Page:Popular Science Monthly Volume 33.djvu/695

Rh This is evidently—in the plant-cell just as in the engine—because things are in the condition to react to slight stimulus. Thus, when a ray of light falls upon a motile swarm-spore, and it swims toward the illuminated side of the drop of water in which it is confined, the undulations of the ether may he held to have caused a more or less continuous change in the molecular structure of the protoplasm; energy is liberated, and ciliary motion in a certain direction is the final resultant. In the same way, when the sun rises in the morning, rays fall upon the stems of the sunflowers, intimate structural changes take place in the cell-protoplasm, and, through mechanical contrivances which will be mentioned later, a slow curving toward the light is effected.

This remarkable instability of protoplasm—and the writer craves the privilege of considering it only as a chemical compound of astonishing complexity—is of deep interest when considered in its relation to growth. Upon this something must be said. The growing part of a plant is, as we know, only the living part. Heart-wood is always dead wood, and is incapable of reacting to the external world except as unorganized matter might. Furthermore—and this need scarcely be mentioned, since the rudiments of botanical knowledge have become so wide-spread—the whole mass of living, growing tissue is made up of cells more or less crowded together, more or less individual in their forms and functions, but all of the same general plan of structure. If one could imagine the Capitol-dome at Washington completely filled with a densely crowded mass of toy balloons, each balloon distended with water, and containing within the water, usually surrounding most of it, a sac-like piece of sponge, it will be a fair idea of what a growing-point would be like if seen upon a sufficiently large scale. The phrase "growing-point" will be understood to have the technical significance, meaning the extreme tip or apical area of a bud or shoot. Each cell-wall contains its cell-sap, or cell-fluid, and its cell-protoplasm, which was compared to the sponges. The protoplasm is, of course, the only essential living part, and the others are but elaborations and mechanisms by which the complicated cell-life, as part of an organic whole, is possible. Or the appearance of a growing-point might be compared to the mound of small bubbles which may be blown in a bottle half filled with suds. Hundreds of bubbles, each full to bursting of air, press each other in every direction, and constitute a more or less conical and coherent mass of bubbles. It is here that the important point is to be noticed—

Growth is possible only when the cells are in a state of tension.

As one may easily discover, a flabby leaf will not increase in size, and a limp and flaccid stem is equally incapable of growth. In other words, growth of a plant-cell is like the growth of a