Page:Popular Science Monthly Volume 20.djvu/679

Rh of the parts can be perceived. Food is taken in on all sides alike, and is worked up in the whole plasma of the cell and applied to the growth of the cell-body, to the multiplication of cells by division, or is stored up as reserve food. This is possible because the cell either lives continuously in a uniform element, water, or, if it dries up, simply suspends its life while the dryness continues. The simplest structure, a uniform medium, the most speedy performance of the functions of life, and the shortest life-term, thus go together. Now, take a plant which no longer consists of a single cell, but is a structure of a larger or smaller number of different cells, which no longer lives in a uniform, constant element, but has its body partly in the ground and partly in the air, so that it is exposed to all the changes to which those elements are subject. In this case a complete differentiation of the organs for different life-functions, of which there is no need in the one-celled plant, becomes essential. There must be a root to take up fluid food; leaves to absorb gaseous matters and elaborate them under the influence of light into new substances to be applied to the growth and strengthening of the plant or to be stored up in particular parts as reserve food; and, besides, particular organs, distinct from the food-organs, for propagation, of the most complicated character, such as are not needed in water-plants, where cross-fertilization, so difficult to secure in the free air, is easily and directly effected. Furthermore, since plants rooted in the ground can not move about, the advantage which change of place offers to the extension of the species is compensated for by the seeds and fruits being endowed with peculiar arrangements by means of which they can spread their kind through a wider circle.

Evidently, all these organs can not be formed in a few days, but a considerable time is needed to bring their development and the fruiting process to perfection. Additional complications now enter into the life-relations of the plant. While the fruit-organs perish, when they are separated from the stem at maturity, the life of the plant itself does not necessarily cease when they are dropped, but has a possibility of continuance dependent on a variety of conditions, the chief of which is, whether the other organs have been exhausted or not.

The prolongation of life is also affected by a great number of outer conditions. The plant has to be subjected to the changes of the seasons; it has to struggle for support in competition with other plants; it has to deal with animals, some of which are beneficial, some damaging to it; and it may or may not find a sufficiency of food in the soil in which it is trying to grow. With respect to all its external relations, it must have the faculty of adapting itself, by extending its organs in one or another direction. And this adaptation is complicated by the fact that it has to be modified to meet different conditions, at different stages of development, and that the plant can flourish only when all its stages of growth are simultaneous with favoring outward circumstances, and can be completed while these continue.