CHAPTER IX
SCIENCE IN THE PLANT WORLD
“Weak with nice sense, the chaste Mimosa stands,
From each rude touch withdraws her timid hands.”
Plants are profound scientists. Their knowledge may not be as broad and far-reaching as that of man, but they are more successful workers than he. With all his wonderful discoveries in physics and chemistry, man as a class has not yet learned to conduct his own body so as to make it yield the highest efficiency. In fact, members of the human race are today wearing out their frames at a faster rate than ever before. Adept at running huge mechanisms of steel, they are neglectful of those most delicate and wonderful machines which are bound up with their own life processes.
Plants are not so prodigal. Whenever they are given a chance, they develop and expand their powers in the most marvelous way. They bring out the latent strength in their beings and so conduct themselves as to conserve their energies. Whether by instinct, reason or blind force they always know just what to do and how to make the most of their heredity and environment. Their efficiency rating is one hundred per cent.
As the whole life of all plants is a scientific progression, we can only consider in the brief limits of this chapter some of the more startling instances of the marvelous sense they exhibit in dealing with Nature’s forces.
Probably one of the reasons we do not always think of plants in the human, sympathetic way we should, is that we are inclined to regard them as quiet, static objects, playthings of every wind that blows upon them. Such is far from the case. Life is motion and the plants are very much alive and very much in motion. From the tiniest cell to the largest tree they exhibit constant, pulsating movements. Many of the movements are described through so small a space as ordinarily to escape our notice, but a little observation makes them quite apparent. They all have a well-directed, scientific purpose.
What is plant growth itself but motion upward and outward? If a telescope or an instrument such as Sir Jaghadish Bose’s crescograph be trained on a healthy plant, it is possible to see the growth actually take place before the eye somewhat as it is managed in motion pictures. Travelers aver that if a Banana Plant be cut off close to the ground and the surrounding soil well supplied with water, the sturdy creature will make such strenuous efforts to destroy the effects of its mutilation that its growth may easily be perceived with the unaided eye, and a full-sized leaf produced in a single day.
Leaves and flowers are usually quite mobile. When they go to sleep, they droop and fold their edges together very carefully, sometimes to such an extent as to make themselves almost invisible. Even such an astute man as Linnaeus was once completely deceived by some sleeping specimens of Lotus. They were very fine red flowers and he was proud of them. Taking a friend to view them one evening by lantern-light, what was his dismay to find that they had completely disappeared. He concluded that they had been stolen or eaten by insects and went away, only to find them in full array on his return the next morning. It took several nocturnal visits to unravel the mystery and discover that the flowers folded themselves and retired so adroitly into the surrounding foliage each evening that they were completely hidden.
The Acacia is a plant which closes up at night; the same phenomenon is very striking in the Oxalis. The common Bean sleeps standing: that is, its leaves close upward instead of downward. The little blue Veronica flower, so strikingly brilliant and attractive in the daytime, tucks itself in so snugly at bedtime that it becomes quite inconspicuous. A Marigold called Calendula Pluvialis even contracts its corolla every time the sun is veiled by a passing cloud. These sleep movements all have a scientific purpose. Their main object, just as in animals, is to reduce bodily activities to a low ebb and so to give the plant a chance to recuperate for another day’s efforts. The contraction of all surfaces cuts down the radiation of heat and moisture and presents less resistance to outside elements. The plant is in a quiescent, somnolent state.
There are other movements of leaves and flowers the object of which is not quite so apparent. For instance, there is the Hedysarum Gyrans or Oscillating Sainfoin. Each of its leaves has three folioles. The center one is very large and stands bolt upright, except at night, when it condescends to bend its head in sleep. The two lateral folioles are in perpetual oscillation both day and night. Nothing but a very hot sun seems able to stop their movement. Possibly, this plant is a fresh air fiend which requires a steady atmospheric flow upon its respiratory surfaces! The two lateral folioles of each leaf are delegated to act as fans and blow a constant supply of air upon their majestic brother.
Similar oscillations have been noticed in some Orchids, where a part of the flower’s corolla rises and falls with a regular rhythm not unlike the beating of a human pulse.
The stamens and pistils of flowers sometimes have the power of movement. If an insect, wandering about in the flower of the Barberry Tree (Berberis Vulgaris), happens to touch the base of a stamen, it bends forward with a quick, spring-like motion and presently straightens up again. The evident intent is to shower some pollen on the little intruder with the hope that he may carry its vital principle to some neighbour of the same species.
In the Parnassia Palustris, fortunate observers have sometimes seen the five stamens bend forward and beat on the head of the pistil in rotation as if on an anvil. Perhaps outside pollen-carrying agencies have passed this particular flower by and, in desperation, it is resorting to self-fertilization.
The Junger Mania, a plant allied to the Mosses, shows knowledge of the laws of mechanics when it uses a natural spring coiled in a small tube to project its seeds out into the world. Seeds of fresh-water Algae swim about for a few hours after leaving their mother-plant, vibrating their cilia with great rapidity. It is the ability of certain one-celled plants to move about freely which causes considerable discussion as to whether they are really not animals. The Diatoms are examples. They propel themselves through the water by oscillating their whole bodies from side to side. To reverse their direction they go backward like a ferryboat.
The ancients as far back as Aristotle recognized the sensitiveness of plants to light and their eager use of its life-giving properties. In fact, one has only to watch the Sun-Flower follow the orb of day across the heavens to realize that there must be something vital in sunlight for the plants. What interests us is that they have the instinct or the knowledge to so present their surfaces to the light that they receive a maximum benefit from its influences. From the aristocratic indoor potted plant to the wild trees and shrubs on the edge of a thicket, we notice a vigorous straining toward the light. Each leaf is tilted at just the right angle to receive the largest possible share of energy, for the leaves are starch factories for which the sun furnishes the motive power.
Botanists tell us that this heliotropism or turning motion toward the light is due to the tendency of most leaves to arrange themselves perpendicularly to the sun’s rays. Tendrils may be apheliotropic or tend to turn away from the light. Morning Glories or Wistaria, which climb up whatever support is handy, exhibit insensibility to light no matter from what angle it strikes. Stems, flower and leaves of all plants each give a different and scientific reaction to light in a way which looks much like directing thought.
Nothing is more scientific than the skill with which plants co-operate with gravity in constructing their root systems. The roots are often trained to grow out horizontally and resist gravity for a certain distance. Then they gracefully yield to its pulling power, and, curving their tips downward, grow straight toward the center of the earth. Any secondary roots which are sent out again start horizontally to repeat the above process on a smaller scale. All this makes for an efficient, well-balanced root-system.
A curious motion which is not thoroughly understood is a slight gyratory movement observable in the tips of all living plants. It is possible that it is connected in some way with the earth’s rotation or is it merely a kind of groping, feeling gesture? In the case of roots, where the same gyrations occur, it undoubtedly serves that purpose. A revolving root tip makes a very efficient drill with which the hardy plant may bore a way through refractory soil. It is claimed that the great whirling sweeps made by tendrils of various climbers are merely amplifications of the circumnutation occurring in all plant terminals.
Before leaving the subject of scientific movement in the plant world, it will be of interest to briefly consider some of the vegetable motions which are called forth by the stimulus of touch. Almost everyone is familiar with the Sensitive Plant and its double rows of tiny leaves. Touch any one of them and the whole group will instantly begin to contract and bend toward the stalk. We say begin, for so slow is the transmission of the impulse that one can readily see its progress, as one after another of the leaves respond.
A motion which has forethought and design behind it occurs in the leaves of the famous and crafty Venus Fly-Trap. Two sections of leaves edged with teeth-like nerve-hairs form the two halves of an enticing-looking bowl and cover. The slightest contact with one of the delicate hairs will cause the trap to shut together and imprison any sweet-toothed member of the insect world which has happened to stray inside. An aquatic form of the same thing occurs in a species of Bladderwort which spreads a leaf-net cunningly shaped to look like a fish’s mouth. Frightened baby-fishes, accustomed to seek their mother’s throat in time of danger, sometimes swim in and, brushing certain nerve-hairs near the entrance, cause the lips to close and leave them to slow dissolution. Both sinister and scientific are the movements of carnivorous plants.
Far from being static or quiescent, the plant world is a kingdom of energetic, vibratory motion—a motion which is cool and calculating and which rarely fails to accomplish its purpose. Even the protoplasm of microscopic plant cells is in constant movement. If a thin slice of Sycamore bark be placed under a microscope, a regular circulation of cell-liquid, suggestive of blood circulation in animals, can be observed.
Plants show great skill in their use of water. It is their storage of liquid in their cells which makes their soft bodies rigid and so makes movement possible. This property sometimes called turgidity was discovered by the scientist De Vries in 1877, the same year that Pfeffer established the theory of osmosis. This latter is a phenomenon which physicists find very difficult to explain and involves the transmutation of one liquid into another through the medium of an intervening membrane.
Some plants have acquired the faculty of storing water in their bodies, on which, camel-like, they can subsist for long periods of time. A certain large tree-cactus of the American desert sometimes stores up as much as seventeen hundred pounds or five barrels of water in the wet season. When drought comes, its roots dry up and it lives entirely on its internal resources. It is said that an eighteen-foot specimen can exist for a year on its stored-up liquid. A branch on such a plant may live and bloom after the trunk is dead. Many ordinary plants, such as Turnips, Carrots, and Beets, store water along with starch and dextrose in their underground tubers. Such subterranean reservoirs are preferable to those above ground.
Plants have paid particular attention to the manipulation of gases. They maintain an internal atmosphere of their own composed of oxygen, nitrogen and carbon dioxide in proportions varying greatly from those of the outside air. If the stem of a Water Lily be broken below the surface of a pond, gas bubbles will often be observed to issue from the wound, indicating that the internal gas pressure of this particular plant is greater than that of the external air. In other cases, the reverse is true and we find partial vacuums within the bodies of plants.
Man long ago found it impossible to “live on air” but the plants have solved the difficulty of aerial existence and have become creatures of the air rather than the earth, so far as their food is concerned. The great bulk of the largest tree is preponderantly composed of carbon, which has been slowly and labouriously extracted from the air. The mineral salts and water which have been filtered out of the ground by the roots are essential but are present in a much lesser quantity.
It is well known that plants breathe in carbon dioxide and breathe out oxygen. This can be graphically demonstrated by placing a plant in a glass jar of carbon dioxide inverted in water. If its life processes are quickened by exposure to sunlight, the plant will replace the CO₂ with oxygen in a day. A more striking example is furnished by any aquatic plant accustomed to growing submerged in ponds and rivers. Placed in a water-filled bottle inverted in a pan of water, it will generate oxygen so rapidly that the bubbles can be seen forming on the leaves when the sun is allowed to strike them fully. The bottle will become filled with oxygen in a few hours, and its presence can be demonstrated with the usual ember test.
Opposed to the absorption of carbon dioxide and the breathing out of oxygen, which is really a digestive operation, the plants, queerly enough, carry on a directly opposite process which involves the absorption of oxygen and the breathing out of carbon dioxide. This is a respiratory process akin to breathing in animals. It is carried on in such a relatively small way that it does not seriously affect the statement that “plants breathe in carbon dioxide and breathe out oxygen” and so are purifiers of the air which man and animals contaminate.
Besides this general use of gases common to nearly all plants, a few of the members of the vegetable world specialize in the production of protective and poisonous vapours of various composition. One of the most interesting of these is the Gas Plant of the South American jungles. This beautiful white-flowered inhabitant of the tropics is entirely protected from leaf-destroying insects and birds by the poisonous vapours it constantly pours forth.
The plants are expert chemists, and the reactions in which they engage are, on the whole, much simpler than those which go on in the bodies of animals. Vegetable tissue is largely carbon, hydrogen, oxygen and nitrogen. It is a curious fact that instead of using the abundant carbon compounds present in decomposed animal and vegetable matter of the soil the plants get most of their carbon from the carbon dioxide of the air. Inversely, they largely disregard the seventy-eight per cent nitrogen of the air, and extract that element from the complicated compounds found in the soil, or take it from the air only by aid of certain Bacteria.
Certain plants manufacture lime and metallic oxides with which to harden the protective armour they wear. Many others generate nitric acid, carbonic acid and ammonia for use in their interior laboratories. Roots nearly always secrete a fluid which aids in the absorption of minerals from the earth. It is so powerful that quartz, flint and limestone are often scratched and corroded by its action. Above and below ground, plants are active chemical laboratories.
The differences of taste, smell and colour which characterize leaves, blossoms and fruits are due to the presence of various organic compounds. These are largely volatile oils which are more complex than the substances involved in the simpler life processes. The slow or rapid evaporation of these oils influences the strength and character of an odour. When a flower or fruit passes through infinite gradations of colour, we can give no adequate account of the chemical changes involved. All we can do is to observe and to note. Sometimes infusions of iron sulphate or other chemicals in the soil darken the hues of flowers. Gardeners profit by this fact in the cultivation of certain varieties of Hortensia.
The chemical activities of plants are of incalculable value to man. They change air, water and mineral salts into forms easily assimilable by the human system. Eliminate all the vegetable life from this planet, and the animals, including man, would perish in a few months. Man has also learned to make abundant use of plant substances for innumerable purposes. Potash is an example of how the plants come to our aid in furnishing us a valuable chemical. It is extracted from wood, Seaweed and Banana stalks. These plants have discovered a way of getting it out of its well-nigh insoluble earth combinations with silica. If it had not been for certain industrious sea plants, man would probably never have been aware of the important chemical twins, bromine and iodine, so important in photography. These plants patiently filter them out of sea water where they exist in microscopic quantities, and build them into their bodies. Beer is possible because germinating grains transform amylum or plant starch into sugar. We find ripe fruits palatable because their acids change into sugar under the influence of sunlight.
Man seems to have outstripped the plants in the use of light, heat, electricity, and other physical forces, but the plants have more engineers among them than we imagine. In the fact that man has just learned to extract nitrogen from the air by the agency of electrical discharges, lies the probable explanation of how the plants have been doing the same thing for years. It is believed that the minute electrical discharges continually going on between the different air strata make small quantities of nitrogen assimilable for the plants. The micro-organisms which also furnish nitrogenous material to the plants may get nitrogen from the air in the same way. It is quite certain that the plants are affected by the chemical state of the atmosphere.
Everyone knows what an important part light plays in plant physiology, but the fact that certain plants produce their own lights, while generally known, is not universally understood. The Austrian naturalist, Heller, was the first to demonstrate that the glowing of decayed wood at night is caused by emanations of light from Fungus growing in the cavities. A similiar organism called Luminous Peridineas (sometimes classed as an animal) is responsible for the phosphorescence of the ocean and the night lights of many flowers.
About three hundred species of Bacteria and fifteen species of Fungus are recognized to be luminous. The dead leaves of the tropical Banibusa, Nephelium and Aglaia often glow at night with the light of these tiny creatures. Ordinary dead Oak and Beech leaves are luminous, sometimes shining in spots, but frequently glowing throughout with a soft, white, steady light. These miniature incandescent lights often shine for days, weeks and months, and with abundant nutriment at hand, sometimes for years. The light is slight in intensity, but uniformly steady and white, green or blue-green in colour. It is strong enough to enable the plants on which the Fungus grows to photograph themselves by long exposure to sensitized plates. The fungus light has also been used to influence the heliotropic movements of plant seedlings. In fact, a colony of Fungus has sometimes been placed in an electric light bulb and made thus to serve as an illuminant.
No matter from what angle we study the plants, we find that they are extremely scientific. They conduct themselves and all their activities in a way to always get the best results. They show knowledge and acquaintance with all of Nature’s laws, and they have learned to apply many of them with startling success.
MODERN NATURE WORSHIPPERS
