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

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

 contents having escaped by the slits sp : (7, contents of a cell (macrozoogonidia) 

 escaping. , macrozoogonidia (7, in the motile state, enclosed in the membrane b. U, 

 the macrozoogonidia arranging themselves in a colony, still enclosed 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- 

 a 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 JB). After a 

 while they lose their pow- 

 er of motion and arrange 

 themselves symmetrically, 

 as in C, Fig. 151. They 

 soon grow together, and 

 thus form a colony like 



opening in cell-wall, by which tfie zoospores it,.., -novonr rmp 

 escape from the cells; c, zoospores (zoogo- L ue> 



nidia).-After OSrsted. 297. In CladopJlOTtt 



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

 filaments break up into ciliated zoospores which directly 



Fig. 152. Portion of the thallns of Ulva. 

 cells filled with zoospores (scoogonidia) ; 



