THALLOPHYTES 



21 



Pleurococ- 

 cus: the single cell with 

 its nucleus and large 

 chloroplast, and cell- 

 groups of various sizes. 



Pleurococcus (fig. 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 represents the sim- 

 plest colony, consisting of 

 two to eight cells lying 

 side by side, the end cells often with more or 

 Figs. 35, 36. — Scemdes- less conspicuous appendages (fig. 35). Each 

 mus: 35, colony of four ce \\ divides internally to form a new colony 

 colony 3 ' ° e W (**S- 3^) > an( l 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. 



-rr , j- . mi • • / Fig. 37- — Pediastrum: a colony of polygonal 



»n. —This is / cells> someof which are forming new colonies . two 



water net, I colonies escaping from the mother colony. 



the well-known 



