Page:Popular Science Monthly Volume 20.djvu/678

658 by the hundred or thousand years. The differences appear conspicuous, not only in the view of the whole vegetable kingdom, but also in single divisions of the same, and whether high or low forms are included. Inasmuch as these conditions are generally known and may be observed by every one who looks into the fields and woods, it may be of some interest to consider more closely the great diversities in the length of the life of plants. Such a consideration will afford a higher interest if we institute in connection with it a comparison of the manner in which the longevity of plants is related to their various systematic affinities; and further, if we include in our survey the question whether the different classes of longevities stand in immediate juxtaposition, or whether there are transitions between them. The results of the latter study will then lead us to the inquiry, by what causes the different periods of life are determined, and how they have been developed.

We begin with a comparison of the life-term and the manner of vegetation of the plant. The shortest periods are found in the cryptogams, among the lower algæ, the growth of which, taking place in an homogeneous and uniform element, water, renders any considerable differentiation of the single organs unnecessary, and thus makes it possible for the whole course of life to be gone through in much less time than can be done in the case of the more complexedly organized air living plants. In some of the simplest algæ the individual, consisting of a single cell, splits into two new cells which are, or quickly grow to be, like the mother, in form, at least; the life of the mother is ended by the formation of its two children, and these in their turn go on to another division and end their lives in the same manner. So short a period of life, lasting for only a few hours or days, is not known in any phanerogamous (or flowering) plant, not even among those which grow entirely in the water. The more complicated structure even of the simplest phanerogam requires at least several weeks before it can bring its fruit to a sufficient degree of maturity to cast its seed—the essential condition which the plant must fulfill before it can bring its life to a close without endangering the continuance of the species. In the short-lived cryptogams, propagation and the end of life come together. The same is the case with a large part of the phanerogams, while other plants, both cryptogams and phanerogams, do not close their lives with a single propagation of posterity, but repeat their fruit-bearings after definite periods of different lengths. Once-fruiting plants may bear seed either in the first or the second year and then die, or after a series of years, as in the case of the American aloe, while the several-times fruiting ones may bear either in the first or second year, or after several years, and then not die; and a number of degrees of transition occur between all of these life-methods. We turn now to a closer consideration of the causes of the differences.

In the simplest plants, consisting of a single cell, no differentiation