THALLOFHYTA: ALGAE 



55 



rounds a large central vacuole. When young, a cell has a large, periph- 

 eral, reticulate chloroplast with many pyrenoids. Later the chloroplast 

 often appears to break up into numerous small chloroplasts, some of 

 which have pyrenoids. 



Many quadriciliate zoospores are formed, usually in cells at or near the 

 ends of branches (Fig. 43i^). The zoospores, which are uninucleate. 



Fig. 43. Cladophora. A, portion of plant, a branching filament, X65; B, a vegetative 

 cell, a sporangium, and two escaped zoospores. X300. Each vegetative cell has many 

 nuclei and a large, peripheral, reticulate chloroplast with a large number of pyrenoids. 



escape singly through a small pore in the cell wall. They develop into 

 new filaments, but these, in turn, produce only isogametes. The gametes 

 may arise in any vegetative cell. They escape into the water and swim 

 by means of two ciha. The gametes pair and fuse, but fusion occurs, as a 

 rule, only between gametes coming from different plants. The zygote, 

 without undergoing a period of rest, gives rise to a new filament directly. 

 This plant produces only zoospores. Although alike vegetatively, the 

 gamete-producing plants are haploid and the spore-producing plants are 

 diploid. The reduction of chromosomes occurs in connection with the 



