4 inches in height. In Pot 2, the clump of
crossed plants was 18 by 9 inches across, with the tallest young stem 8
1/2 inches in height; whilst the clump of self-fertilised plants on the
opposite side of the same pot was 12 by 4 1/2 inches across, with the
tallest young stem 6 inches in height. The crossed plants during this
season, as during the last, flowered first. Both the crossed and
self-fertilised plants being left freely exposed to the visits of bees,
manifestly produced much more seed than their grand-parents,--the plants
of the original clump still growing close by in the same garden, and
equally left to the action of the bees.
5. ACANTHACEAE.--Thunbergia alata.
It appears from Hildebrand’s description (‘Botanische Zeitung’ 1867 page
285) that the conspicuous flowers of this plant are adapted for
cross-fertilisation. Seedlings were twice raised from purchased seed;
but during the early summer, when first experimented on, they were
extremely sterile, many of the anthers containing hardly any pollen.
Nevertheless, during the autumn these same plants spontaneously produced
a good many seeds. Twenty-six flowers during the two years were crossed
with pollen from a distinct plant, but they yielded only eleven
capsules; and these contained very few seeds! Twenty-eight flowers were
fertilised with pollen from the same flower, and these yielded only ten
capsules, which, however, contained rather more seed than the crossed
capsules. Eight pairs of germinating seeds were planted on opposite
sides of five pots; and exactly half the crossed and half the
self-fertilised plants exceeded their opponents in height. Two of the
self-fertilised plants died young, before they were measured, and their
crossed opponents were thrown away. The six remaining pairs of these
grew very unequally, some, both of the crossed and self-fertilised
plants, being more than twice as tall as the others. The average height
of the crossed plants was 60 inches, and that of the self-fertilised
plants 65 inches, or as 100 to 108. A cross, therefore, between distinct
individuals here appears to do no good; but this result deduced from so
few pl
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