THALLOPHYTA: ALGAE 



27 



lage derived from the cell walls (Fig. XQE). Later, when favorable con- 

 ditions return, the cells develop cilia and swim out of the mucilaginous 

 matrix. 



Sexual reproduction in Chlamydomonas occurs by the union of similar 

 gametes. These arise from a (luiescent vegetative cell by division of its 

 protoplast into 16 or 32 daughter protoplasts (Fig. 16F). The gametes 

 are smaller than the zoospores and are usually without a cell wall, but 



Fig. 16. Chlamydomonas, a free-swimming, unicellular green alga, X 1,000. A, vegetative 

 cell, showing large cup-like chloroplast with embedded pyrenoid, nucleus, eyespot, two 

 contractile vacuoles, and two cilia; B and C, formation of zoospores within parent cell 

 wall; D, two escaped zoospores; E, "palmella" stage; F, formation of gametes; G, two 

 escaped gametes; H, gametes fusing; /, zygote; /, four zoospores escaping from zygote. 



otherwise have the same structural features. They escape and swim 

 about in the water. Finally, they come together in pairs and fuse, each 

 pair forming a zygote (Fig. 16G, H). The zygote soon loses its cilia, 

 secretes a heavy wall about itself, and goes into a resting stage (Fig. 16/). 

 While the wall is forming, the two nuclei inside the zygote unite. Upon 

 germination, the protoplast of the zygote divides internally to form four 

 zoospores that escape and enlarge to become new vegetative cells (Fig. 

 16J). The reduction in chromosome number from the diploid to the 

 haploid state occurs in connection with the formation of the four zoo- 

 spores from the zygote. Because, in most species, the pairing gametes 

 are alike in size, Chlamydomonas is said to be isogamous. The fusing of 

 similar gametes (isogametes) is known as conjugation. 



In Chlamydomonas eugametos there are two sexually differentiated 



