340 



GEXERAL BIOLOGY 



The third line represents alternation of generations, as we 

 have studied it among the higher plants ; substitute buds for 

 the spores, and it would represent equally well alternation 

 of hydranth and medusa in the hydroids. The fourth line 

 represents polyembryony, as just described. It also repre- 

 sents the conditions found in the alga Coleochete, in which 

 the fertilized egg breaks up into eight zoospores, each of 

 which then develops an independent bisexual plant. The 

 fifth line represents the production of sex cells upon 

 occasional members in a series of spore-bearing plants. 

 This occurs in Mucor and other molds. The last line 

 represents continuous spore formation, and entire absence 

 of sexual reproduction — a condition that is believed to pre- 

 vail in some of the green algae. 



III. CHANGE OF FORIM WITH ALTERNATION OF HOSTS. 



Among the shifts that organisms make for a place and a 

 living on the earth, none are more remarkable than those 



Fig. 192. The witch hazel pocket -gall aphid (Rormaphis hamamelidis) a 

 young larva; b, grown larva; c, adult (after Pergande). 



