THALLOPHYTES 



2T 



Fig. 



34- 



- Pleurococ- 



cus: the single cell with 

 its nucleus and large 

 chloroplast, and cell- 

 groups of various sizes. 



cells; 36, cell forming a new 

 colony. 



Pleurococcus (&z. 34)- The globular cell has a definite wall, a central 



nucleus, and a large lobed chloroplast whose lobes suggest several chlo- 



roplasts. These cells multiply rapidly by division, 



and they may be seen clinging together in irregular 



groups. No other method of reproduction is 



known, so that the life history is exceedingly 



simple. 



The other forms selected as illustrations are 

 colony formers, this tendency being as striking as 



among the Volvocales. 

 Scenedesmus. — This 



form represe 



plest colony, consisting o f 



two to Piprht rplls Ivinpr 



side by side the end cells often with more n r 

 Figs. 35, 36. — Scenedes- le ss conspicuous ap pendages (fig. 35). Each 

 mus: . 35, colony of four cell divides internally to form a new colony 



(fig. 36), and no other method of reproduction 



is known. 



Pediastrum. — The colony in this form is a floating or suspended, 

 more or less star-shaped plate of polygonal cells, sometimes as many 

 as sixty-four in number (fig. 37). Within any cell distinct zoospores 

 are formed, which escape from the mother cell inclosed by a delicate 

 membrane and then become 

 arranged into a new colony 

 (figs. 37-39). Sexual re- 

 production is also present 

 in its simplest form, certain 

 cells forming zoospore-like 

 cells, smaller and more 

 numerous than the zoo- 

 .spores, which function as 

 gametes (figs. 40-44) . Pedi- 

 astrum, therefore, is isoga- 

 mous, forming zygospores 

 that are resting or protected 

 cells. 



. Fig. 37. — Pediastrum: a colony of polygonal 



HyarodlCtyon. I niS is q^w^^ some of which are forming new colonies ; two 

 the well-known water net, colonies escaping from the mother colony. 



