Part 172. Cycles of life. Every creature which completes its span of life passes through various stages of development from germ to adult, and may in turn give rise to similar germs which may continue the process in endless round. Hence, until we are acquainted with the cycle of changes which normally characterizes a certain kind of plant or animal, we do not know it at all thoroughly; but as with adult structures so with life-histories, the knowledge of a few typical examples gives a general knowledge of many because of the inheritance among kin of fundamental resemblances. Moreover, since the life of the individual, as we have seen, more or less clearly repeats the series of ancestral forms, a knowledge of life-histories throws an important side light upon the relationship of different groups and helps us to picture the earlier stages through which a type has passed in its evolution.

In the comparatively small space here available we cannot hope to do more than glance at the form and behavior of a few typical plants through the various stages of their lives. We shall, however, choose examples exhibiting so wide a range of peculiarities that the student may gain finally a comprehensive view of the vegetable kingdom sufficient for an introduction to more special study.