CHAPTER Vm. 

 BRANCH II. ZYGOPHTTA. 



THE UNISEXUAL PLANTS. 



236. This is an a&semblago of quite diverse plants, rang- 

 ing from minute unicellular species, on the one hand, to 

 large seaweeda of considerable complexity, on the other. 



237. In this branch we find the first examples of un- 

 doubted sexuality. The sexual organs all have this in 

 common, that between the male and the female there is no 

 appreciable difference as to form, size (with a few excep- 

 tions), color, origin, etc. The result of the union of the 

 two sexual cells is the production of a new cell, the resting 

 spore or zygospore, possessing very different characteristics 

 from either. While the sexual cells have only ordinary 

 walls, or none at all, the resting spores are covered with 

 thick, firm walls. 



238. The resting spore is so called because under certain 

 circumstances it remains quiescent, while retaining its vi- 

 tality, often for long periods of time. Thus at the close 

 of the growing season, as upon the advent of the summer 

 drought, or of winter, the resting spores fall to the bottom 

 of the pools (in the fresh- water forms), and in the dried or 

 frozen mud remain uninjured until the return of favorable 

 conditions, when they germinate and give rise to a new 

 generation of plants. 



