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.