Botany: The Science of Plant Life
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Title: Botany: The Science of Plant Life
Author: Norman Taylor
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Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK BOTANY: THE SCIENCE OF PLANT LIFE ***
[Illustration: Courtesy Journal of the International Garden Club]
[Illustration: _The prickly pear (Opuntia chlorotica santarita) of the
desert in the American Southwest._
(_This painting was kindly loaned by Dr. David Griffiths of the United
States Department of Agriculture and reprinted here through the courtesy
of the Journal of the International Garden Club, where it first
appeared._)]
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME THIRTEEN
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
BOTANY
THE SCIENCE OF PLANT LIFE
BY
NORMAN TAYLOR
CURATOR, BROOKLYN BOTANIC GARDEN
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
This book is for those who want some general knowledge of the plant
world, without necessarily caring for the technical details upon which
such knowledge is based. If it leaves the reader with an impulse to
follow the subject further than has been possible here, it will have
more than fulfilled its mission.
Throughout the book, it has often been convenient to refer to plants or
their behavior in terms implying reasoning faculties. Of course, plants
are never reasoning things, reasonable as many of their actions appear
to be, and to ascribe such qualities to them is to saddle them with
attributes perfectly foreign to the plant world. But the description of
them in the terms of our everyday speech, the translation of plant
behavior into the current conceptions of mankind, does so fix these in
our minds that they cease to be among those interesting things that
nearly everyone forgets. I have followed this method deliberately,
understanding perfectly the objections to it, but believing, with the
late C. E. Bessey, that in popular books “it is an admirable way of
looking at some botanical things.”
All of the half-tone illustrations, except two, are from the
photographic collections of the Brooklyn Botanic Garden, and I am under
the greatest obligation to the director of that institution for
permission to publish them here. The illustration of the living and
fossil algæ has been taken from Prof. Henry Fairfield Osborn’s “Origin
and Evolution of Life,” with his kind permission. The illustration of
desert vegetation is from a photograph by the late E. L. Morris, and
kindly loaned from the collection of the Brooklyn Museum. All the line
cuts in the text are from drawings made specially for this book by my
wife, Bertha Fanning Taylor.
While grateful and particular acknowledgments can be made for the
illustrations, it is difficult or impossible to properly express my
indebtedness to all those who, through their books and pamphlets, have
indirectly aided in the making of this book. It would involve the
mention of most of the better known writers of the books found in the
larger botanical libraries. It is a pleasure to acknowledge help from
Dr. M. A. Howe of the New York Botanical Garden on the literature of
fossil and hot-spring algæ, and from Dr. Orland E. White of the Brooklyn
Botanic Garden for helpful criticism of the section dealing with “How
Plants Change Their Characters.”
NORMAN TAYLOR.
BROOKLYN BOTANIC GARDEN
20th October, 1920
CONTENTS
CHAPTER PAGE
INTRODUCTORY--PLANTS AND OUR DAILY NEEDS 9
I. WHAT PLANTS ARE 13
II. PLANT BEHAVIOR 76
III. HOW PLANTS PRODUCE THEIR YOUNG 116
IV. THE FAMILIES OF FLOWERING PLANTS AND THEIR RELATIONSHIP 168
V. USES OF PLANTS TO MAN 208
VI. GARDEN PLANTS 267
VII. HISTORY OF THE PLANT KINGDOM 298
VIII. DISTRIBUTION OF PLANTS 337
LIST OF ILLUSTRATIONS
THE PRICKLY PEAR OF THE DESERT IN THE AMERICAN SOUTHWEST _Frontispiece_
FACING PAGE
BANYAN--A FIG TREE OF INDIA 32
LACELEAF--A SUBMERGED AQUATIC PLANT 33
ROOT HAIRS WITH FINE SOIL PARTICLES ATTACHED 33
AERIAL ROOTS OF FIG TREES 64
VENUS’S FLYTRAP--AN INSECTIVOROUS PLANT 65
INDIAN PIPE--A SAPHROPHYTIC PLANT 96
PARTRIDGE BERRY--A TRAILING VINE 96
RAFFLESIA, THE LARGEST FLOWER IN THE WORLD 97
TALIPOT PALM OF CEYLON 128
TRAVELERS’ TREE, A PLANT OF THE BANANA FAMILY 129
WIND-BLOWN POLLEN OF THE JAPANESE RED PINE 160
COCONUT GROVE IN THE PHILIPPINE ISLANDS 161
TRANSPLANTING RICE IN JAVA 192
TEA ESTATE IN CEYLON 193
BANANA PLANTATION IN FRUIT 224
RICE TERRACES IN CHINA 225
FOREST OF THE TEMPERATE ZONE 256
LUXURIANT VEGETATION OF A TROPICAL RAIN FOREST 257
FOSSIL AND LIVING ALGÆ COMPARED 320
LANDSCAPE OF THE CARBONIFEROUS AGE 321
DESERT PLANTS OF THE SOUTHWEST UNITED STATES 352
CONTEST BETWEEN GRASS LAND AND TREE VEGETATION 353
INTRODUCTORY CHAPTER
PLANTS AND OUR DAILY NEEDS
Perhaps few of us realize that without plants all our modern
civilization would be swept away and that upon plants has been built all
that we have so far accomplished and everything that we may yet become.
The overthrow of any king or republic, the wiping out of all money and
finance or any of the manifold evidences of our modern world could not
for a moment be compared to what would happen to us with the sudden
destruction of plant life from the earth.
Food and drink, the very houses we live in and heat, medicines and
drugs, books and pictures, musical instruments and tires for
automobiles, all these and hundreds of our daily needs depend upon the
fact that plants of many different kinds grow upon the earth and in
sufficient amounts to be of importance. It is easy to say in opposition
to this that we get much food from animals, that we can drink water, and
that neither of these comes from plants. But water would soon be lost to
us if forests did not conserve it, and upon pasturage most of our food
animals depend for their life. The discovery of a single tree in the
mountains of northern South America made possible for white man the
malarial regions unfit for him before the advent of quinine. Just before
Shakespeare’s time sugar and tea and coffee became regular articles of
commerce. Not until the discovery of America was tobacco, Indian corn,
chocolate, the pineapple or the potato known to man. Upon the spruce
forests in the north depends much of our paper supply, from cotton we
get clothes and explosives, from hemp and sisal ropes, from a single
kind of Brazilian tree most of our rubber, and from rice a food that
sustains nearly half the world.
While it is thus plain enough that life depends upon plants now present
upon the earth it may not be so obvious that from certain ancient
forests has come the greatest source of artificial heat in the world.
Coal is nothing but the partial decomposition of vast forests, living
ages before man was first found upon the earth, subsequently buried, and
under the earth’s pressure forming soft coal, or where the pressure was
severe enough hard coal. When it is remembered that a dead, partly
decayed tree is only a fraction of its living size and that coal is
found in many parts of the world in tremendous quantities we get a
partial glimpse of what our debt is to a great forest that lived in
luxuriance no one knows how many millions of years ago, reached its
climax, and upon whose embedded remains we depend for heat.
Later on in this book will be given in greater detail some of the plants
useful to man and just how we have used them. Hardly any part of the
study of the plant kingdom has so much of interest as that dealing with
our utilization of the things that grow about us. From the earliest
struggles of our half-savage ancestors to grow definite crops rescued
from the wild down to our modern nut butter made from the partly
fermented meat of the cocoanut and shipped half round the world before
it is refined, man has constantly striven to use for his advantage the
plants most likely to prove valuable. Countries and empires have been
built upon such facts. Even to-day rubber from the Straits Settlements
and palm oil from Africa are deciding the economic life of those
countries.
But man’s use of plants, in fact his absolute reliance upon them, is not
the only reason for attempting to find out more about them, what they
are, where they came from, how they live and produce their young. A
knowledge of even a small part of such a science opens up a rich field
of inquiry involving a concept of plant life of greater interest than
mere bread and butter. For those with an eye to see and knowledge to
interpret, a landscape with its trees or flowers or marshes may contain
a host of hidden secrets of dramatic import. Unfolded before one may be
found a spectacle of struggle and strife, quiet tragedies of the forest,
the inexorable pressure of plants upon their neighbors, the woods upon
the prairies or an apparently forlorn hope of some plant living in a hot
desert or upon some icy mountain peak. And while these rather obvious
things are happening how much more is hidden of the adjustments that
leaves or flowers or roots or other organs of the plant are constantly
making to the conditions about them. Upon the perfection of such
adjustments to light, heat, or water, for instance, depends their very
existence. Mistakes are fatal, the forces of nature seem peculiarly
relentless, and it is literally a case where many are called but few
chosen. Of the untold millions of seeds produced each year few ever
grow, yet out of this enormous wastage springs all that makes the earth
not only habitable but the beautiful panorama of vegetation to which we
are so accustomed that it is nearly taken for granted.
The study of botany attempts to answer some of the questions raised
above and many others. Subsequent parts of the book will deal with what
plants are, the behavior of them, with the life histories of some of the
better known ones, with the grouping of plants in families and their
relationships one to another, with their uses to man, with the history
of their development from the earliest times, and finally with their
distribution over the earth. The latter will be discussed last because
it is the most important of all the phases of plant life. How plants are
distributed, whether as forest or prairie or thickets or what not,
depends upon the response of individual plants and their organs to the
conditions about them. The type of vegetation in different parts of the
world has been dictated by the success of the survivors in meeting
existing conditions and of having met them in the past. Upon this fact
rests our civilization to-day. Upon this fact there has been reared a
study the cultural, esthetic, and practical value of which may well
outweigh any other.
While the study of botany is necessarily a technical one with a language
all its own, its terms, though generally unfamiliar, are unexcelled for
their purpose. They will be avoided here as much as a clear
understanding of the subject will permit. The few that must be used will
be explained where they first occur and it is assumed that the reader
will understand their subsequent use.
CHAPTER I
WHAT PLANTS ARE
What we commonly call plants, such as corn or buttercups or an oak tree,
are so familiar that a definition of what plants are may seem needless.
It would be quite so if these generally recognized examples comprised
all the plant kingdom. Actually what are ordinarily thought of as plants
make up only a fraction of the great plant world. The fact that our
familiar roadside and garden plants produce blossoms followed by a fruit
and seed, such as peas and beans and all the ordinary flora of any
region, separates them at once from that other large group of plants
that do not. Common examples of the latter class are the green scum on
the ponds, moss, seaweed, the rust on wheat, yeast, disease-causing
bacteria, the smallest of all known plants, and many others. Most of
these organisms are so small that they can be distinguished only under
the higher powers of the microscope. Some of them in their habits and
growth are like the minute animals described in the volume of this
series devoted to that subject. In fact there are organisms about which
scientists are still in doubt as to their animal or vegetable character.
One or two characteristics common to most plants, however, separate them
from animals and these are their method of getting food and their
practically immovable mode of life. Animals, however simple, do eat and
digest their food, plants take various mineral substances from the earth
or air or water in the form of inorganic matter such as oxygen, carbon,
nitrogen, and all the food materials found in the soil, and transform
them, with the aid of sunshine, into the structure that characterizes
each particular form. _Plants, then, may be defined as any living
organism that, with minor exceptions, has the power to assimilate
inorganic substances and transform them into organic matter._ Nothing
else in all the realm of nature has this power. It is a possession
beyond all price, this ability to take from the soil and air and water
simple chemical substances and under the magic of sunlight transform
them into the wonderful plant life of the world. But this faculty has
its handicaps, for it is usually, though not always, associated with
inability to move from place to place, which, in some measure, even the
simplest animals can accomplish.
It will be readily understood that this definition of plants includes
many other things that are commonly attributed to the plant kingdom. For
our purpose the discussion of these relatively unfamiliar forms of plant
life will be left until later. A rough and ready distinction between
those plants that everyone recognizes as such and those others like
yeast and microbes, which are plants to most of us only by virtue of the
definition given above, is that the first group produce flowers and
seeds and the second do not. It should not be forgotten, and it will be
shown later, that this is not a true distinction, but for practical
purposes of dividing the plant kingdom it suffices.
1. FLOWERING PLANTS
The fact of outstanding importance to everyone who really looks at most
plants is that part of them are above ground and part below. This simple
observation carries with it the recognition of a fundamental difference
of plant structure, namely roots and stem. Most plants bear obvious
leaves, and at some time in their life flowers, inevitably followed by
fruits and seeds. The ideally perfect plant would consist, then, of
root, stem, leaves, flowers, fruit, and seed. These are subject to many
changes of form, sometimes they are put to strange uses, and
occasionally one or two may be lacking, as the stem is apparently from
many violets, and leaves from some cactus and from the Tjemoro tree of
Malaya. In fact, so varied are the different forms of these common
attributes of most plants, so important are these differences of
structure that no right understanding of plant life can be had without
examining each in some detail.
THE ROOT
The obvious purpose of the roots of plants is to serve as an anchorage
or holdfast. Their other and equally important function is to secure
food for the plant, a process that will be described in the part devoted
to Plant Behavior. Certain plants bear no roots and attach themselves to
the roots of other plants in which case they literally steal their food,
as does the mistletoe and some others.
Roots are of various kinds, depending upon the soil in which they grow
and upon the kind of plant to which they are attached. In the case of
_annuals_, which live only one year, as does the purslane, and
_biennials_, which live only two years, as does the fringed gentian, the
roots are mostly fibrous (Figure 1) and apt to be only slightly under
the surface. In _perennials_, which live many years, such as the
dandelion, the root is deeper and forms what is known as a _taproot_
(Figure 2). In shrubs and trees they are harder, woodier, and often
penetrate to great depths.
[Illustration: FIG. 1.--FIBROUS ROOTS
As illustrated by the common garden nasturtium.]
If we examine the roots of a tree, we find a large part of them are
woody, often as thick as the smaller branches, and it is only toward
their extremities that they branch out into the multitude of rootlets
[Illustration: FIG. 2.--TAP ROOT OF CARROT
A store of food for the second year.]
that creep through the earth seeking food for the tree. Some, as in the
spruce or hemlock, do not go very deep but spread great distances
through the soil in search of food, others, like the hickory, go nearly
straight down. The interesting feature of these tree roots is that the
part nearest the trunk is all but dead, and acts mostly as an anchor,
while the fibrous rootlets or even finer subdivisions known as _root
hairs_ at the extremities are the food gatherers. At the very end of all
rootlets and of roots is a _rootcap_ (Figure 3), harder than the rest of
the threadlike rootlet. This rootcap is, if not quite dead, at least
useless as a food gatherer, but serves as a tiny pioneer wedge which
forces its way among stones or other obstructions, so allowing the
living root hairs just behind to gather the food to which it leads the
way. In certain of the rocky islands of the Bahamas wild fig trees may
be seen, growing on the bare rocks, their roots sprawling in every
direction in search of a crevice through which the rootcap can force its
way. Such roots may extend thirty or forty feet from the trunk of the
tree over the bare rock in search of a favorable crack where they plunge
to the cool depths and secure the food and water necessary for life.
[Illustration: FIG. 3.--TAP ROOT OF SEEDLING
The figure shows the root cap at the extremity.]
Roots are not always of this common type. Sometimes, particularly in
certain biennials, they are swelled to form great thickened portions,
often weighing many pounds. The sweet potato is a familiar example, and
a related plant, one of our morning-glories, has an enormous perennial
root, known to weigh as much as fifty pounds. This swelling of the
roots of plants is a quite common characteristic of certain kinds and
has great commercial significance. Carrots, turnips, rutabagas, beets,
and parsnips are familiar examples. The purpose of such roots is to
store food for the plant, and this thrifty habit of some roots has been
turned by the gardeners to our advantage.
It is a common sight to see parts of a sidewalk heaved up by a tree root
and their force in this respect is tremendous. One of our common ferns
has been known to raise a weight of over 500 pounds, and even to break
through a concrete walk. Such is the force exerted by the roots of
plants that we can truly think of roots as pushing through the earth
almost regardless of obstacles, binding the soil together and not only
serving the needs of the plants, but actually holding the soil on steep
slopes. Where fire or ruthless lumbering has stopped this natural
process the washing away of the soil and exposure of the bare rocks
leaves desolation behind it.
[Illustration: FIG. 4.--ADVENTITIOUS ROOTS OF THE TOMATO
Produced from joints or injured places.]
While most roots live under the surface many grow in the air, and a few
grow from stems that are injured. The tomato vine often produces roots
at the joints or where it has been injured. Such roots, known as
_adventitious roots_ (Figures 4 and 5), are fairly common in many
plants, the common garden practice of making cuttings, which take root
under favorable conditions, being based upon this fact.
[Illustration: FIG. 5.--PROP ROOTS OF THE INDIAN CORN
Usually produced from the first joint above ground and serving as
additional anchor and food gatherers.]
But some plants produce roots in the air, as in poison ivy and the
trumpet creeper, without injury or the gardener’s skill, and are known
as _aërial roots_. They are some of the most peculiar and fantastic of
nature’s devices for allowing plants to grow in apparently unfavorable
places. In many orchids, some relatives of the pineapple, and a few
other air-inhabiting plants, the roots live wholly in the air, the
plants being fastened to a tree or even to a telegraph wire. Such plants
live on the air and water vapor, and are mostly inhabitants of moist
tropical regions.
Quite the most extraordinary of aërial roots are those produced in
certain kinds of fig trees. Starting perhaps a hundred feet in the air
and no thicker than a lead pencil they appear first as slender vine-like
streamers blown hither and yon by the wind. Eventually they reach the
ground and penetrate it, grow often to a great size and even form
trunklike connections with the tree top. The banyan tree of India is the
best known case of this peculiar habit. One which started from a single
trunk, subsequently sending out great numbers of aërial roots, has now
spread to such a size that it is over 2,000 feet in circumference, has
3,000 trunks, and once sheltered 7,000 soldiers.
A variation of this habit is the case of a giant fig tree of the West
Indies in which a bird may deposit the seed of another tree. The
seedling soon develops, sending out long, at first threadlike, aërial
roots which are wrapped around the tree trunk. As the roots increase in
size and further encircle the trunk they ultimately reach the ground,
where they are frequently a foot in diameter. Then the true nature of
the process becomes evident. For these apparently innocent aërial roots,
as they reach the ground, have so completely inclosed the old trunk and
their pressure is so great that they literally strangle the tree from
which they started. It is slower but just as deadly as the strangulation
of an animal by a boa constrictor, for these encircling roots cut off
by strangulation the ascent of the sap, thus killing the tree. Fate
sometimes overtakes them, however, as it is a common sight to see the
strangler meet the same end. Some of nature’s most ruthless battles are
fought out in this way, very silently, but very effectively.
THE STEM
[Illustration: FIG. 6.--ROOTSTOCK OF SOLOMON’S SEAL
The figure shows roots and bud scars together with the ascending stem of
the year’s growth.]
Nothing dies harder than generally accepted delusions, particularly
those regarding plant lore, and of all such incorrect impressions the
one that a potato is a root, is the hardiest and most difficult to kill.
Yet, the “eyes” of a potato give it away if one stops for a moment to
reflect that the eyes are only buds and buds grow only on stems. That is
one of the chief uses of stems--to support in the air the leaves and
flowers that come from its buds, and no matter if the stem, as in the
potato and many other plants, be ever so deeply buried their true stem
nature cannot be mistaken. Sometimes these underground stems are not
thickened but lengthened out, in which case, notably in common garden
iris, they are called _rootstocks_. Again, these buried stems may be
swollen, as in the potato, when they are known as _tubers_. Onions and
the jack-in-the-pulpit bear still other kinds of underground stems, and
there are many more, but they cannot be mistaken for roots, for it will
be seen from Figures 6-9 that on their under sides they bear roots
themselves. Besides this they bear buds or shoots, which no true root
ever does.
[Illustration: Fig. 7.--CORM OF JACK-IN-THE PULPIT
(After Gray)
Really an underground stem.]
Stems above ground, which is the most usual form for them, are of many
kinds, all serving the purpose of support to the leaves and flowers, and
as a means of carrying sap from the roots or underground stems to the
upper part of the plant, and also to carry certain foods to the roots
from the leaves, of which more anon. In the case of herbs, like
goldenrod or daisy, the stem may be apparently all pith on the inside,
with only a thin outer coating of harder substance, not unlike bark, but
usually green. If we examine the cut-off trunk of a tree, a quite
[Illustration: FIG. 8.--TUBERS OF THE COMMON POTATO
Potatoes are swollen portions of rootstock.]
different structure is apparent. Any lumberman can point out at once
“heartwood” and “sapwood” (Figure 78), and his distinctions are just as
good as those of the scientist, for he says in these two words as
plainly as can be said that heartwood is the oldest and sapwood the
youngest. The sapwood is nearer the bark and is honeycombed with
passages which serve to carry the sap from the roots to the tree top,
while just under the bark is the bright, green, living layer, known as
_cambium_, which is renewed each year. The _phloem_ is the carrier for
the food made in the leaves to the roots. It is the successive layers of
_cambium_, year after year, that gives to tree trunks their annual
rings. The age of almost all trees can be reckoned exactly by counting
these, one representing a year’s growth, and the tree’s rate of growth
estimated from the closeness of the rings. Fires or droughts, perhaps
long forgotten, here find a lasting record in rings so close together as
to be all but invisible. The part nearest the center of the trunk is the
heartwood, usually quite lifeless, yet in its maturity furnishing us
with lumber. It may be and often is completely decayed, without injuring
the flow of sap or the life of the tree for many years.
[Illustration: FIG. 9.--BULB OF THE ONION
Showing root and leaf growth.]
These two streams of sap, one going up and the other returning to the
roots, each in its proper channel, are interspersed with air chambers
that extend from the center of the tree out toward the bark, where they
end in inconspicuous dots called _lenticels_. It is as though nature had
provided an air-cooling device for the constant activity of these
diverse currents. These lenticels are prominent on the bark of cherry,
but whether obvious or not they are found in nearly all woody stems and
insure a constant supply of fresh air to the busy interior.
In palms, sugar cane, corn, bamboo, and many other plants there is not
any distinction between heartwood and sapwood (Figure 82), and in place
of bark there is nothing but an outer rind, harder than the interior
tissue. Such stems do not usually rot first at the center, have no
cambium, and have no annual rings. This method of growth and structure
is associated nearly always with definite leaf and flower forms peculiar
to it and differing from most other plants. So fundamental are these
characteristics, so uniform their occurrence and so clear are the
distinctions between them and other plants that botanists have divided
all flowering plants into those belonging to this group or to some
others. More will be said of this in the chapter on the Families of
Plants and Their Relationship.
The stems of some plants, such as the Big Trees of California, for
instance, are among the oldest and most permanent of living things.
“General Sherman,” one of the biggest in that most famous grove, was
nearly three thousand five hundred years old when Columbus discovered
America; it has lived through all the great periods of modern history,
and to-day it is over 270 feet high and 35 feet in diameter. No living
thing is so large or has lived so long. In Australia are great forests
of blue gum trees even taller than our Californian Big Trees, but not so
old nor so thick.
In the Pacific, off the coast of Oregon and British Columbia, a seaweed
is commonly found with stalks over 500 feet long, and in India the
rattan palm climbs over the tree tops for great distances, a single stem
not much thicker than a broomstick measuring over 700 feet long. The
search by leaves for light and air results in the stems of some plants
performing almost incredible feats. Whether it is one of the Big Trees
with a great massive trunk, or the rattan palm with its sinuous winding
through the topmost heights of the tropical forests of India, the result
is always upward to a “place in the sun.” This struggle for sunlight has
taken many forms in different plants, the ordinary vines like
morning-glory or grapevine, for instance, where the climbing stem is of
great advantage. Some vines always twine to the left, as the hopvine,
others to the right, as in the morning-glory, all seeking support from
something else, each adopting its own most useful way of getting its
leaves in the most advantageous position to catch the life-giving
sunshine. If we could look down on any forest from an aeroplane, the
striking efforts of nearly all plants, whether herbs, shrubs, vines, or
trees, to get the utmost sunshine for their leaves would be evident at
once. No apparently impossible twisting or bending of tree trunks or
reaching out of stems of vines but is to be found in the inexorable
struggle of stems to fulfill their task of giving the plant its chance
to reach “a place in the sun.” Sometimes mere climbing or twining does
not seem sure enough--it seems as though winds or the elements might
break loose the vine from its support and thereby kill its chances. In
certain vines this contingency appears to have been foreseen, and as if
to clinch their opportunity of growing onward they are provided with
special helps. Slender green _tendrils_, delicate prolongations of the
stem, begin, almost insidiously, to catch hold of the nearest support
and by a couple of turns about it and subsequent strengthening of their
tissues make a permanent holdfast. The grapevine is a case in point. And
as if this were not enough, certain other plants, such as the Boston
ivy, have small disks which attach themselves to bare walls or tree
trunks. This is to make assurance doubly sure, and it is this that makes
the Boston ivy so useful to the gardeners for covering walls.
Some stems accomplish their purpose not by holding fast to a support in
the air, but by creeping along the ground, as in the running blackberry,
and often in the Virginia creeper. The purpose is the same, and, as if
to confirm it, a few otherwise quite prostrate vines have their tips
turned upward to the light, notably in the case of the creeping
speedwell.
In certain plants the stem may assume curious forms due to special
conditions under which they live and to which adjustment is necessary
for the plant’s existence. In deserts, for instance, the cactus produces
practically no leaves (Figure 10) and the green stem performs not only
the function of leaves but acts as a storage for water. Where water is
scarce this is of tremendous advantage, a single cactus having been
known to store up 125 gallons. A similar habit of the cactuslike spurges
in South Africa gives as weird an atmosphere to parts of their
landscapes as we find in Arizona. It is as if the stem of such plants,
being unable to push its leaves (it has none) up into the light, takes
over some of the functions of leaves and makes up the deficiency by
adopting other methods
[Illustration: FIG. 10.--COMMON PRICKLY PEAR CACTUS (_Opuntia Opuntia_)
Native along the Atlantic Coast. The green joints of the stem function
as leaves and store water.]
to secure the plant’s survival. Other stems, looking and acting like
leaves, reveal their true nature by producing buds, and the curious
feature of the common butcher’s-broom (Figure 11), often colored scarlet
for Christmas decorations, bearing flowers from the middle of what is
apparently a leaf, but is actually a modified stem, is explained by this
ability of stems to modify their habits to suit conditions. The
butcher’s-broom is an inhabitant of dry regions along the Mediterranean,
where a reduction or
[Illustration: FIG. 11.--BUTCHER’S-BROOM
(_Ruscus aculeatus_)
Note leaflike stems with flowers arising from the center.]
absence of leaf surface is a decided advantage. In many partly desert or
dry regions this production of leaflike stems or branches is common, an
excellent garden example being asparagus, which came originally from
Europe and the feathery growth of which is all stem. In Tasmania a kind
of yew tree produces no leaves, all the foliage being modified stem,
which is true of many kinds of spurge in the West Indies, where an
almost impenetrable scrub is largely made up of a shrub which is
apparently covered with leaves, all actually part of the branches and
stems.
[Illustration: FIG. 12.--DUCKWEED
The smallest known flowering plant, with _no_ leaves and tiny leaflike
stems floating on the surface. Flowers are borne from the margin of the
stem. (Eight times natural size).]
While stems, such as the Big Trees or the giant cactus, may be among the
largest of nature’s creations they may be also the smallest, as the
duckweed that floats on ponds is the tiniest of all flowering plants and
its flat expanded surface is wholly stem. Figure 12 on this page better
illustrates this strange modification of a stem than words could do.
From what has been read it will be seen that stems are not “just
stems”--they are among nature’s most ingenious devices to secure the
survival of the plant. Whether buried in the ground, and producing,
almost by stealth, buds that develop into mature plants, or thrusting
leaves to the utmost limits of their reach, or climbing by an
intricately varied mechanism, or changing their character to suit desert
conditions, or floating on the water--it matters not. Each modification
of form or use secures to the individual plant its chances to survive;
and in most cases its only chance, as anyone may see by the sudden death
which follows a series of changes which prevents a stem from performing
its proper tasks.
THE LEAF
As the palm reader is supposed to be able to tell your history and
future from veins in your hand, and as the veins in the wing of a
butterfly tell their story to an entomologist, so the veins of a leaf
are more significant than almost any other characteristic of a plant.
Most leaves have their veins, or skeleton, with a single midrib and many
branches off it on each side, which themselves break up into a fine
network of veins. Such leaves are _netveined_ (Figures 13-24). Others,
such as corn and grass, have the veins running side by side from one end
of the leaf to the other, sometimes with small branches off them, but
instead of the veins forming a network they are parallel, and such are
called _parallel-veined_ leaves (Figure 38). In the chapter on Plant
Families and Their Relationship more will be said as to the amazing
regularity with which netveined leaves are associated with certain kinds
of flowers and parallel-veined with other kinds, how these distinctions
have been recognized since hundreds of years before Christ, long before
their true import was understood. There are variations from both these
[Illustration: BANYAN TREE (_Ficus bengalensis_). A fig tree of India,
whose adventitious roots make frequent connection between the tree top
and the ground. Starting as thin, whiplike streamers these roots
ultimately form new trunks. (_Courtesy Brooklyn Botanic Garden._)]
[Illustration: LACELEAF (_Aponogeton fenestralis_). A submerged aquatic
plant, with permanently skeletonized leaves, and an inhabitant of forest
pools in Madagascar. (_After Engler & Prantl. Courtesy of Brooklyn
Botanic Garden._)]
[Illustration: ROOT HAIRS WITH FINE SOIL PARTICLES ATTACHED. Note soil,
water, and air spaces. Much magnified. (_After U. S. Department of
Agriculture. Courtesy of Brooklyn Botanic Garden._)]
[Illustration: FIGS. 13-24.--FORMS AND TIPS OF LEAVES
Fig. 13. A linear leaf with an acute tip. Fig. 14. Lanceolate leaf with
an acuminate tip. Fig. 15. Oblanceolate leaf broadest above the middle.
Fig. 16. Ovate, broadest below the middle. Fig. 17. Spatulate, broadest
above the middle and with an elongated base. Fig. 18. Elliptical. Fig.
19. Obovate in which the general shape is ovate, but broadest toward the
tip. Fig. 20. Oblong. Fig. 21. Orbicular or nearly round. Fig. 22.
Deltoid or somewhat triangular, an ovate leaf with a broad base. Fig.
23. Kidney-shaped or reniform with heart-shaped base. Fig. 24. Peltate
leaf of common garden nasturtium; note circular blade with leafstalk
attached to the center.]
types, but in nearly every case, once the difference is noted--and
scarcely any other character of a plant is so much worth notice--they
cannot be mistaken.
During the winter nearly all leaves are folded in various ways in a bud
for protection from the elements. Nature shows herself in some of her
wisest moods in the selection of methods to accomplish this. In some
buds, notably those of the horse-chestnut, the bud is coated with a
sticky substance to protect the tender young leaves inside. In others
there is a hard outer coat, as in the hickory, impregnable to the most
driving sleet, others again have the leaf rolled so tightly and pointed
so sharply at the end, as in the beech, that water cannot cling to the
bud nor soak in, until the warmth of spring gives the signal for the
annual miracle of the bursting out of foliage. Leaf buds are sometimes
hard to find on certain plants, as they are formed at the base of a
leafstalk and covered by it during the growing season. It is only as the
leaf falls in the autumn that the hollow base of its stalk is seen to
have hidden during the summer the young bud for the following season.
The plane tree or sycamore is a good example of a plant where no leaf
buds can be found until the falling of the leaves in autumn.
The forms of leaves are infinite in their variety, and the reasons for
some of their peculiarities in this respect are not yet understood. The
average netveined leaf is obviously composed of a _blade_ (Figure 25),
and at the base a stalk known as a _petiole_. Sometimes at the base of
the petiole--which is lacking in many leaves--there are two tiny
leaflike appendages, called _stipules_, which are of no apparent use to
the plant, and, as if in recognition of this fact, they often fall off
long before autumn. In some plants, however, stipules are permanent,
while in certain others they are never found, as, for instance, in the
horse-chestnut tree.
[Illustration: FIGS. 25-35.--FORMS AND BASES OF LEAVES
Fig. 25. Simple leaf with blade, leafstalk (petiole), and two stipules
at the base. Margins of the leafblade serrate or saw-toothed. Fig. 26.
Leaf with a sagittate base, or shaped like an arrowhead, the lobes
pointing downward, and with entire margins. Fig. 27. Retuse or
emarginate tip, somewhat indented. Fig. 28. With the base auriculate or
with rounded basal lobes. Fig. 29. Hastate, like an arrowhead but the
lobes pointing outward. Fig. 30. With cuneate base (wedge-shaped). Fig.
31. Cuspidate tip with a usually hard and stiff point. Fig. 32.
Perfoliate, the leaf bases joined and the stem passing through them.
Fig. 33. Truncate, the top flattened. Fig. 34. Pinnately lobed, with
deep indentations cut toward the midrib. Fig. 35. Palmately lobed, out
toward the top of the leafstalk.]
The outline of leaves is as varied as nature itself. Some of the common
kinds are shown in drawings (Figures 13-24), which tell more of the
story than pages of description could do. Their margins, too, their
tips, their bases (Figures 25-35), all parts of them, in fact, are so
variable and yet in each kind of plant so uniform, that in the
description of the plants of any region the botanist has used these
characteristics of leaves as one method of identifying the particular
plant in hand.
The terms used to designate these different kinds of leaf margins or
forms of blade are precise, nearly universally used, but need to be
studied only by those who, because of special fondness for the subject,
are likely to need them in using books which are beyond the scope of the
present one. If, for instance, the reader is interested in finding out
what his native roadside plants are, he would need a book describing
them, and there are many for different parts of the country. In such
books he would find these terms, which say so much in a single word
(there are other sets of terms for flowers, fruits, and seeds) totally
unfamiliar and quite likely to disgust him at the start. A little study
may open up to him that most interesting and easily accessible of
recreations, a first-hand familiarity with the wild flowers of one’s own
neighborhood.
All leaves are not as simple as the figures show them to be. In many the
midrib or principal vein is much elongated and there are small
_leaflets_, sometimes even scores of them, all fastened to a common
stalk. Such are called _compound_ leaves (Figures 36-37), which may be
found in ash, hickory, rosebushes, blackberries, peas, beans, and
thousands of other plants.
[Illustration: FIGS. 36-38.--COMPOUND AND PARALLEL-VEINED LEAVES
Fig. 36. Palmately compound leaf, the five leaflets all arising from the
tip of the common leafstalk. Fig. 37. Pinnately compound leaf, the
leaflets arising from the sides of the common leafstalk. Fig 38. A
parallel-veined leaf. All the other leaves figured are netted-veined.]
While leaves are literally factories in which one of the most wonderful
things in the world is produced, it is so much a part of what plants do
or their behavior that the story of it will be given in the chapter on
Plant Behavior. Sunlight is absolutely necessary for the process, and to
reach this sunlight leaves are attached to their stems in a variety of
ways. Some are always opposite each other, as in the common privet,
lilac, or honeysuckle; others always alternate, as in the mustard or the
rose. There are many variations of these simple arrangements, but in
every case the process results in giving each leaf the utmost exposure
to the light without which the plant must wither and die. So vital is
this exposure to light that in some plants parts of the leaves produce
_tendrils_, as in the case of peas, in order that some near-by support
may be used. In one African relative of our lily, this change of leaf
form has been so great that its long slender leaf tip is wonderfully
adapted to reaching up and catching by its curved tip some support to
lift it from the gloom of the tropical forest floor.
Looking down from above on any small plant or bush, or from the sky on a
forest, about all that can be seen are the thousands of leaves, all so
arranged that it is as though some celestial photographer asked every
one of them to so place themselves that they would all be “in the
picture.” The competition between leaves on the same plant and between
leaves on rival plants is infinitely keener than the friendly pushing of
a crowd to get in a picture, _and it lasts forever_. Furthermore,
failure to get in means certain death. So intricate is the method of
leaf arrangement, so marvelous the adjustments that all plants must make
to insure ample light, that it is not inaptly called _leaf mosaic_. As
we shall see in the chapter on Plant Distribution, particularly in
forests, certain variations or partial failures of the process have
far-reaching results.
If leaves did not perform this most important function to perfection,
all animals, including man as well, would perish, and it would almost
seem that their obligation to us and the plant world might stop there as
long as their success in reaching the light is so overwhelming. But
there are no union hours of labor, no regulation as to the kind of work
leaves may perform, and some actually reach out for new tasks to do, and
do them. In one, our common pitcher plant, the leaf, as is implied by
the name, is formed into a slender hollow pitcher, wide at the mouth,
but narrow at the base. Inside the pitcher are slender downward-pointing
hairs so arranged that an insect may crawl in, but never out. The lower
down the luckless insect gets the more certain is its death, and, to
clinch matters, there is a tiny pool at the bottom where it is not only
drowned, but, due to the composition of the mixture in the pool,
digested. Only a very few plants can do this; only a minute fraction of
the world’s vegetation can digest animal matter. Some experiments on the
pitcher plant, which grows in bogs, show that it will digest bits of
beefsteak dropped into the liquid at the base of the pitcher.
In the East Indies and in Africa there is a pitcher plant--in fact,
scores of varieties of them--which grows up on the branches of trees. In
this case the pitcher may be as long as some of our American kinds,
often twelve to eighteen inches, and many of them are attached to a
slender leafstalk two to three feet long, by which they hang suspended.
Insects, literally by the thousands, are caught in these gaudy traps,
for many of the pitchers are beautifully colored, and near the opening
they secrete a sweetish liquid that lures their prey. They are, in fact,
such curious and handsome plants that they are commonly grown in
greenhouse collections.
Nature sometimes finds still other ways of using strange and
curious-shaped leaves, and in our American bogs is a group of plants,
also insect digesters, still more unusual than the pitcher plants. In
bright sunny places in open bogs one may often find small reddish,
glistening plants, called sundews, usually only a few inches tall,
covered with sticky hairs. In fact, the glistening is due to the
secretion of the sticky substance, a tiny drop of which may be found at
the end of each hair. Flying insects are caught in these leaves, and, as
a fly on fly paper, the greater the struggle the more involved does the
insect become among the sticky threads. Once caught by such a plant,
escape is practically impossible.
Lying in ambush for chance insects, as these sundews and pitcher plants
do, may seem nearly the limit of what is to be expected of leaves.
Merely to be always on the job, with a plentiful supply of insect
digester, might seem to be all that could well be expected from what,
after all, are only modified leaves. But nature’s devices are infinite,
and there are still other ways to accomplish the apparently impossible.
In a small section of the southeastern States there grows a plant that
not only lies passively in wait for insects, but actually captures them.
This flycatcher, known as Venus’s flytrap, has two valves to the leaf
blade, supported on a stout broad stalk so arranged that their fringed
surfaces face each other. If an insect--and many do--alights between
these valves, they close together rapidly and the prisoner is hopelessly
caught by the interlocking marginal bristles that fringe each valve. In
this case there are glands on the face of the valves, against which the
live insect is tightly pressed, and which secrete a digestive fluid.
When nothing remains the valves slowly open and are ready for the next
victim. They may be made to close by slight irritation with a lead
pencil, and it is the impact of the insect that releases one of the most
curious examples of movement in leaves known to us. There are a few
other plants in different parts of the world that by still other
modifications of their leaves catch and digest insects, but none of them
are to be considered as “insect eaters,” or other names implying that
they have definite designs on the life of passing insects. The process
is sufficiently remarkable, the success of the operation so sure, that
there is nothing gained by attributing to such plants, as many have done
in the past, malignant characters that are possibly confined only to
man. The whole wonderful process is more reasonably explained by
realizing that all these insectivorous plants are so by virtue of
necessity, that many of them are bog plants, which are often hard put to
it to get suitable food, and that the extraordinary change of shape and
function is but one more contribution of leaves to the economy of
nature.
In dry or desert regions, where the conservation of moisture is
essential to plant growth, water storage by leaves is nearly as great an
aid to the plant as we have seen it to be in the stems of cactus, South
African spurges, etc. Our common century plant, whose leaves are, in
some kinds, a hundred times thicker than in ordinary foliage leaves, is
a good example of leaves adapted to water storage. In our southwestern
deserts hundreds of species of plants can exist only by virtue of the
fact that their leaves are so changed in their form or structure that
they serve as reservoirs for water storage. This may be accomplished by
thickening, or it is more often contrived by a thick coating of hairs.
The surfaces of thousands of different kinds of leaves are clothed with
hairs either on the upper or lower side, or sometimes on both sides. In
many cases they are quite obviously protection from too rapid drying out
of the leaf. In others, as in the nettle, the hairs secrete a stinging
substance which seems to insure the plant against grazing animals.
Leaves, then, are for something more than to provide the beautiful
foliage which is their most spectacular accomplishment. So varied is
this in its beauty, from plain green leaves to the wonderful coloring
found in begonias, coleus, and many other garden plants, that the sheer
beauty of the panorama of foliage is likely to blind us to the more
important uses of leaves. First of all must we consider them the
factories, in which night and day are produced the food of all plants
and most animals. Then in certain cases we have seen that, by every
ingenious device known to nature, they perform other special work, such
as helping the plant to climb where that is necessary, catching or even
capturing insects and digesting them when that peculiar service is
demanded of them, and, finally, serving as storage reservoirs in regions
where water is scarce. Probably no part of the plant works so
unceasingly each season at its varied tasks. In the autumn, dropping to
the forest floor, its decomposition furnishes still other food for the
plant, and, to crown all, this busy life and by no means unprofitable
death leaves behind it, as a promise for the continuance of the work, a
snugly protected leaf bud which will repeat the process the next season.
THE FLOWER
While the plant’s and, consequently, our debt to the leaf is seen to be
tremendous, it cannot be ignored that, if plants produced nothing but
leaves, the end of all plant life would come with the death from old age
or disease of the present generation of plants. Except for those kinds
that reproduce themselves by division or extension of their rootstocks,
which bear buds, there would be no provision for increase. As only a
comparatively small number of plants can reproduce by this method, it is
obvious that something more must be provided to secure new generations
of plants. Flowers, and the fruits and seeds which inevitably follow
them, do this. All plants, with some exceptions to be noted later,
produce flowers at some time in their life. In the case of the century
plant, only once, after which they die. But except for ferns, mushrooms,
seaweed, yeast, bacteria, and some other forms of so-called flowerless
plants, a flower or blossom is to be found at some stage in the life of
all plants.
If we examine the leaves of a goldenrod, we find that they are large
below and diminish in size toward the top. Just below and among the
flower clusters they are so much reduced in size and often changed in
color that they cease to be ordinary foliage leaves, and are known as
_bracts_. The occurrence of bracts is nearly universal in flowering
plants, and they form not only an apparently transitional stage between
leaves and flowers, but an actual one.
In a complete and perfect flower there are, at the bottom of it, a row
of green leaflike sheaths which
[Illustration: FIGS. 39-45.--THE FLOWER
Fig. 39. A perfect and complete flower. _A_, petals, all of them forming
the corolla; _B_, sepals, all of them forming the calyx; _C_, the
stamen, composed of (_C_) the filament, and (_C_^{1}) the anther, which
produces the pollen; _D_, the pistil, consisting of the swollen base
(_D_) the ovary, a slender shank (_D_^{1}) the style, and the swollen or
branched tip (_D_^{2}) the stigma. (H. D. House, “Wild Flowers of New
York.”) Fig. 40. Typical flower of the pea family. Two petals unite to
form the keel (below), two more unite to form the wings (center), the
remaining and larger petal forms the standard. In most plants of this
family the stamens and pistils are concealed within the keel. Fig. 41.
Two-lipped inequilateral flower, common in such plants as Salvia,
Snapdragon, etc. Note the united calyx and corolla. Fig. 42.
Gamopetalous or united and regular corolla of the Fringed Gentian. Figs.
43, 44, and 45, flowers of the _Compositæ_ or daisy family. Many small
flowers grouped in heads and usually surrounded by one or more series of
bracts. Fig. 43. Flowers all tubular, the small one at the left being an
individual flower. Common examples are Boneset and the common garden
Ageratum. Fig. 44. Flowers both tubular and with rays, the tubular in
the center and the rays on the margin. Below is an individual tubular
flower on the right, and on the left an individual ray flower. Note that
its five united divisions correspond to the five petals in other plants.
Common examples are the daisy, sunflower, black-eyed Susan, etc. Fig.
45. Flowers all ray flowers, an individual one at the right. The
_Compositæ_ with only ray flowers usually have a milky juice and have
often been grouped in a separate family, the _Cichoriaceæ_. Common
examples are dandelion, chicory, and lettuce.]
surround and often half inclose the brightly colored petals within. This
outer covering of flowers is called _calyx_ (Figure 39 B), the
individual parts of it, where they are separated, _sepals_. Their chief
use is to protect the interior petals while they are inclosed in the
bud. The calyx may or may not have bracts just underneath it, as it does
very conspicuously in the case of the flowering dogwood, whose white
“flowers” are really only brightly colored bracts. The transition
between bracts and calyx is not difficult to see in many plants, and
where it is impossible the evidence from their internal structure
confirms what our eye might be inclined to doubt.
Just inside the calyx is what most people call the “flower,” which is
really composed of more highly colored sepals, but which we call
_petals_ (Figure 39A). Where these are joined together the collection,
which forms tubular flowers like the lily of the valley, is called a
_corolla_. It is, of course, the petals or corollas of flowering plants
that give our landscapes their greatest beauty, their most gorgeous
coloring. While this from one point of view amply justifies a prodigal
nature in strewing the earth with beautiful flowers, the true value of
the color to the plant is in quite other directions, which will be
explained a little later.
Toward the base of the corolla, or sometimes on the petals or sepals,
may be found a series of slender appendages, usually threadlike or a
little thicker, crowned at the top by a distinctly large knob. The
individual appendage is known as a _stamen_ (Figure 39 C), its
threadlike portion a _filament_ (Figure 39 C), and the knoblike top an
_anther_ (Figure 39 C^{1}).
Directly in the middle of the flower there is still another organ,
usually swollen at the base, slender in the shank, and either thickened
or branched at the tip. This central part of nearly all flowers is
called collectively a pistil (Figure 39 D), its swollen base an _ovary_
(Figure 39 D), the slender shank the _style_ (Figure 39 D^{1}), and the
thickened or branched tip a _stigma_ (Figure 39 D^{2}). A perfect and
complete flower, then, is composed as follows:
Calyx Corolla Stamens consisting Pistil consisting
or + or + of filaments + of ovary, style,
sepals, petals, and anthers, and stigma.
The stamen is the male organ of reproduction and the pistil the female.
The actual process of fertilization, pregnancy, the forming of the fruit
and later the seed, and the latter’s birth of a new plant, comprise one
of the most fascinating of those provisions of nature which secure the
perpetuation of the plant world. In the life history of even the
commonest weed along the roadside there is this constant renewal of life
by sexual reproduction, just as in animals and in man. In the chapter on
“How Plants Produce Their Young” will be found some account of this
supreme function of flowers, after which, as if their usefulness were
over, they wither and perish.
Not all flowers are perfect or complete. Some lack petals, as the
buckwheat, where the colored calyx replaces petals. Others have neither
calyx nor corolla, as in the sycamore or plane tree. Most plants,
however, have both calyx and corolla. In some very few plants certain of
the flowers have no stamens, when they are said to be _pistillate_ or
female flowers, and certain others have no pistils, when they are called
_staminate_ or male flowers. In other words, the sexes are in different
flowers in the same cluster or plant, as is true of the walnut and
hickories, when they are said to be _monœcious_. In still others the
sexes are on entirely different plants, in which case they are
_diœcious_, as in practically all willows. In the latter case there
are _pistillate_ or female plants and _staminate_ or male plants.
While it is a commonplace that peas do not look like daisies, nor a
carnation like a rose, this simple observation does not begin to tell us
of the wonderfully different flower shapes and colors that are to be
found along any roadside. The perfect and complete flower that we have
been studying is quite regular, composed as often as not of four or five
petals, as many sepals, with five or ten stamens and perhaps a single
pistil. Yet there is literally no limit to the variations from this
scheme, and some of these must be understood here in order that the
life-histories and behavior of plants discussed in later chapters may
tell their full story.
The figures on page 44 show a regular flower, with five separate petals
and sepals (Fig. 39). Such flowers are said to be _polypetalous_, i.e.,
separate petals. Sometimes three of the petals are larger, two smaller,
in which case the flower is lopsided or, as it is said, _inequilateral_.
Again all the petals are united to form a regular and equilateral tube,
as in lily of the valley, when they are _gamopetalous_, _i.e._, united
petals (Figure 42). As we shall see in the chapter on Plant Families,
this is a distinction between two great groups of plants, as important
in their classification as negro and white man are in classifying
humans.
In peas, beans, the locust tree, and related plants the petals are much
changed to form an irregular flower, with a keellike or prow-shaped part
made from the uniting of two petals. Two more unite to form the wings,
and the remaining and larger petal forms the standard. Figure 40 and the
explanation under it illustrate this unusual form of flowers.
Our common garden salvia shows still another type of flower, which is
tubular and irregular (Figure 41). There is an arching, hoodlike
structure at the top overhanging a lower lip. This kind of irregularity
is common in thousands of different sorts of plants and, usually, it is
a device to insure fertilization of the flower by insect visitors. So
necessary are these for pregnancy in many plants, that an orchid, once
discovered in Madagascar with a tube eighteen inches deep, puzzled the
botanists, who were unable to understand how the plant produced seed in
the absence of any known insect with a tongue as long as that. Darwin
said at once that such an insect would one day be discovered on that
island. Years after, Baron von Humboldt, a German naturalist, found the
insect and explained the mystery.
Perhaps there is no feature of plant life that shows such an amazing
amount of variation as the forms of flowers, and while only a few of the
simplest deviations from the normal have been discussed here, it must
not be forgotten that this infinite variety is a reflection of the
ingeniousness of nature in securing a plentiful supply of seed. Form,
color, the secretion of sweetish nectar, the night or day blooming of
different kinds of flowers, every device that will make fertilization
certain, by the flower itself, by insects, or even by the wind, is used
in such prodigal fashion, that we come to see the importance of it to
all plants only by a realization of the complexity of it and the
provisions against its failure.
One apparently most lavish method of securing fertilization is the
arrangement of flowers in clusters. While many flowers are quite
solitary, the great mass of individual plants produce a few or dozens,
or even hundreds of flowers--in fact, certain relatives of the common
carrot may produce over a thousand flowers in a single cluster. The form
and plan of arrangement of these clusters follows a rather definite
scheme, and here, as in the case of leaves and parts of individual
flowers, the figures tell the story better than words. In the common
dandelion and daisy, and their thousands of relatives, the “flower”
(Figures 43-45), as commonly understood, is really composed of scores or
even hundreds of true flowers in each head. In the case of the daisy the
yellow center, if picked apart, is seen to be really made up of scores
of tiny tubular flowers, each just as truly a flower as a single rose.
The rays, or what are incorrectly called “petals,” which fringe the
golden center with white, if carefully separated and examined closely,
will be found to be also a complete flower, the true petals of which are
all joined to make the strap-shaped ray. If one looks sharply, the
united edges of these petals may be seen by the ridges or channels that
represent their joined edges. Because plants of this sort produce two
sets of flowers in each head, one conspicuous by its brightly colored
rays and with another tubular set in the center which makes doubly
certain the fertilization and seed supply, they are considered the most
highly developed of all plants. It is not a close aristocracy, nor an
exclusive one, for over eleven thousand different kinds of plants,
scattered all over the world, have their flowers arranged in this
fashion or some slight modification of it. They possess, above all
others, the certainty that there will be no slip in their fertilization,
pregnancy, and subsequent birth of a new generation. Because this is
the great object of all flowers, and these daisylike plants have brought
it to such perfection, they are most surely to be classed as the highest
type upon the earth to-day.
[Illustration: FIGS. 46-50.--TYPES OF FLOWER CLUSTERS
Fig. 46. A spike, the individual flowers attached directly to the common
stalk. Fig. 47. A raceme, a spikelike cluster where individual flowers
are stalked. Fig. 48. An umbel, the individual flower stalks all arising
from one point. Fig. 49. Individual flower stalks of different lengths
but the cluster usually flat-topped (corymb). Fig. 50. A flower cluster
in which the end of the stem is terminated by a flower from the base of
which side branchlets similarly tipped with flowers arise (cyme).]
While highly irregular flowers are common in nature, conspicuous
examples being the orchids in any florist’s window, or the milkweeds
along the roadside, they can nearly always be seen to have various
changes in the shape of their petals, or sepals, or stamens, or pistils,
which are adaptations to their mode of life, but which always result in
fertilization. Some plants, true monstrosities of nature, are not only
far from having the usual arrangement of flower parts, but they even
produce increased numbers of one part at the expense of others.
Double buttercups, and hundreds of our most beautiful garden blossoms,
have been rescued by cultivation or the arts of the gardeners. Some
roses seem to be practically all petals, but for every increase of
petals there must be a decrease of some other part of the flower, and
more often than enough it is the stamens and pistils that lose out in
this transformation. Just as there is a decrease almost to the vanishing
point in the birthrate when people become too effete and cultivated, so
in plants there seems to be a point beyond which they cannot be pushed
without suffering partial or often complete inability to produce young.
The more highly they have been developed, oftentimes the greater their
beauty, the less able are they to see to it that the chief function of
flowers is accomplished. Such garden plants are increased by root
division, cuttings and other arts of the gardener. Naturally true double
flowers are almost unknown in wild plants, and the habit seems to have
been brought about by too easy a time of it, too little struggle, too
much food, or by any other of those things that produce effete but
beautiful things, charming in their way, but of no significance in the
sturdy struggle for existence that all wild plants must meet or perish.
Another curious modification of a flower bud is cauliflower. Here the
bud has been so developed, its calyx, sepals, etc., so transformed that
the large, cabbagelike head, produced at the apex of the main stem of
the plant, has by so much lost all semblance of a flower that it is
actually a vegetable.
[Illustration: FIG. 51.--FLOWER ARRANGEMENT PECULIAR TO THE ARUM FAMILY
Fig. 51. The outer leaflike tubular or hooded spathe surrounds in our
common Jack-in-the-Pulpit a clublike spadix, upon which are crowded the
tiny flowers.]
No feature of a landscape gives us more pleasure than its flowers, over
which poets have sung and artists have painted their most charming
pictures, even a musician has composed a very beautiful piano piece, “To
a Water Lily.” But their true place in the scheme of nature has a deeper
significance: the wonderful color and symmetry of their parts, the plan
of their arrangement, their transformation into curious forms, like the
Madagascar orchid, and hundreds of others--all these point to their
supreme function, an act of self-sacrifice comparable only to the fall
of a leaf when its task is done. Petals, too, wither and die when the
fertilized ovary, already a mother, begins the slow process of maturing
its young and the end of the flowering stage is reached. Such a climax
is this in certain plants that the whole plant dies, as we have already
noted in the case of the century plant. The toddy or wine palm of India,
often sixty or seventy years old and more than a hundred feet tall,
flowers only once, and, as if in recognition of the fact that it has
done that for which it grew, slowly dies as the seed ripens. More humble
_annuals_, like buckwheat, and hundreds of others, live only one brief
growing season, produce flowers and seeds, and then die, leaving behind
them the only means of perpetuating their kind. The dormant seed carries
over the winter the life they were themselves unable to maintain, as
perennials and woody plants do in their buds.
THE FRUIT
The number of different kinds of fruits that one can buy even in the
greatest markets in the world is so small, compared to all fruits that
are annually produced by plants, that they might almost be likened to an
ear of corn as against a Missouri cornfield. If, as we have seen, all
flowering plants must produce fruits, then what we commonly call such
can be only a fraction of what actually makes up nature’s annual
harvest. It follows that fruits often occur in unfamiliar disguises and,
as we shall see presently, some of the things we have been calling
fruits may be so only partly, if at all.
Disregarding what we call fruits and looking at it from the plant’s
point of view, a fruit is anything in which, or upon which, a seed is
developed or ripened quite without regard as to whether it is edible by
man or not. As the ovary is the female organ of reproduction and
contains the yet undeveloped seed, it follows also that fruits are
practically always a development of some part or modification of the
ovary or the upper end of the flower stalk upon which it rests and from
which it is often scarcely separable.
Familiar enough is the distinction between dry fruits, such as a pea pod
and fleshy ones like oranges, and this quality of being fleshy or dry is
practically universal. Among fleshy fruits a few well-known types may be
mentioned, such as the orange, tomato, grape, gooseberry, and cranberry,
all true _berries_. There are, of course, thousands of less familiar
examples of berries, but, whether with a hard rind as in the orange or
not, they are a direct development, or often a mere swelling of the
ovary, with sometimes the adhering calyx, and contain the seed. In
apples and pears, known as _pomes_, the fleshy part is a development of
part calyx and part the receptacle upon which the ovary is supported
while still in the flower. The ovary in these fruits is the
parchmentlike interior which contains the seed. Plums and cherries,
which have a single stone, instead of numerous seeds buried in the
flesh, are known as drupes. These familiar examples are matched by
thousands of others of which we hear nothing, all _drupes_ and all
formed directly from the ripened ovary and without much change, except
the increase of size, juiciness and large development of the tiny
immature seed, now transformed into a stone. In the watermelon, pumpkin,
and related plants, is still another kind of fleshy fruit, called a
_pepo_. All of this, including the hard rind, is transformed ovary and
calyx completely incorporated, and forming in the pumpkin perhaps the
largest fleshy fruit known. In a considerable number of plants there is
not a single ovary, but several, or in some cases many. These
occasionally all develop into what is called an aggregate fruit, of
which examples are the blackberry, mulberry, magnolia, and many others.
While it would be logical to think that these fleshy fruits were
designed to make delicious food for man, that, in the light of what we
have seen to be the real function of the flower, is an assumption which,
while flattering, is far from the truth. It is much more certain that
fleshy fruits help plants in the dispersal of their seeds and that this
fleshy, juicy character is just one more device of nature to see to it
that not only do plants produce seeds, but that the seeds are carried
and so spread the plant over considerable areas. Birds and animals eat
such fruits in enormous quantities and, in fact, bird migrations are
thought to be not so much response to winter cold as to the fact that
fruits are scarce then. When it is remembered that some birds make
tremendous flights, often over 10,000 miles in a few days, their
capacity to spread seeds through their droppings may be imagined. In the
chapter on plant distribution some truly remarkable cases of such seed
dispersal will be given.
The chance of having seed carried great distances, because it is
embedded in a fleshy, often brightly colored fruit, would seem to put
plants having dry fruits at a disadvantage. Birds and animals cannot be
expected to look after the dispersal of those fruits that are neither
tempting to the sight nor to the taste. And it must be confessed that
quite other qualities in dry fruits insure their dispersal. Some are so
nutritious, like the _acorn_, that thrifty squirrels store them over
the winter, as they do many other seeds which are harvested from dry
fruits. Various grains are often so stored by man, and rice, wheat,
buckwheat, and other cereals are common cases. In nearly all grains the
seed fills so completely the fruit that cereals are very generally, but
mistakenly, called seeds. A grain of wheat or corn is just as complete a
fruit as a watermelon. Only its outer coat and inner seed are so closely
welded together as not to be usually recognized as a fruit, with the
seed inside.
One of the commonest types of dry fruit is the _capsule_ (Figure 53),
well named, as it is almost an exact counterpart of the capsule of the
druggist, in that it is in many cases composed of a lower part and an
upper, usually merely a domed lid. Others again, instead of splitting
around the sides, split from top to bottom. Still others, as peas and
beans, known as _legumes_ (Figure 57), are pods that not only split
lengthwise, but have no central partition, as do many other fruits of
the same general type. When the seed is ripe nearly all pods and
_legumes_ finally split open, and the seed or seeds tumble out. A few,
as in the violet and touch-me-not or jewelweed, apparently realizing
that merely to spill out ripe seeds at the proper time will not spread
the species very far, open their fruits with a sudden explosion and
literally shoot their seeds considerable distances. The artillery plant,
commonly grown in greenhouses, a delicate feathery herb from tropical
America, opens its flowers with a report like a toy popgun and shoots
its small pollen grains for several feet, but not its seeds as stated by
some.
But many fruits do not open at all and seem to be at the greatest
disadvantage in the effort to insure
[Illustration: FIGS. 52-60.--TYPES OF DRY FRUITS
Fig. 52. The strawberry. The fleshy part consists of the modified upper
end of the flower stalk or receptacle, while the true fruits are the dry
achenes on or embedded in the surface and popularly called the seeds.
Fig. 53. A three-celled capsule splitting lengthwise as in the common
Iris. Fig. 54. Fruit of the cocklebur, the hooked prickles of which are
admirably adapted for clinging to the fur of animals. Fig. 55. Pods of a
plant of the Mustard family, which split down both edges, unlike the
true peas, which split down only one edge. Fig. 56. Two types of achenes
of the daisy family tipped with plumed bristles, greatly aiding their
carriage by the wind. Fig. 57. Common garden pea--a typical legume. Note
that it splits only on one side. Fig. 58. The samara or two-winged fruit
of the maple. Fig. 59. The samara or single-winged fruit of the ash.
Fig. 60. The dry two-pronged and bristly fruit of the unicorn plant
(_Martynia_), admirably adapted for dispersal by animals.]
dispersal of their seeds. Greater food value to birds and animals
overcomes this in some kinds, and another help is that some fruits of
this sort are covered with hooked prickles or barbs (Figures 54 and 60).
The common weedy burdock, the barbed fruits of which may often be found
sticking to the fur of animals in great quantities, is a case in point.
There are whole groups of plants that rely on this method for seed
dispersal, notably the avens, tick-seed, tick trefoil, and many shrubs
in the tropical regions.
Where the fruits are neither barbed nor very good to eat, and so
apparently doomed to be more or less permanent stay-at-homes, nature has
provided some of them with the proper equipment for flight through the
air. Winged fruits like the maple are to be seen on any windy day during
their season scurrying before the breeze, and consequently spreading
their kind over considerable distances. In the maple there are two
wings, joined at the base where the seeds are embedded in the wings, and
the fruit is known as a _samara_ (Figure 58), or key fruit, from a
slight resemblance to an old-fashioned key. Ash trees bear fruits that
are a slight modification of this type and may be carried considerable
distances by the wind (Figure 59).
In the dandelion, daisy, and nearly all its thousands of relatives, this
faculty of setting sail in the air has been carried to the greatest
perfection, just as we saw its flowers were. In this family of plants,
the largest in the world, the fruit is mostly tipped or surrounded by a
small collection of very fine bristles. The fruit, known as an _achene_
(Figure 56), is so light that with the added buoyancy of this tiny
collection of down it can be transported great distances. Some have
been known to fly hundreds of miles in severe storms, and, as we shall
see in the chapter on plant distribution, these tiny plant balloons have
played a conspicuous part in spreading their kind over the face of the
earth. Cat-tails also, together with many other plants, have this
faculty and make up by its possession for the lack of fleshy or
otherwise desirable fruits that might be carried. All _achenes_ are not
winged, those which dot the surface of the strawberry being imbedded in
the luscious flesh, which is not really fruit at all. Only the achenes
on the strawberry are true fruits, the fleshy part being merely a
development of the upper part of the flower stalk and not of the ovary
(Figure 52).
Fruits, then, cannot be restricted to the common understanding of them.
They are transformations of the ovary, in which or upon which seeds are
nursed, and upon which most plants depend for the dispersal of their
seeds. We shall see later on how fruits have fulfilled their destiny,
how some are fit for their true function only when they have been eaten
by birds, and when some digestive juice has released them from the
impotence they would suffer without being eaten, how a whole forest has
been changed in the West by the busy activity of squirrels upon the
fruits and seeds of a single kind of fir tree; how the fruit of the
coconut palm has been spread throughout the tropical world because it
can float in the sea securely protected from injury from salt by the
impervious coverings of its fruits.
THE SEED
As the final stage in the development of all plants is their seed, with
the dropping of which they bid good-by to their fellows, it is not
perhaps remarkable that in the seed of all flowering plants is the germ
for the new generation. To seeds which may be as small as the mustard,
so often mentioned in the Bible, or as large as the coco de mer, or
double coconut, from the Seychelles Islands, often fifty pounds or over,
is intrusted by cunning nature the one final and most important act in
the whole kingdom of the plant world. Nearly all plants would die off
forever if seeds did not have in them the germ of life, apparently quite
dead, but actually only dormant. This living germ may persist for years,
sometimes even a hundred years, and yet with the proper conditions it
never fails to sprout.
Seeds have inside them a tiny plantlet folded and ready to grow when the
seed splits to release it. Also, in the seed is stored up food to
sustain the new plant until such time as its own roots begin to act.
This young plantlet is known as the _embryo_, and to this all actions of
the seed are subservient.
As the seed splits, and the young plant develops its first leaves and
rootlets, there is shown one of the most remarkably uniform tendencies
in plant life. In all plants with net-veined leaves the young plantlet
starts life with two leaves, or _cotyledons_, as these first leaves are
called, and this whole group of plants are thus known as _dicotyledons_.
In plants with parallel-veined leaves the young plantlets start out with
a single cotyledon and are therefore called _monocotyledons_. In only
the pines, spruce, and a few other evergreen trees the seedling plants
have several cotyledons and are known as _polycotyledons_. All the
flowering plants in the world belong to one of these groups, so that
merely to see the germinating seed tells the story at once. The linking
of parallel-veined leaves and a single seed leaf, and net-veined leaves
with two seed leaves, is also associated with very definite arrangement
of their flower parts, their method of growth and other characters.
Something has already been said of this in the discussion of stems and
leaves, and more will be found in the chapter on plant families. No more
beautiful example of the plan or scheme of nature is to be found than
these characteristics of all plants, and in seeds we find the first hint
as to which army the plant will join, under which banner it will fight,
and under what generalship it will develop. Nothing tells us so much as
these first seed leaves, pushing their way up through the soil and
revealing, as they burst above ground, to what place in nature their
destiny will consign them.
Flowering plants, which make up the bulk of the vegetation of the earth,
have been discussed in some detail, not only because they furnish us
with all the things that make life possible, but also because they show
perhaps better than anything else the division of labor, all striving
for one end. Roots, the food gatherers. Stems, the framework for the
foliage and its means of reaching the light, or as a storage house for
reserve food. Leaves of many kinds, all factories working night and day
to make the necessary food. Flowers of every hue and shape to lure
insects, or by other means secure union of male and female. Fruits to
ripen the result of this mating of the sexes. And, finally, the seed
carrying with it the yet unborn life. Each part occasionally losing
itself in order that the end may be accomplished, many of them changing
their form or even their function where that is of advantage, all in
their separate ways doing their task, the end of which they cannot see,
and the fruits of which they will never enjoy. Nowhere is it so true as
in plants that to save oneself there must be the capacity to give
oneself. Untold millions of leaves fall, or trees crash down, or seeds
are developed, each fulfilling their destiny which is to insure the
perpetuation of their kind. As we shall see later on, there are many
mistakes, many apparently futile attempts, thousands are wiped out that
one may be saved, and in the past multitudes have gone out forever. Yet
the result of it all is the plant world as we know it to-day, each kind
struggling to increase its sphere of influence, or to cover more of the
earth’s area. The combat between different kinds is inexorable, yet the
capacity for sacrifice on the part of different organs, in order that a
certain individual kind may win, is literally beyond belief.
2. FLOWERLESS PLANTS
In the light of what has been said about flowers it may well be
questioned how anything can be a plant and still have no flower. The
fact is that flowers as we commonly understand them are unknown in the
plants about to be discussed, but that what _corresponds_ to a flower,
and performs the _function_ of a flower all plants must and do have. In
the case of most flowering plants the possession of flowers is one of
the beauties of nature in its most resplendent mood, while in the
so-called flowerless plants the functions of flowers are performed by
tiny microscopic organs, even the existence of which has been only
recently discovered. Because flowering plants produce their sexual
organs in such a gorgeous setting, for all the world to see their
matings they have been called _phanerogams_, which means literally
visible marriage, while the flowerless plants which perform similar
functions in more secret ways are called _cryptogams_, meaning hidden
marriage.
These _cryptogams_ or flowerless plants occur in far greater numbers in
the world than flowering plants, but their size in most cases is very
much less. Many individuals are so small, as in the case of bacteria,
that a single one can only be seen after it has been magnified many
hundreds of times by the microscope. Of the cryptogams some of the
largest, certainly the most beautiful, and probably the best known are
THE FERNS
In nearly all woods one may find delicate feathery plants with graceful,
usually much divided leaves that nearly always start up from the ground
like a slowly opening, but somewhat fuzzy coil. (Figure 62.) Ferns, at
least most of those that grow in America, uncoil their leaves in this
way, almost without exception. The accompanying figure shows the
procedure, and in addition to this character one may hunt in vain for
flowers.
While they bear no flowers we already know that nature could not leave
them with no means of reproduction without abandoning them to a
childless old age and the consequent extinction of the race of ferns. So
far from the truth is this that ferns make up a goodly proportion of the
world’s vegetation, and there are many hundreds of different kinds
known. The lack of flowers, of course, explains why ferns do not bear
seeds which are matured in a fruit or ripened ovary.
On the back of the leaves of most ferns, along or near the edges of the
finer subdivisions, one may
[Illustration: FIGS. 61-63.--COMMON WOODLAND FERN
Fig. 61. A general view. Fig. 62. Its uncoiling spring condition. Fig.
63. The back of one of the smaller divisions of the leaf showing the
collection of spore cases (sori). These are sometimes borne on special
leaves, but in most of our American kinds on the backs of ordinary
foliage leaves.]
find, at the proper season, collections or rows of tiny, usually
brownish dots. These contain often thousands of microscopic objects
known generally as _spores_, and from this fact the dots are called
spore-cases, or more technically _sori_. (Figure 63.) The process by
which new plants are formed is a
[Illustration: AERIAL ROOTS OF FIG TREES HANGING OVER THE EDGE OF A CAVE
IN THE RAIN FOREST, SAN LORENZO, SANTO DOMINGO. (_Photo by the author.
Courtesy of Brooklyn Botanic Garden._)]
[Illustration: VENUS’S FLYTRAP, AN INSECTIVOROUS PLANT OF THE
SOUTHEASTERN UNITED STATES. The fringed valves of its leaves close
together when an insect alights between them. (_Courtesy of Brooklyn
Botanic Garden._)]
somewhat complicated one, but the spores in these brown dots are the
agency which makes reproduction possible, and the actual mechanism of
it, one of the most interesting achievements in plant life, will be
described in the chapter on “How Plants Produce Their Young.” Sometimes
the spores are not borne on the backs of ordinary foliage leaves but on
special leaves that bear, very often, nothing else.
Ferns are much like ordinary flowering plants; except for their lack of
flowers, they have all the root, stem, and leaf characters of their more
showy neighbors. While most of them have compound leaves, even sometimes
twice or thrice compounded, a few have simple, narrow leaves without
teeth, and one kind in tropical America has threadlike leaves. In many
tropical rain forests, so called from their dripping wet condition,
ferns form large trees, and these tree ferns are among the most graceful
and feathery of all plants. There are, too, a few climbing kinds--one,
called the climbing fern, is a native of the eastern United States. Then
there is the walking fern, that seems to upset the statement that plants
do not move as animals do. It sends out delicate runners that, rooting
at the tips, form new plants, often several feet from the parent plant.
The characteristic of having, even in the simplest form, stems, leaves,
and roots, with all that this implies in their internal structure, marks
them off at once from all other flowerless plants. In ferns there is
always some internal equipment for carrying food from one part of the
plant, the roots, to another, and this ability is possessed by virtue of
ducts or vessels through the stem and leaves. This system, found in all
flowering plants and ferns, but _nowhere else in the plant world_, is
called the _vascular system_, or literally, a vessel system. We shall
see how important was the acquirement of this system of vessels, when we
get to the chapter on the History of the Plant Kingdom. Its appearance
upon the earth marks as important a stage in the development of plants
as the dawn of a definite backbone did upon animal life.
Ferns, then, are _vascular cryptogams_ because they do have conducting
vessels in their stems, and they produce their young by a process of
hidden marriage which will be described later. All other cryptogams or
flowerless plants are without this system of vessels and are called
therefore _non-vascular cryptogams_. Numerically they are tremendously
important; upon them depend many manufacturing processes like bread
making, brewing, and all arts using fermentation. But they are hardly
recognized as plants by the general reader, and because of their size
and the necessity of studying them with a microscope in order to
understand their structure they will be treated here only briefly.
OTHER FLOWERLESS PLANTS
The remaining flowerless plants, having no duct system in their make-up,
are, as we know, called _non-vascular cryptogams_. This is a general
term for a very large group of plants, some quite obvious and well known
like a mushroom, for instance; others so small or of such uncertain
structure that they are not even well known by experts. This great mass
of plant life, more numerous than all the other kinds of plants
combined, contains many different forms, some of which are of gigantic
size. A single plant of a certain Pacific Coast seaweed regularly
exceeds in length the height of the tallest
[Illustration: FIGS. 64-67.--TYPES OF FLOWERLESS PLANTS
Fig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi,
which include also puffballs, molds, and many disease-causing
microscopic organisms. Fig. 66. A common seaweed, a representative of
the algæ, which include the green scum on the top of ponds, and the kelp
from which fertilizer is now being made. Fig. 67. A lichen, a common
cryptogamous plant on logs and rocks. Our native kinds are usually
grayish-green in color.]
known trees. And yet other inhabitants of the water, certain kinds that
float freely, are microscopic in size. The latter occur in such enormous
numbers that their tiny decomposed skeletons after dropping to the
bottom of the sea form the diatomaceous earth, so much used in polishing
machinery. The commercial product now comes from deposits of these
skeletons laid down in past ages, which, due to changes in the land and
water surfaces of the earth, are now found in Virginia, Nevada,
California, and in Bohemia. All these must have been in the bed of
waters long since gone, which teemed with these microscopic organisms.
To-day there are over ten thousand different kinds known, yet so small
are they that their dimensions are measured in thousandths of an inch!
Somewhat lower in the scale of life--and by this we mean simpler in
structure--than the ferns are the _mosses_. (Figure 64.) There are
thousands of different kinds, but everyone is familiar with the
collective growth of the commoner sorts which makes the velvety mossy
carpet in our woods. The individual plants are small, but in many kinds
sufficiently large to be seen without a microscope. Most important of
all, practically every one of them has the ordinary green color of the
better known plants, and as we shall see in the section devoted to
“Leaves as Factories for the Making of Food,” that stamps them at once
as plants, if other things did not.
Mosses are almost infinite in their habits, some growing on the dry
rocks or trunks of trees, many growing in moist woods, some in the
water, and immense quantities of certain kinds in bogs. The peculiar bog
mosses, known as sphagnum, play an important part in forming peat and
perhaps coal. While mosses are otherwise not of much commercial
importance, they are among nature’s most beautiful ground covers,
carpeting many a nook and dell with a soft, velvety, almost cushionlike
growth.
Although they are rather small, they appear to have a somewhat definite
stem and tiny leaflike appendages of it, without, however, having the
vascular system found in all ferns. Mosses might almost be considered
miniature ferns, of which they are perhaps only simple ancestors. Their
vegetative or green parts vary much in shape, size, and the arrangement
of the tiny leaflike appendages, and while most of them are a beautiful
bright green, nearly all the bog or sphagnum mosses are rather ashy gray
in color. In most of the typical mosses there arises from among the
vegetative growth of them a slender stalk, at the top of which is a
small capsulelike organ. This contains the spores, and it is upon this
long slender stalk and its spore-filled capsule, really marvelous in its
internal structure and mechanism for the discharge of the spores, that
mosses depend for their reproduction. As in the case of the ferns this
process will be considered later, along with that of some other plants.
This whole story of how plants produce their young, perhaps the most
fascinating of any part of the study of plant life, is so fundamentally
a part of their history and shows nature in her most maternal moods,
that a special chapter will be devoted to it. There we shall see, as a
whole, how these vastly different acts of fertilization and reproduction
are, in different groups of plants, all responses to that insistent
command for life, more life, in a never-ending stream.
The chief characters to remember about mosses are that they are very
simple, but practically always green plants that have some
differentiation into stem and leaf; that, while they have no vascular
system, their structure and particularly the mode of reproduction
suggests that they are not very distant from the ferns, and quite likely
simple ancestors of them. These characters are of more importance than
appears on the surface, as we shall presently see, for they mark mosses
off from many other nonvascular flowerless plants which have quite
different structure and altogether different mode of life.
If you will turn to the chapter on Plant Behavior and read particularly
the sections on “Leaves as Factories for the Manufacture of Food” and
“Borrowing from the Living and Robbing from the Dead,” you will see in
the food habits of the plants there noted the great difference that
exists between plants, like mosses and ferns, that have green coloring
matter in them, and those we are about to mention that never do. The
lack of this green coloring substance tells us at once that plants of
this sort live only on the dead remains of other plants. In the case of
these nonvascular flowerless plants there are certain modes of growth
that, in some forms at least, are always associated with this
scavenger-like food habit.
The common mushroom (Figure 65) is the best known of that large group of
plants, called generally fungi, which produce no green coloring matter,
have no leaves attached to a stem, and _always_ live on decayed
vegetable, or sometimes inhabit living animals, even man himself. The
mushroom with its brownish stalk and buttonlike dome is familiar enough,
but there are literally thousands of different kinds, a common sort
forming “brackets” on the trunks of trees. While perhaps everyone would
recognize these as plants, peculiar as they are in their often weird
shapes and unusual as they nearly always are in their color, there are
many minute kinds of fungi that scarcely anyone would even think of as a
plant, and yet for better or worse they are incomparably the most
powerful plants in the world. For upon these microscopic fungi man
depends for many things. It is certain kinds of them that make the
manufacture of cheese possible. They turn milk sour (pasteurizing milk
is merely stopping their work), give to yeast its power of “raising”
bread, all brewing depends upon them, every process of fermenting the
juice of fruits for wine making or for whatever else, the decay of
wood--all these processes and scores of others, whether for the good or
evil of mankind, depend upon the work of these plants, any one of which
is so small that a single individual must be magnified hundreds of times
to detect it. Many of them are the “germs”--better called bacteria--that
cause diseases like tuberculosis, cholera, typhoid, anthrax, and
diphtheria. All surgeons wage incessant warfare against a host of them
that attack wounds and form pus. They live in our intestines and have
much to do with digestion, and unhappily with indigestion, so that we
may be said to carry about with us a whole flora of them! Nearly all the
diseases of plants, like the blight of potato and the rust on wheat, are
caused by them. Some other kinds live in the soil, and many flowering
plants depend absolutely for getting their food upon the work of these
fungi. Unfortunately their minute size and consequently obscure mode of
life demand technical skill and the use of the microscope to detect
them, so we must leave them here, always keeping in mind that these
smallest of all plants are charged with a power for good or evil; so far
as man’s life is concerned, greater perhaps than all other plants.
While most fungi, particularly those familiar ones like mushrooms and
puffballs, are inhabitants of the land, the remaining group of
nonvascular flowerless plants are nearly all water plants. Most of the
better known ones live in the sea, and as the wrack or tangle washed up
on the shore we recognize them as seaweed. The _algæ_ (Figure 66), which
is a general name for such plants--and they live in the sea, in fresh
water, and even on dry land--are, so far as structure is concerned, the
simplest of all plants.
Those that are fastened to rocks are often beautifully colored, much
branched, and many kinds bear small bladders that act as buoys. These
coast seaweeds are generally of different colors, those nearest the
surface being generally greenish, the deeper water kinds reddish or
brown. None of these seaweeds are found at great depths, because the
really deep parts of the ocean are almost, if not quite, dark. Seaweeds,
and in fact all the algæ, have green coloring matter in them, even where
this is masked by reds and browns, as is the case in some particularly
showy kinds. As you will find in the section on “Leaves as Factories for
the Making of Food,” no plant with green coloring matter can live in the
dark. That is why seaweeds are not found in the great deeps of the sea,
some of which are several miles below the shore line along the coasts,
and are so cold and dark that neither plants nor animals can grow in
them.
Those seaweeds that grow along the coast, and are uncovered by the
retreating tides, are well known by everyone, but by far the greater
number of algæ float without anchorage of any kind. One kind that has
been torn from its anchorage occurs in such enormous quantities that off
the coast of America it has formed literally a floating island composed
entirely of dense mats of a species of seaweed. This place, known as
the “Sargasso Sea” from the name of the seaweed forming it, was the
terror of old mariners and Columbus’s ship was fouled in it for two
weeks. The area occupied by the weed is several hundred miles long and
wide, and while old sea yarns about ships being caught in it and never
escaping are gross exaggerations, it is certainly one of the most
curious of plant growths, due entirely to a nonvascular cryptogam.
Of those kinds that are never anchored the number is legion, and in
addition to those forming the diatomaceous earth, already mentioned,
there are many more. They form almost the only food of hosts of
creatures of the sea, but because of their floating freely in the water,
the consequent difficulty of collecting them, and their unusually minute
size, little is likely to be known of them, except by the experts.
Other algæ are always found in fresh water and form the scum found on
stagnant pools. Individuals of any of these are so minute that, while
under the microscope they are of the greatest beauty, their structure
must remain for most of us a sealed book.
SUMMARY OF WHAT PLANTS ARE
We have now traced, in only the briefest fashion, the outlines of what
plants are, reversing the order of nature in beginning with those most
complex but best known, the flowering plants. As we shall see later,
these are the climax of prodigal nature and are to be considered the end
rather than the beginning of plant life on the earth. Then, and still
more briefly, have we stopped to see those less known plants that
produce no flowers, such as the ferns, mosses, fungi, and finally the
seaweeds or algæ. These are all to be considered as the ancestors of
flowering plants, the ferns the nearest to them and the algæ probably
the most distant relatives. The development of plants from the minutest
alga up to our most gorgeous flowering plant, is an infinitely slow and
painful process. With many mistakes, with its pathway strewn with the
wreckage of forlorn hopes and false starts, it is incomparably the most
dramatic story in the plant world. Some of its details will be told in
the chapter on the “History of the Plant Kingdom.”
Nor can we leave the discussion of what plants are without some mention
of the thing that really makes up their structure, whether it be a
microscopic bacterial organism or the Big Tree of California. For the
unit of all animal and plant life is the _cell_. In its simplest form it
is merely a minute sac with a definite wall and inside the wall is a
substance known as _protoplasm_, literally _protos_, first, and
_plasma_, thing formed. It is protoplasm that forms the living tissue of
all plants and animals; it is life itself. No one has ever succeeded in
making any, notwithstanding that many learned men have tried for years.
Its inclosure in the cell wall, its power of self-division and
consequent multiplication of the units, make up those first things about
which most of us can never know much, but the end of which we recognize
in the beauty of plant life all about us. For only under the highest
powers of the microscope may cells be seen and studied. Just as bankers
reckon mills as a definite unit of a cent, and yet none of them has ever
seen a mill, so we must think of cells as the definite unit of all
living things, although most of us will never see a cell. But, unlike
the mill, cells may be seen by those equipped to see them, and this
study, the development and grouping of them to form all the varied
objects that inhabit the plant world, is known as _histology_. It is
literally the internal history of plants and animals, and lies quite
outside the scope of this book. What we must never forget is that
whatever knowledge we have gained, either from the foregoing account of
what plants are, or from our observation of them, is, after all, only a
partial notion of them, as unsatisfactory as our estimate of what people
really are, from merely looking at the outside of the houses in which
they live. The outer form we may know and admire, the inner substance
must ever remain for most of us a secret treasure house the value of
which is certain, but the key to which we do not possess.
CHAPTER II
PLANT BEHAVIOR
1. LIGHT AND ITS IMPORTANCE TO THE PLANT
Practically all that has been said in the first chapter relates to what
plants are, their organs, or what we may call the architecture or plan
of their framework. But what they do with this elaborate structure is as
important as what we do with a house that may contain every modern
improvement but is never a home until these things have been put to use.
One of the chief concerns of any architect is to see to it that the
house has as much sunshine by day and as attractive illumination by
night as possible. Nature, that greatest of all architects, also sees to
it that plants get the utmost necessary sunlight, but for a much more
important reason than the mere attractiveness of sunshine, be that ever
so beautiful. For light, the life-giver of all green things, is so
absolutely essential to plants that experiments to grow them in the dark
have always failed, and many gardeners now use electric light in
greenhouses in order to prolong the short daylight of winter. It is the
lack of light that makes celery blanch.
Plants grown in the house inevitably turn toward the windows, even
plants growing against a wall turn their leaves away from it--nowhere
can one find living green things that do not find the light as surely
and persistently as men try to get their food or their mates. Many
examples of this could be given and must have been noticed by everyone.
Sometimes seeds germinate under a barn floor for instance, and the puny
pale little plantling reaches out slender stems, all of which turn, as a
compass turns to the north, to perhaps a crack of light in one corner of
the building. We have already seen how the search for light will carry
the slender rattan palms of India hundreds of feet to the topmost leaves
of the forest. Individual plants, and, as we shall see later, whole
forests make desperate efforts to get to the light. We know already,
that the struggle for light is just as bitter as the struggle for food
by roots. And finally if, as we have many times proved by experiments,
plants die when grown in a dark room, what is it that light does for
plants and how is a process carried on that everything leads us to think
is of the greatest possible importance? Quite obviously it is not the
mere beauty of sunshine dancing upon the landscape, as entrancing a
picture as that may be any summer afternoon, with the play of sunshine
and shadow on the tracery of foliage. That green color of the foliage,
the almost universal green of so much of the earth’s vegetation, restful
to tired eyes, providing us with the most pleasant shade, has wrapped up
within it the secret of just what sunshine does for plants. For under
the magic of light acting upon this greenery one of the most important
industries in the world, the manufacture of food, is constantly going
on.
LEAVES AS FACTORIES FOR THE MAKING OF FOOD
It must be clear enough from the start that to call a leaf a factory for
the making of food forces us to decide at once whether this is a mere
way of speaking, or whether, incredible as it may seem, anything as
thin as a leaf _can_ really produce food. As we eat lettuce, and
millions of cattle graze every day, leaves as food producers win handily
on that score. But to understand how food is produced in such a tiny
factory demands that we walk about in it for a bit, study the inside of
it and especially its many small chambers within which is not only the
machinery, but some of the finished product stored up for later use.
Unlike modern factories there are many entrances, from any one of which
we can begin our tour of inspection. On the under side of nearly all
leaves and on the upper side of some there are scores or even hundreds
of small pores called _stoma_, so small that only with a microscope can
they be seen. These entrances through the factory wall, are carefully
guarded by a pair of watchmen whose business it is to see neither too
much dry air gets in nor too much of the product of the factory gets
out. They see to it, also, that waste products are thrown out at the
proper time. These watchmen, or _guard cells_, as they are called, are
constantly on the job, work almost automatically, but their chief
function is connected with the proper ventilation of the place, and will
be discussed later under “How Plants Breathe.”
Once past the entrance it is obvious that we are in one of the strangest
of all factories, for none of the rooms are truly square or oblong and
their irregularity as to outline would drive your average foreman into
profanity. Yet they are certainly divided into distinct classes, at
least as to size and as to what the rooms contain. Some are apparently
filled with nothing but air and have direct connection through the stoma
with the outdoors. These are called _intercellular spaces_. Others, and
these are most important, are filled mostly with the green coloring
matter that gives the leaf its color. This substance is known as
_chlorophyll_, its individual units as _chloroplasts_, or literally,
chlorophyll bodies. Quite independently of these chlorophyll cells or
rooms, or the intercellular spaces which correspond to halls, there are
some large and many small tubes. These are the veins of the leaf and
their finer branches and by their direct connection through the stem to
the roots, serve as the ducts through which some of the raw materials
are brought into the factory.
This green coloring matter or chlorophyll is perhaps the most important
substance in nature. Without it all except a very few plants would die,
and even in those beautifully colored leaves like coleus or caladium
chlorophyll is always found, but in these colored leaves it is merely
obscured by other coloring substances. It is in the chlorophyll that the
ability resides to take the inorganic substances through the roots or
from the air, and by the aid of sunlight transform them into organic
substances like starch and sugar. Nothing else in all nature can do it;
without this faculty, which the commonest green leaf possesses, the
earth would prove uninhabitable within a single year. Just what
chlorophyll is chemically is not yet thoroughly known, but the thing of
chief interest is that it is hardly ever found in parts of the leaf not
exposed directly to the sunlight, and that during the autumnal coloring
and before the fall of the leaf chlorophyll is carried to other parts of
the plant, and quite possibly stored for use the following season.
While the composition of chlorophyll is not surely known, iron is
certainly one of its constituents, as plants deprived of iron lose
their green color. It also is known to contain oxygen, carbon, hydrogen,
and nitrogen, but merely to catalog what we know about its make-up does
not tell us that it is a living green substance and that sunshine sets
it in motion. Just exactly how light acts on chlorophyll no one really
knows; we merely know that it does so act and that the result is one of
the marvelous secret processes of nature, perhaps like the secret of
life itself forever hidden from man. In our tiny factory, then, we have
raw products coming from the roots and through the stoma from the air;
machinery of the most efficient type, for chlorophyll works night and
day, and constantly renews itself while producing the finished products;
energy from the sun; and finally the complete manufactured products
which are foods in the shape of starch and sugar. During the growing
season there is no banking of the fires, no stoppage of this most
important of all industries, no strikes or lockouts. Each part of the
whole works smoothly and with the nicest precision--in fact so perfectly
does this process keep on going, so complete is the orderliness of the
place, and so regular are the completed products turned out, that no
modern factory manager or workman but can learn something from a rather
close study of this smallest but most efficient factory in the world.
Some of the raw products are delivered to the leaf from the roots where
they have been absorbed by another process that will be considered a
little later. These consist of water and the inorganic substances
dissolved in it, popularly called sap. Carbon and oxygen come mostly
from the air, sometimes separately, more often in the form of a
combination called carbon dioxide which is one of the chief
constituents of the gas thrown off by man as he breathes _out_. Now
these inorganic substances, contained in the sap or derived from the
air, are literally mixed by the chlorophyll and form, always with the
aid of sunlight, substances known as _carbohydrates_, the commonest
example of which is sugar. Some form of sugar is one of the earliest
results of this process, but sugar is quite easily dissolved in the sap
which has contributed to its manufacture, and the excess sugar is thus
removed. Otherwise it would clog the machinery and prevent the
production of fresh supplies. This first step in the manufacturing
process has not inaptly been called _photosynthesis_, the meaning of
which _photos_, light, and _synthesis_, combining by means of, suggests
in a word the necessity of light and the combination of the inorganic
substances mentioned above. Of course this process of photosynthesis is
not as simple as the brief account of it suggests, for it is actually a
complicated chemical process only part of which is yet understood. It is
fairly certain that it goes step by step; it is quite certain that the
beginning is inorganic and the end organic compounds like sugar.
Something is known also of the wear and tear on the chlorophyll, its
waste products, and how it keeps itself not only fit but provides for
its own constant renewal. One of the excess or by-products in this
initial manufacture of sugar is oxygen. This is either used in other
ways by the plant, or more generally it is thrown off through the stoma
into the outer air. Oxygen, as one of the necessary constituents of the
air that man breathes _in_, is thus thrown off, while, as we have seen,
carbon dioxide, a poisonous gas which we breathe _out_, is a necessity
for this manufacturing process in all green plants. Hardly any trick of
nature so completely fulfills the wants of animal and plant life as this
mutual exchange of by-products--in the case of animals it is the waste
of respiration, in plants it is the wastage of sugar making and some
other changes that go on in the plant just after this stage.
The amount of sugar made, carbon dioxide taken in, and oxygen given off
by this process suggests that while leaves may be very tiny factories
they are among the most efficient in the world. Assuming an area of leaf
surface equal to about a square yard the amount of sugar made would be
about one-third of an ounce in a day or nearly three pounds in a single
growing season. Carbon dioxide withdrawn from the air would average from
the same area of leaf surface about two gallons a day or over three
hundred gallons for the season. As an equal amount of oxygen is given
off by the leaf, it becomes clear that as all of this interchange must
go through the stoma the functioning of these and their guardians must
be nearly one hundred per cent perfect. As we shall see a little later,
they perform still other duties with even greater perfection. When we
stop to reflect what an absurdly minute fraction one square yard of leaf
surface is to the total leaf surface in the world, we come to some
realization of the gigantic proportions of this process of manufacturing
sugar and exchange of gases mutually useful to animals and plants. While
in the United States most of the leaves fall in the autumn, the great
bulk of the vegetation of the world holds the greater part of its leaves
all the year, notably in the vast evergreen forests in the north, and of
course practically all tropical vegetation. Chlorophyll in such places
works continually and what the total of sugar production may be no man
can even guess.
Sugar, although the first step in the process, is not the final one, and
the leaf has still other tasks to complete. Some of the sugar is used up
in the process of renewing the chlorophyll, some of it is moved to other
parts of the plant where in sugar cane it forms the world’s chief sugar
supply; but the remainder is transformed into starch, a substance that
is not dissolved by the water of the sap, and is therefore capable of
permanent storage either in the leaf itself or in other parts of the
plant, notably in the tubers of the potato, the solid part of which is
nearly all starch. The conversion of sugar to starch, which is really a
means of contriving to properly store the product of the factory, is
done by certain ferments known as enzymes. Just what enzymes are or even
how they work is not well known, but apparently they have the faculty of
converting certain substances like sugar, and in the process they
neither use up nor materially change their own composition. It is
certain that the conversion of sugar to starch is an elaborate chemical
process, but it is accomplished by these enzymes, the very existence of
which has only recently been discovered. Enzymes not only do this, but
they convert starch which is insoluble into a kind that may be dissolved
and thus carried to different parts of the plant. Upon this power
depends the storage of starch in roots, tubers, seeds, or wherever else
it is found in the plant, and it is of course upon this power man
depends for the food supply of the world. Wheat or corn, potatoes, rice,
all the foods that are rich in starch produce none in that part of the
plant harvested by man. All of it has come by the process which is only
sketched in its briefest outlines in the foregoing paragraphs. All of it
must come from that green coloring matter of nearly all plants which,
while mostly confined to leaves, is not always so. And wherever
chlorophyll is found this process goes on even in the simplest plants.
Because it is so overwhelmingly a characteristic of leaves and, as we
have seen, leaves are the one organ of the plant upon which man pins his
only hope of future food supply, the leaves of all plants may be truly
likened to a factory the work of which is never ending, the product of
which the leaf will never use, but the result of which has far-reaching
consequences to us all.
EFFECT OF LIGHT AND DARKNESS ON INDIVIDUAL PLANTS AND VEGETATION AS A
WHOLE
Now that we understand the importance of light to all except a very few
plants, and its very close relationship to the green coloring matter of
all leaves, many things about the arrangement and position of leaves,
and indeed of the whole plant, may be understood, which, without this
knowledge, seems the result of mere caprice or chance. It would seem as
though the habit of plants growing toward the light, and against the
pull of gravity, a character almost universal, no matter from what
mountain declivity or rocky cliff it may spring, might be the result of
the “pull” exerted by light on the green coloring matter in the leaves.
While light does aid in plants having a generally erect habit it is not
the cause of it, as we have many times proved by experiments. As a seed
sprouts and the roots go down into the earth, the shoot, before it has
broken through the surface and while _still in the dark_, always grows
upward. This property of growing in two opposite directions at the same
time, the roots always with gravity and the shoot nearly always against
it, is known as _geotropism_. In the case of vines or other trailing
plants there is the same tendency exhibited, even though the plant is
not erect. We must think of geotropism as a growth habit of all plants,
not caused by light, for it has been shown to act in the dark, but of
the greatest advantage to all plants in their initial start toward the
light. If this were not the case, it may be imagined into what chaos the
vegetable world would be thrown. We are so accustomed to roots going
down and shoots going up that we are not apt to think of it as the
result of two antagonistic growth habits, the true cause of which is not
understood, the result of which is common knowledge. Geotropism is one
of those mysteries with which the book of nature is crowded, and merely
to describe it and realize its force is by no means to arrive at its
true inwardness.
But, quite independently of this peculiar growth habit, the stems and
often whole plants do show response to light and many times the
response, in its effects, cannot be distinguished from geotropism.
Perhaps the most homely illustration of this is the common house
geranium which, no matter how often it is turned, always grows toward
the window, and if not turned at all becomes hopelessly lopsided, with
the leaves all bending sharply toward the light. Trees growing on a
cliffside, while always growing upward, nearly always may be seen
bending away from the cliff where light is scarce and toward the
unobstructed light. The position of hundreds of twigs and branches on
any tree have been dictated by their exposure to light, and the habit of
practically all trees in the forest of being clear of branches for many
feet from the ground is another illustration of the profound effect of
light. In the latter case the taller the trees the farther from the
ground are the first branches, and in the big trees of California the
first branches are frequently over a hundred feet from the ground. In
their young stages all these trees were furnished with branches, the
leaves of which in their day performed their appointed tasks. But in the
strife and hurry of the crowns of the forest to overreach their
neighbors these lower branches, from the bottom upward, gradually die
off. So inexorable is the plant’s demand for light, that these lower
branches, in spite of being nearest the source of their food from the
roots, are doomed to be killed. Nature plays no favorites and these
lower branches, once the pride and support of the young tree, are
ruthlessly dropped off when they can no longer play the game. This
wholesale slaughter of lower branches in a forest, more complete than
any pruning by man could ever be, gives us, if the story of the factory
leaf has not already done so, some conception of the part played by
light in the plant world.
The shade of certain trees is so much denser than others that they have
been planted for this purpose, notably the horse-chestnut and Norway
maple. Foresters have long recognized this difference in trees and it
would be strange if nature had not taken advantage of it also. If
certain trees can still maintain themselves in the forest without
producing a dense crowd of leaves, such as the silver maple for
instance, they would have a decided advantage over a tree like the sugar
maple which casts a much denser shade. A walk through any forest will
show scores of examples of trees that live and produce seeds by virtue
of the fact, not that they demand all available light, as their more
vigorous neighbors do, but that by a compromise, by an almost diabolical
cunning, their light demands, and of course their leaf exposure, have
been cut down to a point where the tree can grow in a place impossible
for trees that lack this ability. It is, of course, not a trick which
any individual tree can perform at will. Rather is it a characteristic
found in all individuals of certain kinds, where the comparative
disadvantage of making less food and having less leaf exposure is more
than overcome by the enormous advantage of being able to fight their way
into a forest that would otherwise be impossible for them. We shall see,
in the chapter on Plant Distribution, how this peculiar response to
light has had effects of considerable significance upon forests,
particularly after forest fires, lumbering, or other disturbance of the
natural conditions. Trees in the forest, and the shrubs and herbs under
them are not the quiet stately things about which the poets are so fond
of singing. They are places, on the contrary, of intense warfare, and
perhaps some of the greatest casualties occur in the battle for light.
Leaves, as being the most directly involved in the matter of utmost
exposure to light, show the greatest amount of response to it, by their
shape sometimes, by their position nearly always, and very often by the
character of their leafstalks. In many herbs the first young leaves are
relatively short-stalked, while as the plant grows upward the lower
leaves are progressively longer stalked, which is a direct response to
the fact that the upper leaves take their full share of light, leaving
little or nothing for the lower ones. To avoid complete shading their
leafstalks are often many times the length of their more fortunately
placed neighbors above them. In those plants like the garden primrose or
common weedy plantain, which bear all their leaves in a close cluster or
rosette at the level of the ground, we see an almost fiendish cleverness
in their earlier and later habits of growth. When the leaves first
start, as they nearly always do among grasslike vegetation in which
these plants usually have to fight for a chance of life, the leaves grow
straight up, so that they may get above the level of the surrounding
grass. Once there, and the precious light an assured fact, they
gradually flatten out their leaves to form a rosette, of course cutting
off the light from the grass about them and killing it just as certainly
as though it were pulled up by the roots. Hundreds of different kinds of
plants do this, apparently with the utmost cruelty to their inoffensive
neighbors, with whom they start upon nearly equal terms in the race for
life. If they began at once to spread out their rosette while it was
still in its small spring state, the upward pointing grasses would
smother it, and as if in anticipation of this the leaves grow up with
the grass, only to flatten out when the proper time comes for them to
show their true colors.
Light not only affects leaves in their habits of growth but it actually
causes movements in some leaves which are as regular as clockwork. The
best known cases are those in the pea family and wood sorrels, all of
which bear compound leaves. During the day these leaflets are spread out
in the ordinary way and catch the light, but at sundown, as though this
were a quite useless exertion for the night, they fold up and the leaf
“goes to sleep.” On cloudy days they partly fold up, as if in
recognition of the fact that for their business of getting light it is
an off day; but also if the sun comes out they hurriedly expand their
leaflets. It is not yet certain whether these apparently intelligent
movements of leaves in relation to light are of any real advantage to
the plant as a whole or not. They are surely one of the most interesting
things to watch and may be seen in locust trees and wood sorrel any
night.
Just as we can have too much of a good thing, it is possible for plants
to have too much direct sunlight. In open spaces, where the struggle for
life centers not about the fight for light but over other matters, we
find leaves actually protecting themselves against too much exposure,
and by a variety of ingenious ways. The texture of the upper or lower
side, the kind of hair growing on their surface, and the number and size
of their pores, are the most usual ways of leaves arming themselves
against an oversupply of the one thing that their neighbors in the cool
forest fight to the death to obtain. There seems to be a fatality
against which plants, like ourselves, are nearly helpless. Their
attempts to overcome it, again like our own struggles against an
apparently overmastering fate, develop those characteristics that insure
survival to the fittest, death to the puny or unaccommodating.
We could hardly leave the subject of light and plants’ relation to it
without mentioning, perhaps, the most remarkable case of adaptation to
peculiar light conditions. All those aquatic plants that grow beneath
the surface of the water need and get much less light than ordinary land
plants. But from the island of Madagascar comes the lace leaf or water
yam, which grows in quiet pools that are mostly in the depths of the
tropical forest. Add to the dense shade cast upon the gloomy surface of
such ponds the amount of light naturally lost in its passage through the
water, and we get some notion of the singularly secluded home of this
aquatic plant. What, now, is nature’s response to these peculiar
conditions? How do the leaves of this well-shaded inhabitant of quiet
pools behave? Their leaves are about a foot long and three or four
inches wide, quite unnecessarily large for a submersed aquatic, but they
consist _wholly of veins_. There is no “meat” to the leaf, none of that
soft, green tissue so familiar in ordinary leaves. The conditions under
which it is doomed to live almost seem as if it recognized the futility
of having a broad expanse of the usually constituted leaf blade to
expose to a light which is not there. It is significant that this
skeletonized condition is permanent, the leaf functions much as ordinary
aquatic leaves do, but its network of quite naked veins almost seems a
mute protest against its fate. The delicate, lacelike “foliage” of this
aquatic adds a touch of beauty to one of the most curious plants in the
world.
2. HOW PLANTS GET THEIR FOOD AND WATER FROM THE EARTH
If we could stretch an apparently impervious membrane, like the inner
white skin just inside an eggshell, or a piece of parchment, and so form
a wall through the middle of a glass box, and then pour into one of the
compartments pure water and in the other a mixture of water and
molasses, a very curious result would follow within a comparatively
short period. We should find that presently there would be a gentle
filtering of the water through the membrane toward the molasses water,
and similar gentle current in the other direction. In other words,
fluids of different density, if separated by a membrane, tend to
equalize each other. This equalization may not be very rapid, and at
first it will be more speedy from the less dense to the more dense, but
eventually it will make the different fluids of a common density. This
purely mechanical property of the equalization of fluids separated by a
membrane is known as _osmosis_, and it is upon the possession of the
equipment necessary for this that roots depend for getting food and
water from the soil.
In our discussion of roots in Chapter I, we found that they end in very
fine subdivisions, which are themselves split up into practically
invisible root hairs. These root hairs are the only way that roots can
absorb the food and water in the soil, and they are able to do this
because they are provided with a membrane which permits osmosis to act
between the solution inside the root hair and the water in the soil. The
solution in the root hair is mostly a sugary liquid, some of that
surplus sugar made in the leaves, and it is denser than the soil water,
so there is apparently nothing to prevent an equalization of the liquids
on different sides of the membrane. If this actually happened, as it
would in the case of the simple experiment noted above, then roots would
exchange a fairly rich sugary liquid for a much more watery one, and we
should find that plants did not get their food from the soil, but really
have it drained away from them by osmosis. But nature has a cunning
device for stopping such robbery, which is prevented by the membranes of
root hairs being only permeable to the extent of letting water _in_, not
permeable enough to allow sugar to escape. As we have seen, osmosis is
a purely mechanical process which, if left to operate without
interference, would not aid but injure the plant. Surely, nothing with
which plants are provided is so important to them as this delicate
membrane of the root hairs which, while allowing osmosis to act in a
one-sided fashion, preserves to the plant the sugary liquid that alone
makes the absorption of soil water possible.
As root hairs are very much alive and work constantly, they must be
provided with air, without which no living thing can exist. And here,
again, it seems as though nature, with almost uncanny foresight, had
deliberately planned for this requirement of roots. And, in this case,
not by interfering with a physical process by an adjustment of plant
structure, but by the arrangement of soil particles and the way in which
water is found in all soils. Soil particles, even in the most compact
clay, do not fill _all_ the space occupied by the soil as a whole. There
are tiny air spaces all through the soil, which insures a constant
supply of fresh air. That is one reason why gardens are cultivated, to
see to it that plenty of fresh air is allowed to permeate the soil.
Around the finest soil particles there is always an almost incredibly
thin film of water, which is renewed as soon as it is lost by its
absorption by the root hairs or by evaporation. This renewal of the
water film is itself a mechanical process, called capillarity, best
illustrated by putting a few drops of water on a plate and placing on
them a lump of sugar. The water will spread all through the lump of
sugar in a few seconds and the capillarity that forces it up through the
lump is the same as sees to it that the tiny film of water surrounding
the finest soil particles is constantly renewed from the lower levels
of the soil.
Little do we dream, as we walk over the commonest weed, that buried at
its roots are these delicate arrangements for securing food and water.
Osmosis allowed to act so that the “exchange” of liquids is all to the
advantage of the plant, capillarity providing a constant water supply,
and the very piling together of the soil so contrived that the
life-giving air filters all through it--does it not seem as if all this
were, if not a deliberate plan, certainly a more perfect one than mere
man could have devised?
If you will turn back for a moment to the beginning of the description
of how plants get their food, you will find that in osmosis the weaker
liquid tends to permeate the denser one more rapidly than the denser one
does the weaker. As we have just seen, the sugary liquid in the root
hairs is denser than the soil water outside, and, furthermore, _none of
it is allowed to escape_. This comparatively greedy process of taking
everything and giving nothing results in a constant flow of soil water
into the root hairs. When the flow of liquids in osmosis is not at once
equalized, a gentle pressure is brought to bear to make them so. This is
what is called _osmotic pressure_, and it is this pressure that forces
the absorbed liquid through the roots and part way up the trunk of even
the tallest trees. While we have just said it is a gentle pressure, that
is true only in the case where the osmosis has free play, and the
pressure is stopped with the perfect mixing of the two liquids. But what
if they can never mix? What may not the accumulated osmotic pressure
amount to in such a one-sided process as goes on in root hairs with
everything coming in and nothing going out. Cut-off stems, with a
pressure gauge attached to them, indicate that in some plants the
pressure is from 60 up to 170 pounds!
Another result of this pressure is that it keeps leaves and the fleshy
stems of plants in their ordinary position. The actual solid part of
nearly all leaves is scarcely 5 per cent of their bulk and all the rest
is water. The constant pressure of this water from the roots is
sufficient to keep leaves comparatively stiff and rigid, how stiff is
quickly realized if the pressure stops and the leaf wilts or withers.
Sometimes this osmotic pressure, particularly during rainy weather,
becomes so great as to cause injury to the plant, the splitting of
tomatoes and occasionally of plums, being due to it. This osmotic
pressure, together with the extra pull given by the leaves, is
sufficient to account for the rise of water to the tops of the tallest
trees. The tallest trees in the world are certain kinds of blue gum in
Australia which frequently reach a height exceeding 300 feet. What the
combined osmotic pressure and leaf pull must be to carry such a heavy
thing as water to such a great height is easier to imagine than to
calculate.
The root hairs, then, by the process already described, absorb the water
from the soil, but plants can no more live on water alone than we can.
As we have seen, the membrane in the root hairs cannot allow the passage
of even the tiniest particle of solid matter. In fact the root hair
itself is so small that it can only be seen through the microscope, and
of course the membrane is smaller still. Plant foods, then, can never be
solids, but must always be such materials as can be dissolved in water.
The chief of these are chemical substances, such as lime, potash,
nitrogen, magnesium, phosphorus, sulphur, and iron. Hydrogen is also
necessary, but as this makes up half the composition of water, there is
a permanent supply of that provided by the soil water. These things make
up the great part of plant foods taken in through the roots, and it is
from these that leaves, by a process you already understand in its
essential details, manufacture sugar and starch.
But neither starch nor sugar, important as they are to the plant, and
absolutely necessary as they are to us, are the only things made by
plants. Leaves may well be called factories, but plants are themselves
the most wonderful chemical laboratories, beside which any built by man
are as play-things. For plants, by processes too complicated to be
explained here, work over their accumulation of starch and sugar,
recombine some of their constituents, and store up in various parts of
the plant the results, which are often such food ingredients as protein.
This is the really essential food substance in wheat, as it is in eggs
and meat. No chemist has ever succeeded in making a single scrap of it,
yet it is such an everyday occurrence in practically all plants that it,
with starch and sugar, forms the great food supply of the world. Not
protein alone, but all the amazing plant products like the oils from the
olive and the resin from pine, rubber, the drugs of plant origin, even
tobacco--all these and hundreds of others are made by plants from those
few simple foods absorbed through the roots, literally pumped up to the
leaves and there, under the magic of sunlight, combined and recombined,
worked over and changed utterly in their make-up. Nothing could be more
perfect than the marshaling of forces and contrivances to secure the
result; let there be even the least bungling, and for us the world
would cease to be worth fighting for.
Nor does the work of plants stop here. If it did, they would be not
unlike a commission merchant who had gathered from the four corners of
the earth a supply of eggs only to find he could not or more likely
would not sell them all at once, and yet had failed to provide himself
with proper storage. Plants, too, have times in their life when adequate
storage is necessary for them. So true is this that unless there is food
enough stored in seeds to give a start to seedlings before their own
roots begin to work, they would die almost at once. In seeds and in many
nearly dormant parts of plants these foods are stored away for future
use. The tubers of potatoes and all our root crops, like beet and
parsnip, are common examples of this. Even the manufacture of wood in
the trunks of trees is a storage appliance on the part of the plant, for
wood is just as much one of the food products of a plant as wheat or
rice.
3. BORROWING FROM THE LIVING AND ROBBING FROM THE DEAD
With such a beautifully perfected mechanism for getting food it might
seem as though all plants would be satisfied to lead that life of
independence for which they are so splendidly equipped. Some of them,
however, are like men in one respect: there seems to be no end to the
chase after getting something for nothing. Those that stand on their own
roots, get their food honestly, and take nothing for which they do not
make prodigal returns, make up the great bulk of the vegetation of the
earth. Their independence has dubbed them with the title
[Illustration: INDIAN PIPE. (_Monotropa uniflora_). A saprophytic plant
inhabiting rich woods in eastern North America. (_Courtesy of Brooklyn
Botanic Garden._)]
[Illustration: THE PARTRIDGE BERRY (_Mitchella repens_), a trailing vine
of northern forests. (_Courtesy of Brooklyn Botanic Garden._)]
[Illustration: RAFFLESIA. ONE OF THE STRANGEST PRODUCTS OF THE RAIN
FOREST. It consists only of a giant flower, the largest in the world,
which is attached directly to the roots or stems of relatives of the
grape, upon which it is parasitic. (_After Kerner and Oliver. Courtesy
of Brooklyn Botanic Garden._)]
autophytes_, literally solitary or self-providing plants, and this
thrifty mode of life is called _autophytic_. But a few kinds of plants,
actually many millions of individuals, have more devious ways of getting
their food and provide strong contrast to their sturdier associates.
These baser modes of life appear to have been rather insidiously
developed, as though there had been some hesitation at even the smallest
departure from the normal. Of course we must not forget that plants,
while living things, are never reasoning ones, and that good and evil
and all other qualities that are ascribed to plants are perfectly
foreign to them. Throughout this book, and in many others, the habits of
plants are spoken of as base, for instance, or good. What is actually
the fact is that nature works in truly marvelous ways, and to our
reasoning faculties these adjustments seem clothed with attributes they
do not really possess. But the description of them in the terms of our
everyday speech, the translation of their behavior into the current
conceptions of mankind, does so fix them in our minds that they cease to
be “just plants,” and we no longer put their habits in the category of
those interesting things that nearly everyone forgets.
One of the first signs of departure from the usual methods of getting
food is the association of certain minute organisms at the roots upon
which plants, otherwise autophytic, depend for aid in securing
nourishment. This characteristic is fairly common, notably in all the
plants of the pea family, such as peas, beans, locust trees, vetch,
clover, and hundreds of others. If the roots of any of these be
examined, it will be seen that attached to the smaller divisions of
them are small tubercles from the size of a pinhead to a pea, depending
on the kind. These tubercles or galls are caused by and infested with
bacteria, the smallest of all plants. The bacteria have the
extraordinary power of changing nitrogen into nitrates, which is the
only form in which nitrogen can be absorbed by roots. Not only do they
accomplish this, but excess nitrogen is stored in the roots by the same
agency. It is this fact that has resulted in the planting of vetch and
kindred plants for soil enrichment, as each year there is a residue of
nitrogen left in their roots and by so much they add plant food to the
soil. For hundreds of years farmers have done this, but only quite
recently have we known why they did so. The occurrence of bacteria or
microbes at the roots of plants is much more common than was formerly
supposed to be the case, and many other plants than those of the pea
family depend, at least in part, upon them in getting food from the
soil. While not wholly autophytic, such plants do make some return for
what they gain, as some of them at least pay dividends in extra
nitrogen, and all of them provide opportunity for the bacteria to live.
The latter play an important part in populating the soil, which is not
the comparatively sterile thing it appears to be. Actually it is
infested with organisms that play a mighty, if rather inconspicuous,
part in the work of preparing the soil for plant growth. These organisms
are so minute and the chemical nature of their work is so complicated
that merely to mention their existence must suffice here. This close
association of certain roots and bacteria, which, as we have seen, is of
mutual advantage, is known as _symbiosis_. It is really only a kind of
exchange, not unlike the storybook community that helped out by taking
in each other’s washing. Unlike that community the association between
the two works to the actual advantage of both, but the process is
undeniably a step away from those wholly autophytic plants which live
free and independent of such aid.
A much more gruesome habit of certain plants is their reliance for food
only upon the dead. In the Indian pipe, some kinds of shinleaf, and in
many other plants their roots and root hairs are changed or often nearly
lacking, and we find them growing only on the dead bodies of other
plants. One peculiarly repulsive characteristic of such plants is that
they secrete at their roots a substance that hastens the decay of the
dead, and, as if this were not rapid enough, there are associated with
them certain kinds of minute fungus organisms that also speed up
decomposition. Plants with this charming mode of life are known as
_saprophytes_, literally _sapros_, rotten, and _phytes_, plants. “Rotten
plants” they may be in their mode of life, but the pearly white stems
and flowers of the Indian pipe have a certain ghostly charm, an almost
statuesque beauty among the normal greenery of the gloomy dark woods in
which they always grow. It is not without significance that Indian pipe
bears no leaves, has none or almost none of the life-giving green
coloring matter which we have seen to be the almost priceless possession
of plants which lead a different, and perhaps a better life. The great
bulk of saprophytes bear no leaves, and some only partially wedded to
the habit appear to be midway between bearing normal green leaves and
bearing none, or much reduced ones that are quite unlike the busy
factories we know normal green leaves to be. Plants with this method of
getting their food, must of course grow in places where dead and
decaying vegetation is plentiful, and often as such soil is turned up
there may be noted a peculiar dank odor, suggestive not only of its
origin, but of the fact that these “rotten plants” make their home in
it. Some of our most beautiful orchids grow in this fashion, but even
there, in spite of flowers that for beauty of form are without rivals,
the plants have no green coloring matter in their leaves, which are
often reduced or even wanting altogether.
It might almost seem as if demoralization, so far as food habits are
concerned, had reached its lowest point in these plants that literally
rob the dead, but there are still lower depths to which certain plants
have been reduced. This consists of robbing the living, and such plants
are called _parasites_, a word perfectly familiar in other connections.
Parasitic plants have no roots, but attach themselves to the roots of
other plants, somewhat generously called _hosts_, from which they derive
their food. The best known case is the common Christmas mistletoe, and
the dodder (Figure 68), but there are hundreds of others. Nothing in all
the realm of plant life so perfectly fits the action to the word as
plants of this type, flourishing when the host flourishes, dying when it
dies. Producing flowers and seeds, and often, by an irony of fate,
perfectly green leaves, they are nevertheless the most debased of all
plants in their mode of life.
These successive steps in the degradation of food habits, are not always
the clean-cut things they might be inferred to be from the foregoing.
There are many intermediate stages; it may even prove to be the case
that some plants are wholly autophytic at certain stages of their life,
and slip partially into more devious practices at other stages. The
whole affair is not yet thoroughly understood and may well be the result
of competition, as it is quite conceivable that if the getting of food
in normal ways became difficult or impossible plants may have had to
resort to other methods.
[Illustration: FIG. 68.--THE DODDER
A leafless parasitic vine which steals its food from the plants to which
it is attached.]
4. WHAT PLANTS DO WITH WATER AND HOW THEY BREATHE
Some one has said that one day without water would make men liars, in
two days they become thieves, and after the third or fourth day they
would kill to get water. In the Army Records at Washington is a report
of one of our expeditions, which in chasing Indians got lost in a
desert, and in which the soldiers fought among themselves for even the
most repulsive liquids. It hardly needs these gruesome examples,
however, to confirm what everyone who has ever been mildly thirsty
knows, that water is an essential for all animals, and that to be
without it is to suffer torture. Air of the proper kind is just as
important, and because its absence or impurity causes more sudden agony
and a quicker death, the need of it is that much more acute. Plants rely
even more upon these two essentials of life, and in getting them they
behave in ways just as ruthless as do men who are suddenly deprived of
either of them.
As we have already seen in “How Plants Get Their Food and Water from the
Soil,” the water is the carrier of the food elements from the soil, but
water as such does much more for the plant than act as a carrier.
Osmotic pressure, a never-ending pump, keeps sending up a steady stream
of water to the limits of its power. In everything except trees it seems
fairly certain that this pressure is sufficient to drive water into the
remotest leaves. It finally reaches these tiny rooms in the leaf about
which we read in the account of Leaves as Factories. And just here a
very curious thing happens. Each room is, as we have seen, a very busy
place, crowded with all the necessary equipment to make sugar, and yet
there is still room for water which is just as necessary as the other
fittings; in fact so necessary is it that the whole interior of the room
is bathed in water. This irrigation system works so well that the walls
of the room literally bulge with the pressure of the water in them. If
they did not--a condition known as turgor--the plant would at once wilt,
and if no new supply came it would wither and die.
But water cannot stay in this condition of pressure and stagnation for
even a brief period. That would be as if a leaf were like a toy balloon
which, after inflation, had the entrance pinched and so remained
inflated. And while we have all along spoken of factories for making
sugar, and pressure pumps for forcing up food and water, it must never
be forgotten that this marvelously adjusted mechanism is a living thing.
Constantly growing, even producing their own means of falling in the
autumn, leaves must be thought of as living machines, just as we are
still more highly developed machines. In other words the accumulated
water in the cells of the leaf must be removed, after it has served its
use, and replaced by fresh supplies. The removal is carried on by its
evaporation into the halls, or, in the more precise terms of our account
of leaves as factories, into the _intercellular spaces_. It will be
recalled that these are connected with the outside air through the pores
or stoma. When the air outside is hot and dry it might easily suck out
by evaporation all the water vapor in these intercellular spaces and
wilting follow at once. This would actually happen if the guard cells,
already mentioned, were not constantly on the job. They control the size
of the opening just as certainly as a steam valve does, and maintain,
with a few exceptions, just the proper amount of water loss not only to
maintain turgor, but to see to it that transpiration, as this process is
called, goes on rapidly enough to insure fresh supplies of water being
sent to the leaf. The opening and closing of the stoma by the guard
cells is a nicely balanced operation dependent upon root pressure,
turgor, and atmospheric conditions. Guard cells have, because of this,
been much studied in spite of the fact of their microscopic size. We now
know that they allow greater openings during the night and reduce them
during the day. When we reflect that the constant removal of water in
the leaf, both as such, and as the only carrier of food supplies from
the roots, depends in such large measure upon the functioning of these
guard cells, then we come to some realization of their importance to the
plant.
They do not always work unaided, for in many places the transpiration,
even with their best efforts, would exceed the rise of water in the
plant and death must follow if such a condition exists for long. This
may be the case in certain bog plants, where, even with their roots in
the water, they actually are in danger of drying out because the
composition of bog water makes it partially unfit for most plants. And,
again, in very open dry or windy places, such as deserts or the mountain
tops above timber line, the actual supply of water may be insufficient.
Many plants growing in such places have their leaves, particularly the
under surfaces of them, clothed with various kinds of hairs. These may
be quite velvety or cottony, but in any event, either by their texture
or their color, they tend to reduce transpiration. An extreme case is a
desert plant from Arizona where the whole leaf surface is covered with
an ashy gray velvety coating, which, of course, absorbs less heat than a
normal green leaf, and in addition there are much fewer pores through
which transpiration could be carried on. In ever so many leaves nature
has provided them with a thick coating of hairs in early spring, which
they lose later in the summer. Shrubs and herbs, especially those that
start earlier than the trees under which they grow, very often may be
found with a dense woolly or silky covering in early spring. As the
shade becomes denser and the need of the protection less, the wool or
silk is shed, sometimes completely. Some of the most conspicuous cases
of this are certain kinds of our common shadbush, which in April are
covered with a beautiful grayish-white silky coat, but by August are
practically the ordinary green color of other leaves. The hairy covering
of leaves is well worth observation, as it may hide not a few facts
about transpiration and, in some leaves, has had much to do with their
preservation from grazing animals. Some, like the common mullein, are
never touched, and may be found standing like sentinels in fields
otherwise cropped short.
In many leaves there is conflict between those forces that result in the
leaf getting the utmost possible exposure to light and those that
prevent too rapid transpiration. On the one hand there is the absolute
necessity for light, on the other the ever-present danger that the
response of leaves to this necessity will result in a transpiration rate
too rapid to be held in check by the guard cells. The compromise between
these two forces, each pulling in opposite directions, gives to some
leaves a series of movements that are among the most interesting things
in nature. One of the most marked examples is the common wild lettuce, a
weedy plant of our roadsides introduced from Europe. In bright sunlight
the leaves are turned so that the edge of the blade faces upward, and
the surface is thus protected from the direct rays of the sun, but
during cloudy weather or in the shade the leaves turn into the ordinary
position of most foliage leaves. It is difficult to avoid the inference
that photosynthesis, which, as we have seen is an absolute necessity to
the leaf, is in the wild lettuce retarded by transpiration, to avoid the
too rapid rate of which the leaf is turned on edge. In this plant the
leaf base, as though to be ready for whatever change transpiration or
photosynthesis may demand, is so attached to the stem that such changes
are made with the least possible delay or wrenching. In one of the many
kinds of blue gum trees of Australia all the leaves turn one way in the
light, and another in shade or on cloudy days. Ever so many plants have
partial movements of their leaves, a good many of which are in response
to these opposing demands, one pulling the leaf into the greatest
possible light, the other holding it away from that condition. There are
other movements of leaves, of parts of the flower, or even of the whole
plant that are not so certainly the result of the conflict between light
requirements and the necessity of conserving water supply. They will be
considered presently.
While most plants are well provided with methods of losing water, so
well provided in fact that in very hot or very long dry periods it is a
common sight to see many plants literally panting for more water, there
are some apparently more cautious individuals, who reverse this process.
All throughout tropical America hundreds of relatives of the pineapple
have their leaves so formed and arranged that they catch and hold
considerable quantities of water. In one kind, called _Hohenbergia_, the
long leaves are joined together toward their base into a water-tight
funnel, which will hold a quart or two of water over a period of
drought. In Africa the extraordinary traveler’s-tree, a giant herb
growing twenty to thirty feet tall, has the overlapping leaf bases so
arranged that they hold many gallons of water. And we have already seen
how the giant cactus of our own Southwest will hold 125 gallons. The
most remarkable case is the Ibervillea from the deserts of Arizona. In
riding over this country one may find objects that look not unlike a
burned pudding, about two feet in diameter and nearly as high. From the
center comes a delicate stalk with the finest feathery foliage and tiny
flowers. Of roots there appear to be almost none, and these curious
objects, which are very hard and woody, might almost be taken for
stones. But they are actually plants not distantly related to squash and
pumpkin, and one of them collected years ago and brought into a museum
behaved in quite the most thrifty fashion of any plant yet discovered.
It was carefully cleaned and put in a museum case and locked up as a
curiosity for the wondering public to gaze at. But suddenly, almost
miraculously, it sent out its delicate growth which grew its appointed
time and then withered. Imagine the astonishment of the curators of this
museum to find it doing the same thing the next year, and the next.
Finally after putting forth its shoot for five years it actually died
and is now a peaceful museum specimen. No other such case of water
storage is known, but thousands of plants have this remarkable ability
to a less degree, all in response to conditions that would mean
destruction to plants not so providently equipped.
This conservation of water on such a great scale offers striking
contrast to the truly prodigal habits of certain plants that actually
drip water, so charged are they with this precious liquid, and so little
stress do their conditions of life put them under in this respect. Where
water is plentiful and turgor maintained almost to the bursting point,
evaporation in a moist or chilly atmosphere does not suck out water
vapor fast enough. Sometimes, around the edges of the leaves of the
common garden nasturtium, drops of water may be found, literally forced
out as drops, rather than transpired as water vapor. This happens to a
considerable number of plants, during the night when transpiration is
laggard, and such drops are usually mistaken for dew. The latter is
actually the condensation of moisture in the air upon the leaves of
plants which cool down more rapidly than the air, and seldom due to the
forcing out of drops of water from leaves, although in rare cases it may
be. In tropical forests, where the humidity is very heavy and water
supply from the roots copious, certain leaves leak water so fast and are
so constructed that this excess is prevented from accumulating on the
leaf. The pipal tree of India has long drip tips to its leaves that
conduct the excess water from the blade to the end of the slender tip
where it drips off. The advantage of these dripping points is obvious,
for in regions so humid that water is forced out of the leaf, the
coating of the leaf with this extra moisture would by that much retard
transpiration. Dripping points, which in less exaggerated forms than in
the pipal tree are common in many parts of the world, are thus of
decided advantage.
Whether it be desirous to retain water or to lose it by gradual
evaporation, or expel an excess of it, each species of plant has
developed the apparatus to best preserve its individual life. While only
the barest outline of these adjustments to the water requirements of
plants has been given here, the details form an almost dramatic picture
of struggle of the different kinds of plants for survival. The extremes
are the desert plants on the one hand and those of the rain forests in
the tropics on the other. The chapter on Plant Distribution will show
how important these water requirements of plants have been in
determining what grows on the earth to-day.
With carbon dioxide going in, oxygen, water vapor and, as we have seen,
even liquid water coming out of the stoma of leaves, it might be
surmised that these busy little pores and their guard cells had done
work enough for the plant. And yet there is still one more act to play
and the stoma have much to do with it. For this process of
photosynthesis and the closely related one of supplying food and water
to the leaf cannot go on without respiration, which is quite another
thing. In plants respiration or breathing has no more to do with
digestion than it does in man. Digestion in man is not unlike
photosynthesis in plants, except that plants make food in the process
while men destroy it. But plants must breathe just as we do, and, as we
need oxygen to renew our vital processes, so do they. While respiration
is a necessary part of plant activity it is not such an important part
as photosynthesis, for which it is often mistaken. The thing to fix in
our minds is that photosynthesis makes food, uses the sun’s energy and
releases oxygen in the process, while respiration uses oxygen and might
almost be likened to the oil of a machine--necessary but producing
nothing.
5. RESTLESS AND IRRITABLE PLANTS
In walking through the quiet cathedrallike stillness of a deep forest or
over the fields and moors, perhaps our chief thought is how restful the
scene is, and what a contrast the quiet, patient plants make to the
darting insects or flitting birds that our walk disturbs. We found at
the beginning of this book that ability to get about is one of the main
differences between animals and plants. Like so many first thoughts,
this is, however, only a half truth, for while most plants, seemingly by
a kind of fatality, are anchored forever to the place of their birth,
many of them do move certain parts of themselves and that quite
regularly. While some of these movements have already been hinted at as
a possible response to transpiration or too intense light, there are
others where the advantage to the plant, if any, has yet to be
demonstrated. These other movements, perhaps because their cause has
never been discovered, seem the more mysterious as they certainly are
more weird and interesting than almost any other of the curious things
that plants do.
Perhaps the most difficult thing in the world is to keep an active
growing child perfectly still for more than a few moments at a time.
There seems to be some impelling force that makes young growing things
in a constant state of restlessness, and it is perhaps not so
extraordinary, after all, that practically all young plants are restless
in the sense that they are never quite still. And, like many grown-up
people who do not know what repose in their waking moments really means,
there are a goodly number of plants that are restless until the day they
die.
Charles Darwin, perhaps the greatest man that the last century produced,
wrote a book in two volumes on these restless plants, and proved by a
series of experiments illustrated by charts which the plants themselves
drew for him, that there were perhaps no plants that do not move at
least some part of themselves during the early stages of their career.
While he never could explain the cause of these movements he left in
that book an imperishable record of the amount and direction of these
mysterious movements, which are almost to be likened to the growing
pains of young children.
The tips or growing shoots of many plants will point in one direction in
the early morning, a different way at noon and still a different one by
nightfall. Hundreds of totally unrelated plants seem to have this habit
of moving their tips through a definite cycle during each day and this
restlessness does not appear to be of the slightest use to them. It
cannot be response to the moving of the sun through the sky, for often
the movement may be away from the direct sunshine, and sometimes the
motion goes on in the dark, as experiments have proved.
It is hard to see the movement of the whole upper part of a plant,
although it is well known that they do move in many cases. But in the
tendrils the movement is often easy to observe and even to induce. Some
of these slender aids to climbing plants, if they happen to be swinging
freely in the air, do actually make slow circular movements, that even
if they were designed for the purpose could not more perfectly
accomplish their obvious intent, which is to catch the nearest favorable
support. These circular movements are to the left in the hop,
honeysuckle and many other plants, to the right in the climbing beans,
morning-glory and some others. When the tendril reaches a support it
almost immediately turns about it, in the same direction as its free
movements through the air have been. It is thus this apparently aimless
swinging of tendrils through space that determines whether the vine is
going to twine to the right or left. The speed with which a tendril will
take its first turns about a support is so comparatively rapid that,
once the support is caught there is scarcely a chance of the vine being
torn away by the wind or other agency as would surely happen if tendril
movements were the leisurely things that some folks think they are. In
the case of one Passion-flower vine, which are gorgeous climbers mostly
from the tropics, the tendril made a complete turn in two minutes after
it first touched a possible support. And there is a quite noticeable
movement in thirty seconds if the tip of the tendril be ever so lightly
touched. Teasing tendrils to see how much or how fast they will coil has
resulted in some extraordinary cases of the “comeback” of some of them.
Darwin irritated a tendril for a few moments and induced a partial
coiling which straightened out when the object causing it was withdrawn.
To see how long the plant would stand this sort of thing and still not
be literally tired of coiling he succeeded in making the plant partially
coil, and by withdrawing the incentive uncoil again, over twenty times
in fifty-four hours. An impulse to coil of such persistence as this
naturally results in vines forming the impenetrable thickets they do in
many forests. It emphasizes how restless are the growing points of these
climbers, and serves as a striking illustration of those gradual
movements of many other plants that seem to have some relation to
growth, but in a way not yet understood. For while it is an obvious
advantage for the vine to swing its tendrils through the air this
advantage has not yet been proved the cause of the swinging. In fact if
all possible supports are removed the tendril will often coil anyway, a
perfectly futile proceeding, that looks almost like disgust.
This general restlessness, which by the imaginative has been thought of
as a mild protest by plants at their otherwise fixed position, is not so
spectacular as that of certain other plants, notably the poplars. A
flattened instead of a round leafstalk makes the leaves of these trees
flutter in the lightest air and in a gale the tree is a mass of animated
foliage. No use has ever been found for this curious habit and it is not
certain that it is of the least advantage to the tree. If anything, the
constant movement may have the decided disadvantage of increasing
transpiration.
In our common wood sorrel the leaflets on cloudy days or during the
night regularly “go to sleep.” That is, they are folded at such times,
rather than spread out in the ordinary way. These sleep movements may
have something to do with transpiration, but whether or not this is true
they are very regular and in certain plants the habit is remarkably and
rather mysteriously uniform. Why, for instance, do the leaflets of these
wood sorrels, the beans, lupine, locust tree and licorice plant, always
fold downward while the clovers, vetch, peas, and bird’s-foot trefoil
are always folded upward? Such movements and their direction are among
the unsolved problems of botany, and merely to know of them or observe
them leads us nowhere as to their true inwardness.
But quite apart from these merely restless plants, and there are
thousands of different kinds which are known to move slightly, at least
during their young stages, are a few more decidedly active ones that are
seemingly irritable. At least they show peculiar movements if touched,
and at night. One of the best known is the sensitive plant from tropical
America. Its twice compound leaf is composed of many tiny leaflets
which upon the slightest touch close up and apparently wither on their
stalk at once. In five seconds after the leaf is touched it will appear
like a wilted wreck. If the jar is sharp enough the whole plant will
droop, and the response to a sudden jar is almost electrically quick in
its action. And yet all this sudden wilting, actually caused by a quick
loss of turgor, is slowly repaired and the plant carries on quite
normally again until another shock renews its irritable response. This
plant does the same thing gradually during the night, except that the
leaflets recover their normal position only with the rise of the sun.
From India comes the most remarkable of all plants so far as movements
are concerned. For in the telegraph plant the movements are so regular
and long continued that irritability might almost be said to be
continuous. The plant is a low shrub or herb with compound leaves, and
the terminal leaflet, which is much larger than its neighbors on either
side of their common stalk, performs a motion that describes with its
tip an irregular oval or ellipse. But the movement is not steady; it
goes by a series of slight but perfectly distinct jerks. It takes about
two minutes for the leaf to complete its cycle, and it is this jerky
movement that has given the plant its name. During the night its
leaflets stop this apparently quite useless performance, the cause of
which is quite unknown. It is often grown in greenhouse collections
where its strange movements may be seen on any sunny day.
Many other cases of the restlessness or irritability of plants could be
given, and nothing has been said here of the curious movements of some
insectivorous plants as they have already been mentioned. The very
considerable movements of certain flower and fruit organs will also be
considered elsewhere.
* * * * *
It cannot have escaped the thoughtful reader that all of this chapter on
plant behavior has dealt with those functions of plants in which roots,
stems, or leaves play the chief part. These purely vegetative actions of
plants, what might almost be called their bread and butter activities,
would never lead to perpetuating their kind. For while all of these
functions are necessary, except certain apparently wayward movements
which still remain unexplained, they are in a sense only the preparation
for an infinitely more important act, the reproduction of their kind.
What the poetic have called the love of the flowers, or in more prosaic
but perhaps more truthful words the fertilization, pregnancy, and birth
of the new race, will be considered in a separate chapter. No other act
of the plant world is so interesting as the mechanism of reproduction,
the almost endless devices for securing it, and the ingenuity of nature
in seeing to it that there are no flukes.
CHAPTER III
HOW PLANTS PRODUCE THEIR YOUNG
There is perhaps no device of nature that more perfectly accomplishes
its purpose than the one with which all living things are endowed--the
instinct for the renewal of life. In man the dawn of the mating instinct
has ever been the theme of poets, and some of its manifestations are the
despair of ascetics. Through it some of the noblest of man’s emotions
have arisen, and because of its perversion our daily newspapers
chronicle the basest and most sordid tragedies.
But whether noble or ignoble, this instinct for mating is, in its
simplest terms, only a provision of nature that all life contains within
itself the means of renewing life. Without this, life, so far as we know
it, would end with the present generation. Perhaps our understanding of
this decree of an all-wise nature to increase and multiply will be
heightened by looking at it not only from its familiar manifestations in
man, but more broadly. Seen from this broader viewpoint, it is the
inherent legacy of all living things from the dawn of life on the earth
down to the present. Even the simplest one-celled organisms have the
faculty of increasing. In all plants, both the flowerless ones and those
producing flowers, the process is carried to a perfection almost
unbelievable in its intricacy and in provisions against its failure.
From the matings of flowers much may be gleaned; even man himself can
learn from them the capacity for sacrifice, the sinking of individual
aims and pleasures in the greater scheme of conforming to that necessity
for renewal of the race upon which all progress must be based.
The equipment which different flowers have developed for this purpose,
their almost uncanny devices to make certain that only the distant and
foreign male can ever impregnate the female, the enormous wastage of
both unfertilized females and males that will never become fathers, and
the overwhelming effectiveness of it all, in spite of this
prodigality--these manifestations of the production of young in the
plant world will take up the rest of this chapter. All the first part
will tell of this process in flowering plants, while the second shows
how flowerless plants accomplish the same end in more secret ways.
Finally, in a brief third part, we shall see how, without mating of the
sexes, nature has still one other way to see to it that there is a
constant supply of young.
We have already made clear that all plants are divided upon the basis of
whether they bear flowers and their mating goes on before the world, or
whether they bear none and the process is accomplished in more secret
ways. Because flowers are so much better known, and it is simpler to see
how the act is consummated in them than in the _cryptogamous plants_, we
shall first consider the _phanerogams_ or flowering plants, and in the
second section of this chapter the _cryptogams_ or flowerless plants.
1. VISIBLE MARRIAGE OF FLOWERING PLANTS
In the first chapter, under the section devoted to flowers, we found
that the stamens are the male and the pistils the female organs of
reproduction. As the period for mating draws near there is developed in
the _anther_, which is the enlarged tip of the stamen, a fine, usually
yellow, powder known as _pollen_. This matures in the anther, and when
ripe is discharged from tiny pores.
Pollen is made up of individual _pollen grains_, which are very often
stuck together so that we see only the mass, not the individual pollen
grain. Sometimes the pollen is not sticky, as in the case of pine trees
or in the ragweed--a fertile cause of hay fever. In these, and hundreds
of other plants, the wind will blow great clouds of pollen through the
air. When we stop to consider that a single, or at most a very few
pollen grains are all that are necessary--in fact, are all that _can_ be
of real service--the enormous wastage of the male fertilizing substance,
in order that mating be secured, gives us some idea of how prodigal is
nature in this supreme function.
The pistil, or female organ of reproduction, is more cautious in the
expenditure of its resources. As we have seen, it is composed of a
swollen base, the _ovary_, a slender shank, the _style_, and a swollen
or branched tip, the _stigma_. In some plants the ovary is divided into
several compartments or cells, each with one or more _ovules_, which are
only immature or unfertilized seeds, often very tiny, but usually quite
easily seen if the ovary is cut open. It is the entrance of the pollen
grain into this ovule that consummates the act of fertilization. As the
ovule is carefully secreted within the ovary of the flower, and as the
male fertilizing stuff or pollen is found only on the anther, it is
obvious that some method of bringing the two together must be provided
for.
In some plants this is accomplished by the anthers being just above the
stigma, and when the pollen is ripe and the ovule ready, the stigma is
found to be covered with a sticky substance. As the falling pollen
grains touch the stigma, they are caught in this sticky substance just
as surely as flies are caught once they touch a fly paper. But just here
one of the most wonderful processes of nature begins. The pollen grain
begins, slowly at first, to grow, and in the act it penetrates the outer
coat of the stigma with a minute _pollen tube_. This slender threadlike
tube, carrying with it the male germ, grows straight down through the
stigma, into the narrowed style, and through this to the ovule. Once the
pollen is caught on the stigma, nothing is so sure of fulfillment as
that this male fertilizing stuff will ultimately reach the ovule. For
the hitherto virgin ovule this impregnation starts a new phase in life.
It means the beginning of the end, but in the process fruit and seed
will be developed, and the young bride, already a mother, has
triumphantly accomplished that for which she exists.
If fertilization of all flowers were as simple as this, there would be
no need of what follows, but actually in surprisingly few plants are the
stamens and pistils so arranged, the ripening of the pollen and
readiness of the ovule for impregnation so timed that the act can be
accomplished in such direct fashion. For it is quite obvious that in
flowers in which the whole drama of mating goes on within the petals,
without the interference or help of any outside agency, the result will
be a crop of young who know no other characters than those of the
parents, and have nothing to look forward to but a closely inbreeding
progeny, very little, if at all different from themselves. In other
words, such plants are pure bred, they lack the usually obvious virility
that comes from crossing the male of one plant with the female of
another. There are so many devices to prevent self-fertilization in
flowers, so marvelous are the contrivances to see to it that only
cross-fertilization can be effective, and, finally, the experience of
breeders that strength and virility often or usually result from
impregnating the ovules of one plant with the pollen of another, that we
are forced to the conclusion that absolute purity in the sexual
relations of flowers is rare indeed. It occurs, without peradventure of
a doubt, only in those flowers whose petals never open and where
fertilization is consummated, if not in private, at any rate without
external help. In many violets the showy violet blossoms are often
nearly infertile, while down near the ground are inconspicuous flowers
which never open, but within which fertilization is so successful that
the crop of seeds is far more plentiful than in the more showy ones that
most people think are the only flowers ever borne by violets. These
flowers that never open, or at any rate open so slightly that their
sexual processes are modestly completed without intrusion, are known as
_cleistogamous_ flowers (Figure 69). They have been found in a few
plants, but overwhelmingly the greater number of flowers not only do,
but must, rely on some outside agency to insure fertilization.
Certain structural features of flowers have been so developed that
fertilization of the ovary by the pollen of the same flower is
impossible. The commonest case is in those flowers where the stamens are
shorter than the pistils, as they always are in the common snowdrop,
hyacinth, the sassafras tree, and in hundreds of others. There can be
no consummation of the reproductive process in such flowers without some
outside aid. More futile still without this aid are those flowers where
the stamens are well above the pistils, but the time of maturing in both
differs by a few days or even hours. Nothing could be more
helpless than the pistil under these circumstances, for if
its instinct for maternity were ever so strong, it would be doomed to
barren sterility by the premature development of the males. Sometimes,
too, the female is prematurely ripe for impregnation, and the stamens
lag behind a day or two. Her time passes and with it her only chance of
fertilization--by her own haste she has rendered impotent the now
useless pollen which appears doomed to fall aimlessly upon the
unreceptive stigma.
[Illustration: FIG. 69--THE VIOLET
Note the showy, often partly infertile upper flowers and the much
smaller cleistogamous ones at the base, which never open and yet produce
a good crop of seeds.]
But, perhaps, the most hopeless of all is the well-known partridge
berry, whose red berries are common in the woods during August and
September. This seems as though it fought off any chance of securing a
mate by a flower structure and behavior that would certainly so result
if some way out of the difficulty were not at hand. The partridge berry
bears two kinds of flowers that outwardly look much alike, but whose
sexual organs differ in this way: in some flowers the stamens are all
shorter than the pistil, and in others the pistil is much outtopped by
the stamens. The extraordinary feature of it is not so much this
structural difference, however, but the fact that pollen from the
short-stamened flower is useful only to its neighboring short-styled
relative, while the pollen from this long-stamened but short-styled
neighbor is nearly useless where it is found and really useful only on
the long-styled plant. By this device, but again only with outside aid,
this plant does not prevent maternity, but increases its chances of
being fruitful, for, as we have already seen, cross-fertilization
appears to be the rule rather than the exception, and the partridge
berry not only needs it, but can exist only when its offspring are the
result of such crosses.
In all those plants that bear the different sexes in different flowers
on the same plant, as in the hickory, or even on different plants, as in
the willow, there must, of course, be some method arranged for
cross-fertilization or they would promptly die out. So general is this
cross-fertilization, so much a part of the economy of nature does it
appear to be, that we can only think that there must be in the
production of this vast horde of the cross-fertilized some advantage.
Besides securing greater virility, which almost certainly results from
this promiscuity, greater variability is promoted. If virility is the
result, the price paid for it is tremendous, for the hindrances to
self-fertilization are so many and so effective that most flowers would
inevitably die as perfectly pure but ineffectual virgins if that
fatality were not prevented. How they are saved from such a sterile
fate, how they finally secure a mate by devices that outshine the most
bewitching tricks of the daughters of Eve, is one of the most
fascinating stories in all the history of the plant world.
For, of course, flowers do secure a mate, and they are aided in this
enterprise by the most formidable array of helpers, one might almost
call them conspirators. The chief of these are insects, thousands of
different kinds of which are constant flower visitors. Some of the
smaller birds, and even snails, also help flowers to meet their mates.
The wind, too, bears pollen through the air to some expectant
bride-to-be. And, finally, in the water, by a series of acts the like of
which no one could improve for cunning, the cross-fertilization of
certain aquatic plants is consummated. It would take a book larger than
the present one to give even the briefest account of how these different
aids to maternity do their work and how the flower responds to this
help. As that is quite out of the question, only some of the best-known
examples of cross-fertilization will be given, and these will be grouped
according to what agency the flower is indebted for its chance of doing
that for which it is created.
INSECTS AS FLOWER VISITORS
On any summer day, especially when the sun is shining brightly, we may
see bees and butterflies flitting from flower to flower, busy as the
proverbial bee. We already know enough about nature’s ways of doing
things to be certain that these, and hundreds of other kinds of insects,
do not come for nothing, and that the flower must have something to
offer. Bees, especially, are thrifty creatures whose business demands
exacting and prolonged toil. They would not waste five seconds upon idle
flower calling if the blossoms did not yield a rich store. And thousands
of flowers do yield the sweetest and richest kinds of stores of nectar
or honey, which is, in fact, by the help of insects who alone can
extract it, our sole source of honey. Many flowers which produce no
nectar do have such plentiful stocks of pollen that the bees come for
that alone. In the peony, for instance, over three million pollen grains
are produced in each flower, only a minute fraction of which can ever
fertilize an ovule. All the rest would be wasted were not pollen in
itself a particularly nutritious food for young bees, and consequently
much sought after by the careful bee mothers. They are the only insects
that feed their young on pollen, or beebread, as it is called by the
beekeepers, so that the enormous overproduction of this male fertilizing
agent, from the point of view of the flower, is a decided attraction,
one might almost call it a trap, to insure constant visitations from
bees. For they are perhaps the most useful of all insects in the great
game of securing cross-fertilization, as we shall presently see. Many
other kinds of adult insects eat pollen directly and so add to the
number of insect visitors.
No one has ever been able to explain the beautiful coloring of flowers,
except that it serves as an attraction for insects and small birds. Like
the honey or nectar, it seems to play no real part in the home economy
of the flower, to be of not the least use otherwise. While honey and the
gorgeous colors of flowers are a delight to man, that would be no
sufficient reason for the ability to produce them. Both of these
attributes of flowers, as attractions for insect visitors, are, however,
so absolutely essential to cross-fertilization that we must think of
them as having grown up out of that demand. As we shall see a little
later, even the structure of some insects has been modified so that they
can reach the nectar or pollen only by automatically doing for the
flower what it cannot do itself.
While color of flowers seems as though it were attraction enough, it is
very likely that their fragrance or perfume is still more seductive in
its power of luring insect visitors and repelling useless ones. Poets
have called this perfume the soul of the flower, and in its almost
intangible beauty it might well be so called were it not for the fact
that it appears to be of not the slightest use, except as a lure. In all
the equipment of seduction there is none like this fragrance of flowers
for attracting insects.
Flowers, then, have things to offer to insects which the latter need.
Nectar and pollen are the chief, and where these merely bread and butter
objects are not enough, or sometimes in addition to them, the flower is
dressed out in gorgeous colors or perfumed with a fragrance beyond the
dreams of the fairest bride. What insects do to complete the
fertilization of such a legion of beauties makes up the romance of the
flowers. Perhaps not even in man himself is this creation of new life so
surrounded with beautiful ideas. Also, as in man, it sometimes is bound
up with an almost fiendish cruelty and cunning. Some of these visitors
and what they do, but unfortunately only a very few, can be mentioned
here. They must serve as types or examples of what is going on all about
us on any summer day.
The common blue columbine, much grown in gardens for its beautiful
blossoms, always has the flowers hanging upside down, a habit that
admirably serves to keep its pollen from rain. The opening and closing
of many flowers in cloudy weather, or at night, may be for the same
reason. Everyone knows the five blue spurs into which the petals of
columbine are produced. At the very end of each spur, which is always
curved, the flower secretes a considerable quantity of honey. This, one
of the greatest attractions to bees, leads inevitably to a visit from
one. The bee, in order to reach the honey, hangs on to the inverted
flowers, clutching the base of the spur with its foreleg, and further
securing itself by the mid or hind legs, which grasp the slender column
into which, in the columbine, the stamens and pistils are crowded. In
its anxiety to reach the honey the bee pokes its head as far into the
spur as possible, but it gets in only a fraction of the full length of
the tube. To reach the honey it extends its sucking apparatus, which is
a complicated mechanism for this purpose on the head of nearly all
insects, and which will hereafter be called by its true name of
_proboscis_. It happens that bees can easily bend the proboscis downward
or toward their own body, but only with considerable difficulty can they
bend it in the opposite way. And yet the honey in the curved tip of the
columbine can _only_ be reached by curving the proboscis to fit the
tube, and in this process the bee’s body for nearly half its length is
forced to touch the anthers. While these are close to the stigma, they
produce pollen only on their outside surface, where it is, of course,
scarcely likely to reach the stigma, but must be brushed off by the
contortions of the bee’s body in reaching the honey. The hairy body of
the bee, coated with pollen, goes next to perhaps an older flower of the
columbine. Heedless of any change in the flower the bee goes straight
for the honey in one of the spurs, again catches hold of the only
available support in the center of the flower. But this time, instead of
brushing pollen off the exposed anthers, it brushes it off its own body
to the stigmas, which, at a slightly later stage than in the one just
described, are branched and perfectly adapted for collecting the pollen
with which the bepowdered bee can hardly avoid dusting them.
Cross-fertilization is of course assured, but it seems a precarious
business at best, as the number of bees with a proboscis long enough to
do the work is limited. The columbine, by a kind of uncanny foresight,
is so constructed that bees or other insects that try to reach the honey
and are not provided with a sufficiently long proboscis, nevertheless in
further attempts upon other flowers, inevitably cross-fertilize them
without reaping their reward. One or two kinds of bees, as though in
retaliation for this subterfuge of the columbine, make short work of the
honey by biting a hole in the spur and forthwith sucking out the honey
without so much as touching anther, pollen, or stigma. The reply of the
columbine to this ravaging of its chief attraction is that finally, as a
last resort, and by a new movement of its reproductive organs, it is
self-fertilized. Here the shape of the flower, the original position of
the pistil and anthers, the exposure of pollen only in such a direction
that, while a chance of cross-fertilization still exists, it can hardly
ever fertilize its own stigmas, all point to cross-fertilization as the
plant’s greatest requirement. And yet failing this, it falls back on
self-fertilization rather than endure barren sterility.
[Illustration: FIG. 70.--COMMON BARBERRY
The stamens of this, two at a time, drive off bees by sharp blows, thus
preventing self-fertilization.]
While the columbine by its spurs and other interior structure succeeds
nearly always in holding a bee long enough to insure its being dusted
with pollen, the common barberry bush of Europe (Figure 70), also much
planted in American gardens for ornament, actually drives bees away by
sharp blows of its stamens, so that self-fertilization shall not result
from the visit. In this shrub the petals partly cover the stamens unless
the latter are disturbed, and, in fact, the curved tip of the petals
forms a kind of socket into which each of the six stamens are fitted.
The position of these is such that any insect
[Illustration: PLANTS OF THE PALM FAMILY, _Palmaceæ_, in Ceylon. They
are the talipot palm (_Corypha umbraculifera_), the fiber from the wood
of which is used in India for paper making. (_After Reinhardt. Courtesy
of Brooklyn Botanic Garden._)]
[Illustration: A GIANT HERB OF THE BANANA FAMILY, _Musaceæ_. It is the
travelers’ tree (_Ravenala Madagascariensis_), the sheathing leafstalks
of which hold considerable quantities of water--hence the name. Grown
throughout the tropical world. (_Courtesy of Brooklyn Botanic
Garden._)]
can go straight to the honey glands which are at the base of the flower,
without touching the anthers. But their filaments are broadened out at
the base, so much so that their edges touch. The honey glands are so
placed that the insect must touch the broad bases of at least two
filaments, between which, in fact, it must force its proboscis in order
to reach the honey. The moment any particular pair of filaments are
irritated by the bee, two pollen-dusted stamens fly out from their
position among the petals and the anthers strike the bee with a sharp
blow. Many observations prove that almost never does the bee go on with
his honey sucking after this rude interruption, which has resulted in at
least its head being dusted with pollen. The low, sticky stigma is so
placed that it is one of the first things the bee’s head strikes as it
reaches the center of the flower. Because of the position of the
stamens, while they are undisturbed, it is impossible that pollen from
them could have been brushed off at the bee’s entrance of the flower.
And by an almost miraculous adjustment of the power of the blow by the
irritated stamens, this drives off the intruder only after he has
brushed his pollen-laden head over the stigma. His head at this stage
is, of course, covered only with foreign pollen gathered elsewhere, but
just as soon as the bee tries to get what he came for, sometimes even
before he gets his reward, out fly the pair of stamens, thoroughly
dusting the bee, and seeing to it that the blow is just sufficient to
drive off the pollen-laden insect. No device to secure
cross-fertilization could be more effective. If the blow of the stamen
were only ever such a slight fraction less than it is, the bee would
only stop a moment and then go on honey sucking; which, because of the
release of the pollen by even the gentlest blow, would result in
self-fertilization by the aid of the bee, rather than
cross-fertilization. Very few, if any, bees will stand this gentle
reminder to go, however, and it is a little curious that such
intelligent creatures as they are supposed to be, should not realize
that it is all the clever but quite harmless trick of an apparently
still more intelligent flower to secure fertilization from any pollen
but its own.
To attract insects and then repel them seems a little like using them as
some flirts notoriously use men, only to throw them over when they are
no longer interesting. In the large-flowered magnolia tree from the
southeastern United States, insects, however, fare somewhat better than
this. In this magnolia, which has flowers several inches long,
self-fertilization is impossible as the stigmas are ready to receive a
mate several days before the laggard stamens are provided with the
wherewithal. Without some insect or other outside help there would be
only a childless old age for this particular tree. The flower opens
rather early in the season, while the nights are still cool, and as a
protection from the cold, rose beetles habitually fly into them. They
find a pleasant shelter under the three inner petals which arch over the
honey-coated stigma, and form a snug little chamber so much warmer than
the outer air that its heat is appreciable to the touch. The rose
beetles, once they are inside this warm shelter, cannot get out and are
often held for a few days. Then, as the stigma passes its period and the
stamens are furnished with pollen, the chamber opens by the gradual
withering of the petals. But the insects, in their efforts to get out,
have raised a perfect dust storm of pollen with which they are
naturally covered. Just as soon as they are released they are free to
seek another warm shelter where the process is repeated. Thus they
always enter the flower with foreign pollen, use it up impregnating the
waiting ovule, and are held until the flower’s own pollen gives the
signal for their release properly dusted for a renewal of the work. The
premature timing of the stigma, the tardiness of the anther in producing
pollen, the generation of heat and secretion of honey which frequently
covers the whole stigma--there could scarcely be a better equipment for
securing cross-fertilization. And without it the magnolia would be
simply sterile.
There are some other flowers that hold visiting insects in a trap until
cross-fertilization
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