THALLOPHYTA: ALGAE 



35 



noids, the chloroplast becoming reticulate and diffuse (Fig. 255). 

 Mature cells have a peripheral layer of cytoplasm surrounding a large 

 central vacuole. In asexual reproduction as many as 7,000 to 20,000 

 biciliate zoospores may arise from a single vegetative cell by progressive 

 cleavage of its protoplast. These do not escape but swim around 

 within the parent cell, finally coming together to form a new net (Fig. 

 25C). Later the cell walls of the old net dissolve and the young colonies 

 are set free. These grow to the adult size without any cell division. 



Fig. 24. Pediastrum horyanum. A, formation of zoospores and escape of one group in a 

 common vesicle; B and C, zoospores forming a new colony; D, a thick-walled resting cell 

 (polyhedron); E, F, G, zoospores within a polyhedron forming a new colony. {A, B, C, 

 after A. Braun; D to G, after Askenasy.) 



In sexual reproduction a single protoplast may give rise to as many as 

 30,000 to 100,000 biciliate isogametes. These escape from the parent 

 cell through a small pore and fuse in pairs to form thin-walled zygotes 

 (Fig. 25D--F). After undergoing a short resting period, the zygote 

 turns green and increases in size. It then produces four large zoospores 

 and, in connection with their formation, the number of chromosomes 

 is reduced one-half (Fig. 25G, H). As in Pediastrum, the zoospores 

 escape into the water, settle down, and become large heavy-walled 

 polyhedrons (Fig. 25/). These remain dormant until the following 

 spring and represent the real resting stage. Upon germination, a poly- 

 hedron produces 200 to 300 small zoospores that escape enclosed in a 

 membrane, where they arrange themselves to form a new net (Fig. 25 J). 

 These nets are much smaller than the ones developed later by the zoo- 

 spores arising within the vegetative cells of the colony. 



