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 zoospores 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 zoospores. First, gametes and zoospores 

 grade into each other in size. Second, it has been observed that 

 small zoospores may fuse and, therefore, behave as gametes when 

 poorly nourished, or grow directly into new plants and, therefore, 

 function as zoospores when well nourished. Thus a zoospore-like 

 cell may be a zoospore or gamete according to conditions. Such 

 is the evidence supporting the theory that sexuality arose through 

 the fusion of zoospores 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 



