304 THALLOPHYTES 
and, since they are alike, they are isogametes. The zygospore, 
a spore formed by the fusion of similar gametes as the prefix 
(zygo) suggests, is commonly a well-protected spore and, there- 
fore, able to resist conditions that are destructive to the zo- 
ospores or vegetative cells of the plant. The approach of unfavor- 
able conditions commonly induces the formation of gametes and 
zygospores. The zygospore remains dormant until favorable 
conditions return and then produces a new plant. The zygo- 
spore is, therefore, a stage in the round of life in which the plant 
is able to survive unfavorable conditions. 
Gametes are supposed to be zoéspores that are too small to 
function alone. By pairing and fusing, the energies of two 
gametes are combined in a zygospore which is able to produce 
a new plant that neither of the gametes could produce alone. 
Thus the zygospore may also be regarded as a cell in which gam- 
etes combine their energies, so that they may be effective in pro- 
ducing new plants. There are two things which indicate that 
gametes are miniature zoéspores. First, gametes and zodspores 
grade into each other in size. Second, it has been observed that 
small zoéspores may fuse and, therefore, behave as gametes when 
poorly nourished, or grow directly into new plants and, therefore, 
function as zoéspores when well nourished. Thus a zodspore-like 
cell may be a zodspore or gamete according to conditions. Such 
is the evidence supporting the theory that sexuality arose through 
the fusion of zoédspores which, on account of size, or conditions 
of light, temperature, food, etc., were unable to function alone. 
From this simple isogamous sexuality the more complex heterog- 
amous forms of sexuality have followed. Even in some forms 
of Chlamydomonas, the gametes pairing often differ some in size 
and, therefore, suggest heterogamous sexuality. 
Pandorina. — The colony, which is one of the notable features 
of the Volvocales, varies widely in different genera, ranging from 
16 cells or less up to 20,000 or more. Pandorina, shown in Figure 
260, is one of the forms producing simple colonies. 
The cells or individuals of which the colony of Pandorina is 
formed are similar in structure to Chlamydomonas. Commonly 
the spherical colony consists of 16 individuals, held together in 
a mucilaginous matrix. 
Reproduction differs in some ways from that of Chlamydomonas 
on account of the colony formation. Any individual of the 
