THALLOPHYTES 265 



ally lose the cilia and become dormant. During this period 

 the contents divide into two, four or more cells, which take the 

 form of the motile parent and escape as zoospores. They 1 in- 

 crease in size and the process ma,y be repeated indefinitely. Some 

 individuals produce many very small zoospores, which unite in 

 pairs to form new plants. They are called gametes (sex cells) 

 and since they are all alike, they are said to be isogamous. This 

 union or fusion (conjugation) of sex cells is called fertilization, 

 and the resulting cell is a zygospore or zygote. 



Pandorina (Fig. 119, e) is a colony usually composed of six- 

 teen cells, each, of which is of the same general character as 

 Chlamydomonas and Sphcerella. The only important difference 

 is that they hold together in groups by means of a gelatinous 

 substance, and that each cell has 

 the power of producing a new 

 colony of sixteen cells. They 

 produce gametes which are 

 more or less variable in size and 

 motility. However, they fuse 

 without regard to either of these 

 characters and, therefore, must Fia - 119 --^ 

 be regarded as isogamous. 



Eudorinu is a larger colony than Pandorina, and differs from 

 it in that the gametes are heterogamous ; i.e., of two kinds, large 

 and small. The large gametes may be considered as female 

 or egg cells, while the small cells may be considered as male or 

 sperm cells. The) sperms unite with the eggs (oospheres) to 

 form oospores, which in turn give rise to new colonies. 



Volvox is the highest type of the motile forms. It consists 

 of many cells arranged so as to form a hollow sphere and held 

 together by a gelatinous substance. The entire colony may be 

 large enough to be visible to the naked eye. . Each cell is of the 

 same general character and the ciliai project outward. They 

 are connected by cytoplasmic strands. The reproduction may 

 be non-sexual or sexual. In the former case a cell becomes en- 



