“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.
IF THIS AGED CEDAR COULD TELL ITS LIFE’S STORY, WE WOULD FIND IT FULL OF ROMANCE AND ADVENTURE
