Personality of plants

by Royal Dixon; Franklyn Everett Fitch

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.