THALLOPHYTES 



21 



Pleurocoecus (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 Volvocale's. 



FIG. 34. Pleurococ- 



Scenedesmus. - This cus . the single cell with 

 form represents the sim- its nucleus and large 



plest colony, consisting of cWor P last . and cell ~ 

 . groups of various sizes, 

 two to eight cells lying 



side by side, the end cells often with more or 

 FIGS. 35, tf.Scenedes- less conspicuous appendages (fig. 35). Each 

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

 cells; 36, cell forming a new //- .,,. j .1 i j / j 



colony W 1 3 6 )> an( * no otner 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 



HydrodlCtyon. - This IS ce]1S) g^^f which are forming new colonies; two 

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



