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. 106. A fern (Anisosorus hirsutus], 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 life-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 



