CHAPTER XII 
LIFE-HISTORIES 
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. 
173. The blue alge (Class Cyanophycez). Among the 
useful plants we have studied the only alga is the so-called 
carrageen or “Irish Moss” (see page 112), and this, as we 
shall see, belongs to one of the most highly developed classes 
of seaweeds. It agrees with the great majority of alge, 
however, in being aquatic and containing chlorophyll, and 
in being without true stem-, leaf-, or root-members. Before 
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