THE THALLOPHYTA 



269 



There are a number of other algae similar to Chlamydomonas 

 except that they arc united into loose colonies of ciliated indi- 

 viduals, and the group culminates in the large, hollow, spherical 

 colonies of Volvox (Fig, 144) which often consist of thousands 



Fig. 144. — Volvox. Globular colony of bi- 

 ciliate individuals, with three daughter- 

 colonies developing in its interior. 



^^-B 



— Volvox. A, group 

 of sperms developed within a 

 single cell. B, sperms. C, 

 fertilized egg or oospore. {After 

 Cohn) . 



of cells. In these higher forms the sperms are markedly different 

 from the eggs, and the cell formed by their union is termed (as 

 in all such cases) a fertilized egg or oospore (Fig, 145), Sexual 



^.M^. 



nw 



Tfi '■ 



Fig. 146. — Pediastrum. A group 

 of sixteen cells united into a plate- 

 like colony. X 260. 



Fig. 147. — Hydrodictyon. Portion of 

 a net-like colony, individual cells forming 

 the meshes of the net. X 310. 



reproduction of this type is known as heterogamy. It should be 

 noted that both zygospores and oospores are typically thick- 

 walled cells which are capable of resisting such unfavorable condi- 

 tions as drought and low temperature, and of germinating to 



