148 



STRUCTURE AND LIFE HISTORIES 



nature of an organ, no matter how it may be disguised, is 

 termed the science of form, or morphology. 

 "^ 140. Life History. — Every plant, in the course of its 

 existence, passes through a series of changes in orderly 

 sequence. Like an animal, every plant begins life as a 

 single cell, the egg, or the equivalent of an egg. Except 

 in some of the lower forms, the egg develops into an 



Fig. io6. — A fern {Anisosorus kirsutus), showing portion of the stem 

 above ground. 



embryo, and the embryo matures into an adult. By a 

 series of more or less complicated processes the adult 

 eventually gives rise to another egg, like the one from 

 which it came, thus completing one Kfe-cycle and initiat- 

 ing another. These various changes constitute the life 

 history of the individual. The various stages of life 

 history common to most plants are nowhere more clearly 



