THALLOPHYTA: ALGAE 



33 



on photosynthesis. The cells are spherical and solitary. They have a 

 single nucleus and a cup-shaped peripheral chloroplast usually without 

 a pyrenoid. Chlorella resembles Chlorococcum except that it produces 

 only aplanospores, no motile cells of any kind. A protoplast divides to 

 form as many as 16 daughter protoplasts, each of which, before escaping, 

 secretes a cell wall. Gametes are unknown. 



Fig. 21. Chlorococcum infusionum. A, section of vegetative cell with single nucleus and 

 pyrenoid; B, multinucleate stage; C, appearance of cleavage furrows, isolating uninucleate 

 protoplasts with a pyrenoid fragment; D, section of nearly mature sporangium; E, escape 

 of zoospores in a gelatinous vesicle; F, two zoospores; A, X 2,000; others, X 2,700. {After 

 Bold.) 



Scenedesmus. This alga is common and widely distributed in fresh 

 water. It is a colonial form with generally four or eight cells arranged 

 in a short row (Fig. 22). The end cells often bear conspicuous spine- 

 like projections. Each cell contains a single nucleus, a large peripheral 

 chloroplast, and a pyrenoid. In reproduction, a protoplast divides 

 within its own cell wall to form a new colony that escapes as a whole. 

 Neither zoospores nor gametes are produced. 



Pediastrum. Pediastrum is a free-floating form widely distributed 

 in fresh water. It consists of a colony of cells symmetrically arranged 

 in a flat plate (Fig. 23). The number of cells may be 2, 4, 8, etc., up to 

 128, but is most commonly 16 or 32. The cells are nearly all alike, 

 except that the peripheral ones often bear short spine-like projections. 



