268 



BOTANY: PRINCIPLES AND PROBLEMS 



Chlamijdomonas (Fig. 143) is a motile type, its cells containing a 

 single chloroplastid, a red pigment spot, some contractile vacuoles, 

 and two cilia.* The cell may lose its cilia and divide into several 

 zoospores, each of which is capable of producing a new plant. 

 Toward the end of the growing season, however, smaller motile 



.1 



Fig. 143. — Chlamydomonas. A, individual plant, showing the large chloro- 

 plast, the pyrenoid, the nucleus and the two cilia. B, union of two gametes. 

 In most instances these gametes are equal in size, but in the case figured they are 

 distinctly different. (B after Goroschankin) . 



cells are formed which swim about and unite in pairs. These 

 are the gametes, and in this case their evolution from ordinary 

 zoospores seems to be very clear. The cell formed by their 

 union is known as a zygospore, a term applied to all cells produced 

 by the fusion of similar gametes. Such a form of sexual reproduc- 

 tion is known as isogamy. 



* This plant is very similar to members of the interesting group of Flagel- 

 lates, sometimes classed with animals and sometimes with plants, and which 

 are evidently intermediate between the two. 



