TtiALLOPHYTES 



^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 . 



w hich is composed usually 



of sixteen similar rp)l s 



h eld top;ether bv a s^elat i- 



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. — I n 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 differen t 

 from ttifl vpgrptativp rplk, assume the function of eirgs. Other ceHs 



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. 



divi de to fo^m p; rotips of s-berms . which hang together in a plate, but 

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

 (egg and sperm) h ave become very dissimila r, 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 anlherozoid or spermatozoid, is better called the sperm. The 



