224 



BOTANY. 



296. Closely related to Hydrodictyon is Pediastrum (Fig. 

 151), which consists of a number of cells arranged into a 

 flat, thallus-like mass. The cells at a certain stage produce, by 



Fig. 151. A, a colony of cells constituting a so-called individual of Pediastrum 

 graiiulatum ; t, cells with their contents remaining ; the white cells are empty, their 

 contents having escaped by the slits sp : a. contents of a cell (mnci ozoogronidia) 

 escaping. B, macrozoogonidia g, in the motile state, enclosed in the membrane b. C, 

 the macrozoogonidia arranging themselves in a colony, still unclosed by the mem- 

 brane b. X 400. After Braun. 



internal cell-division, a large number of daughter- cells, which 

 are of two sizes. The function of the smaller ones is un- 

 known ; the larger ones 



(macrozoogonidia) escape 

 by a slit in the wall of the 

 mother-cell, surrounded by 

 a thin membrane, in which 

 they swim freely for a time 

 (Fig. 151 B). After a 

 while they lose their pow- 

 er of motion and arrange 

 themselves symmetrically, 

 as in C, Fig. 151. They 

 soon grow together, and 



n V. !,. Kl 



tllUS form a COlOll) like 



.] n . iron j- OT1P 

 u >lle ' 



297. 111 ClofophWA 



(one of the common Confervaceae) the cells of the branching 

 filaments break up into ciliated /.oospoivs which directly 



Fig. 152. Portion of the thailus of Viva, a, 

 cells filled with /oospores (zoognnidia) ; b, 

 opening in cell-wall by which the zoospores 

 escape from the cells; c, zoospores (zoogo- 

 nidia).-After O3r*ted. 



