CHAPTER VIII. 
BRANCH II. ZYGOPHYTA. 
THE UNISEXUAL PLANTS. 
236. This is an assemblage of quite diverse plants, rang- 
ing from minute unicellular species, on the one hand, to 
large seaweeds 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, ete. 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. 
288. 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, 
