658 THE POPULAR SCIENCE MONTHLY. 



by the hundred or thousand years. The differences appear conspicu- 

 ous, 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 in- 

 cluded. 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 ques- 

 tion 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 cryp- 

 togams, among the lower alga?, 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 j)os- 

 sible 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 alga? 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 foi*mation 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 to- 

 gether. 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 de- 

 grees 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 



