THALLOPHYTES 



'7 



the case of Sphaerella, when the zygospore germinates, the protoplast 

 divides, forming two or four cells, which escape as free-swimming cells. 



It should be remarked that reproduction by zoospores (an asexual 

 method) results in multiplying plants during the growing season; while 

 reproduction by zygospores (the sexual method) is connected with the 

 formation of a protected cell which endures unfavorable conditions. 



Pandorina. — Among Volvocales, as among the previously described 

 groups, there is a prevailing tendency to colony formation, which fi- 

 nally reaches an extreme 

 expression. Pandorina 

 illustrates a simple colony, 

 which is composed usually 

 of sixteen similar cells 

 held together by a gelati- 

 nous matrix (fig. 23). 

 The protoplast of any cell 

 of the colony may divide 

 into sixteen daughter cells, 

 which form a new colony 

 that escapes from the 

 mother cell and from the 

 mother colony. There is 

 also sexual reproduction 

 as described above, the 

 gametes being produced 

 just as are the daughter 

 colonies. The pairing gametes are similar, and therefore Pandorina 

 is isogamous and the sexually formed spore is a zygospore (figs. 

 24-27). 



Eudorina. — In this form there is a larger colony or cell family. New 

 families are formed as described for Pandorina, but sexual reproduc- 

 tion is different (fig. 28). Certain cells of the colony, not much different 

 from the vegetative cells, assume the function of eggs. Other cells 

 divide to form groups of sperms, which hang together in a plate, but 

 finally break up and fuse with the eggs. In this case the pairing gametes 

 (egg and sperm) have become very dissimilar, and hence the condition 

 is called heterogamy, the plant being heterogamous. The larger gamete, 

 often called the oosphere, is better called the egg; and the smaller one, 

 often called antlicrozoid or spermatozoid, is better called the sperm. The 



Figs. 23-27. — Pandorina: 23, the free-swimming 

 colony; 24, a gamete; 25, two gametes beginning to 

 fuse; 26, fusion of gametes almost complete; 27, the 

 zygospore. — After Pringsheim. 



