CHAPTER XV. 



j 



ZYGOPHYTA. 



289. This is an assemblage of quite simple plants, none 

 of its members attaining any great degree of complexity. 

 For the most part the plant-body consists of an elongated 

 filament composed of united cells ; sometimes, however, 

 they form surfaces, and in other cases the plants are unicell- 

 ular, or aggregated into communities. In these plants we 

 find the first examples of undoubted sexuality, and through- 

 out the group, the organs and methods of fertilization are 

 nearly enough uniform to enable us to use them as distin- 

 guishing characters. The sexual organs all have this in com- 

 mon, that between the male and the female there is no ap- 

 preciable difference as to form, size (with a few exceptions), 

 color, origin, etc. In the sexual processes, likewise, there is 

 this in common, that the result of the union of the two 

 sexual cells is the production of a new cell, the zygospore, 

 possessing very different characteristics from either. While 

 the sexual cells have only ordinary walls, or none at all, the 

 zygospores are covered with thick, firm walls. 



290. The zygospore is frequently called the "resting 

 spore," because under certain circumstances it remains quies- 

 cent, while retaining its vitality, 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 zygo- 

 spores fall to the bottom- of the pools (in the aquatic 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. 



291. Nearly all the plants of this group contain chloro- 

 phyll, only one order being destitute of it. The green forms 

 are all aquatic, and inhabit either fresh or salt water. They 



