324 



BOTANY. 



296. — Closely related to Hydrodidyon is Pediastrum (Fig. 

 151),w}iich consists of a number of cells arranged into a 

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



Fig. 16t. — A, a colony of cells constitnting a so-called iDdividnal of Pedia8i7nim> 

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

 contents having escaped by the slits ap : p, contents of a cell (macrozoogonidia) 

 escaping. 5, macrozoogonidia gr, in the motile state, enclosed in the membrane 5. C> 

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

 brane 6. 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- 

 o 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 gi'ow together, and 

 thus form a colony like 

 the parent one. 

 297. — In Cladophora 

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

 filaments break up into ciliated zoospores which dirfectly 



Fig. 152.— Portion of the thallus of TJlva. a, 

 cells filled with zoospores (zoogonidia) ; ft, 

 opening in cell-wall, by which the zoospores 

 escape from the cells; c, zoospores (zoogo- 

 nidia).— After CErsted. 



