LIFE OF A PLANT
“_We cannot pass a blade of grass unheeded by the way,
For it whispers to our thoughts and we its silent voice obey._”
--_J. E. Carpenter_
The growth and development of a plant, though such a common thing,
is full of very real wonder and mystery. It takes only a little
observation to discover the various stages in the process, but how they
are brought about and by what laws they are governed, not even the most
astute investigators can always say.
To the lay mind, the statement that the plants depend upon the soil
for their nourishment is quite self-evident, yet it is extremely
inaccurate. It is now quite certain that the vegetable world relies
upon the _air_ for its largest and most important food supply. The
great mass of carbon which is the chief constituent of all plant
structure is drawn almost exclusively from the atmosphere. While it is
true that many vital elements are obtained from the earth, all green
plants manufacture the greater part of their solid material out of
the carbon dioxide of the air. Of what the plants do obtain from the
soil, water makes up the largest bulk. The bread and meat of the plant
world is carbon dioxide; the drink is soil water in which is dissolved
certain essential salts and condiments.
A chemical analysis of a Green Pea will show approximately 46.5% of
carbon, 4.2% of nitrogen and 3.1% of all other elements, exclusive of
the hydrogen and oxygen which make up the water permeating all tissue.
This is truly a startling fact. Instead of belonging to the earth,
the plants then belong primarily to the air. The air is their natural
habitat; the earth serves to give them a fixed place in the world and
provide them with flavoured water to drink.
Plants are born from seeds, the joint product of two previous
individuals; they live by eating and drinking; they marry and in turn
rear families of their own. It is our purpose in this chapter to show,
in a very definite way, that this is not mere figurative language but a
common-sense statement of fact.
The cycle of plant life can be illustrated by any dicotyledonous,
herbaceous annual. If one is so inclined he may hark back to his high
school days and plant a few Beans in a box as a practical illustration
of the facts stated here.
The first action of the planted Bean is to absorb water to a prodigious
amount, and so wake the quiescent life forces which may have been
slumbering within it for years. It is a law of animal and vegetable
life that all vital processes must be performed in solution. Without
water, life is dead or somnolent.
When Nature made the Bean, she left a small opening or window in
its skin-wall called the micropyle. Through this opening of the
water-swollen seed, now issue two pale sprouts. One is long and
pointed; it is the radicle or incipient root. The other is stubbier
and is tipped by a cluster of folded, yellow-green leaves; it is the
plumule or incipient stem. With unerring exactness, the radicle grows
down into the soil and the plumule feels its way up into the air.
By this time, the seed has burst its walls and split into two halves,
which indicates that it belongs to the dicotyledonous group of plants.
As the seedling continues to grow, these cotyledons begin to shrink
and shrivel. The plant is living on their substance until it can begin
to make its own. In the case of the Bean, the stem lifts the emaciated
cotyledons up into the air, where they act as leaves until the tiny
green things at the stem’s tip have expanded into those important
organs.
When the first leaves have fully opened and the spent cotyledons have
dropped off as mere empty shells, the independent life of the plant may
be said to have begun. We are now in a position to examine its methods
of living.
Examining the root, we find that by this time it has expanded into
many branches. Each tip is a tiny mouth through which the plant drinks
the all-important water and mineral salts. Root tips exercise great
ingenuity; they feel their way underground, touching here, recoiling
there, and searching out the elements necessary to the plant’s economy
with wonderful sagacity.
The actual absorption is done by minute filaments or hairs which take
in water and its dissolved contents by osmotic action. They secrete
a digestive fluid which renders certain minerals soluble, and by a
strange intelligence, select the kind and amount of material they
take in. In certain groups of plants, notably the Legumes, colonies
of Bacteria take the place of root hairs, and by a reciprocal action,
provide the plant with the nitrogenous elements which it craves.
The principal food of most vital importance taken in by the roots is
nitrogen. Nitrogen is one of the basic elements of protoplasm, the
life fluid of the living cell. Where there is life, there is nitrogen.
Sulphur, phosphorous, silica, iron and other elements are also needed
in small quantities.
The root hairs are constructed so as to allow fluids to pass in but not
out. The continual absorption of water results in a mechanical pressure
which automatically forces the sap up through the stem to all parts of
the plant. The process is aided by the evaporation of water from the
leaves, through the partial vacuum created by them at the top of the
system. Pushed from below and pulled from above, the sap of a tree,
for instance, moves with a propulsive power greater than the blood
pressure of the strongest animal.
Above the roots and the stem of the developing plant are the branches.
Their function is too well known to need much comment. They raise
the leaves up into the air and the light. They act as conduits for
ascending and descending sap. They give the plant strength and
rigidity. Each main stem is a clever bit of plant engineering, so built
as to withstand all kinds of heavy strains and stresses.
The leaves of our seedling are extremely important parts of its
anatomy. Pluck them off and it will die in a few hours. They are
mouths, stomachs and lungs all in one. Their surfaces are broad and
flat, in order that they may catch and devour every particle of
carbon dioxide which comes their way. To us, carbon dioxide is a
negligible part of the atmosphere, but out of this intangible product
of combustion, arising from fires, breathed out by animals and expelled
by volcanoes and hot springs, the tallest tree builds its greatest
structure. Is it any wonder that it takes so long!
In the inner tissue of each leaf is a substance called chlorophyll.
It is the material which gives leaves their green colour. It is one
of the most important substances in plantdom. Under the influence of
sunlight, this chlorophyll takes the carbon dioxide of the air, and,
with water and certain minerals, makes starch, the raw material of
plant construction. This process, called photosynthesis, goes on while
the sun shines, and stops with the approach of darkness. The necessity
of plenty of light cannot be overestimated.
In the manufacture of starch, oxygen occurs as a by-product. As
the plant has no use for this element, it is breathed out from
the surface of the leaves. From the standpoint of man, this makes
plants atmospheric purifiers. At night, when the making of starch is
suspended, there is often a superabundance of carbon dioxide within
plant structures. It is this gas which is now exhaled, though in very
small amounts. Some authorities maintain that the excess of carbon
dioxide is contained in water absorbed by the roots. In the daytime
this is welcomed as additional starch material, but at night there is
no use for it.
Another substance which is always present in excess of plant needs is
water. It is essential as a tissue builder and also as a carrier of
nourishment. Its continual evaporation from the leaf surfaces furnishes
one of the sources of motive power for the circulatory system. The rate
of evaporation is controlled by the stomata, little pores or mouths
which have contractible lips. In the Lilac there are as many as one
hundred and twenty thousand stomata to the square inch. They are nearly
always located on the under surface of the leaves.
Certain plants like the Cacti seem to be able to get along without
leaves, but thick, fleshy sections of stem perform all their functions.
The Fungi and other parasites differ from most plants in that they have
no chlorophyll for starch-making but live on the already elaborated
tissue of living or dead neighbors.
When our seedling grows old enough, it marries and has a family. Among
the higher plants, the sexes are quite distinct. There are such things
as male plants and such things as female plants, but more often both
sexes occur in the same individual and frequently in the same flowers.
The Hop, Nettle, and Date Palm are one-sex plants. Maize has flowers of
different sexes on the same stem.
Flowers are the reproductive organs. In the blossom of the Bean, the
stamens are the male organs and the pistil is the female organ. The
stamens produce dust-like pollen which is conveyed by the wind to the
pistil of some other flower. Pollen grains deposited on the stigma of
the pistil are held there by a sticky secretion until they can grow
a long tube which travels down the style, eventually reaching and
fertilizing the tiny ovules or eggs.
The ovules then develop into seeds and the pistil grows into a pod, on
both of which the parent plant bends all its energies to give a good
start in the world.
The cycle is now complete. We have another Bean and are back to where
we started, ready for some other fellow to plant the new Bean and
perform the experiment all over again.
This is the story in brief, but there are many other details. The
different plants have invented and perfected all kinds of devices
to secure the effective propagation of the race. The Hazel and the
Grasses hang their stamens out in the wind in order that it may blow
their pollen to some other plant, which is waiting with feathered
pistil to catch it. Most garden plants depend on the insects to act as
pollen carriers and display gorgeous flower-petals and nectar pits with
which to attract them. Many plants aim to prevent self-fertilization by
having the stamens and the pistil come to maturity at different times.
The plants go to great lengths to secure an advantageous distribution
of their offspring. The nature of a plant is to live by growing. When
it has reached a prescribed height, it must continue the process by
producing new individuals to carry on the cycle. It gives its children
a start in the world by providing them with wings, bladders, feathers,
spikes, thorns, sticky secretions, submarines, boats, and kites,
according to the method of travel they are to use. Sometimes the
matured pistil or fruit is dispersed entire. Sometimes it opens and
shoots the seeds out. The Violet and Oxalis act like veritable guns, so
vigorously do they expel their seeds. There are seed-capsules, like
those of the Primrose and Xanthium Spinosum, which open at the top so
that only a high and efficient wind can dislodge the seeds.
The problem of food storage is an important one in plantdom. Annuals
die when they have flowered and produced seed. Perennials wither but
persist for a number of seasons and sometimes many years. Those whose
stems or trunks are permanent withdraw their starch and chlorophyll
into their cambium layer where it is safe from freezing. Those which
die down to the ground each fall store up food material in underground
stems and roots in sufficient amount to get a good start the following
season. The Potato is an enlargement of the underground stem, but
Carrots, Beets, and Turnips are bulbous roots. Hyacinths, Tulips,
Daffodils, Snowdrops, Crocuses, and Buttercups all store food material
in bulbs. Practically all wild flowers which come up early in the
spring, feed upon the nutriment manufactured during the previous season.
Buds represent the foliage of the coming season. Each fall, trees and
bushes prepare for next year’s growth by putting forth miniature
shoots and leaves folded up in warm brown overcoats. At spring’s urgent
call, the buds have merely to cast aside their coverings and step out
into the warm sunlight. These buds really make a tree a community of
individuals, because each one is capable of reproducing everything that
has occurred on the plant up to that point. This is the principle on
which grafting is carried on.
The most wonderful thing in all plant structure is the plant cell.
There are anywhere from six thousand to twelve thousand of these living
units to the square inch. In their restless, moving protoplasm lies
the mystery of life--the directing energy which controls the plant’s
activities and makes it a conscious, intelligent organism.
[Illustration: IF THIS AGED CEDAR COULD TELL ITS LIFE’S STORY, WE WOULD
FIND IT FULL OF ROMANCE AND ADVENTURE]