CHAPTER XII 

 LIFE-HISTORIES 



172. Cycles of life. Every creuture 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 n(.irmally characterizes a certain kind of plant or 

 animal, we do not know it at all thor(jughly; 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, sinc(.> 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 ilo more than glance at the form antl behavior of a 

 few typical plants through the various stages of their lives. 

 We shall, however, choose examples exhibiting so \vide a 

 range of peculiarities that the student may gain finally a 

 comprehensive view of the vegetable kingdom sufficient for 

 an introduction t<.) more special study. 



173. The blue algae (Class Cyanophyceje). Among the 

 useful plants we have stuched the only alga is the so-called 

 carrageen or "Irish JNIoss" (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 algae, 

 however, in being aquatic and containing chlorophyll, and 

 in being without true stem-, leaf-, or root-members. Before 



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