Page:Popular Science Monthly Volume 15.djvu/761

 are restored to the light the starch reappears in the chlorophyl of the cells.

With this dependence of assimilation on the presence of chlorophyl a new physiological division of labor is introduced into the life of plants. In the higher plants, while the work of assimilation is allocated to the chlorophyl-containing cells, that of cell division and growth devolves on another set of cells, which, lying deeper in the plant, are removed from the direct action of light, and in which chlorophyl is therefore never produced. In certain lower plants, in consequence of their simplicity of structure and the fact that all the cells are equally exposed to the influence of light, this physiological division of labor shows itself in a somewhat different fashion. Thus in some of the simple green algae, such as Spirogyra and Hydrodictyon, assimilation takes place as in other cases during the day, while their cell division and growth takes place chiefly, if not exclusively, at night. Strasburger, in his remarkable observations on cell-divisions in Spirogyra, was obliged to adopt an artificial device in order to compel the Spirogyra to postpone the division of its cells to the morning.

Here the functions of assimilation and growth devolve on one and the same cell, but, while one of these functions is exercised only during the day, the time for the other is the night. It seems impossible for the same cell at the same time to exercise both functions, and these are here accordingly divided between different periods of the twenty-four hours.

The action of chlorophyl in bringing about the decomposition of carbonic acid is not, as was recently believed, absolutely confined to plants. In some of the lower animals, such as Stentor and other infusoria, the Green Hydra, and certain green planarise and other worms, chlorophyl is differentiated in their protoplasm, and probably always acts here under the influence of light exactly as in plants.

Indeed, it has been proved by some recent researches of Mr. Geddes, that the green planarias when placed in water and exposed to the sunlight give out bubbles of gas which contain from forty-four to fifty-five per cent, of oxygen. Mr. Geddes has further shown that these animals contain granules of starch in their tissues, and in this fact we have another striking point of resemblance between them and plants.

A similar approximation of the two organic kingdoms has been shown by the beautiful researches of Mr. Darwin—confirmed and extended by his son, Mr. Francis Darwin—on Drosera and other so called carnivorous plants. These researches, as is now well known, have shown that in all carnivorous plants there is a mechanism fitted for the capture of living prey, and that the animal matter of the prey is absorbed by the plant after having been digested by a secretion which acts like the gastric juice of animals.