24 



EVOLUTION AND GENETICS 



duction give rise to individuals 

 which develop into new colonies. 

 Ferns are asexual plants which pro- 

 duce many spores, small reproduc- 

 tive bodies which are able to de- 

 velop under favorable conditions 

 into plants of entirely different 

 appearance. The fern of common 

 parlance is the sporophyte (Fig. 9), 

 and the plant produced from its 

 spores the gametophyte or pro- 

 thallus (Fig. 10). The latter, hke 

 the medusoids of Obelia, produces 

 germ cells which fuse to give rise 

 by sexual reproduction to a new 

 sporophyte. 



Metamorphosis resembles this 

 process only in that the different 

 forms appear in succession; all 

 forms are a part of a single genera- 



Fio. 9.— The sensitive fern, Ono- 

 clea seiiaibilis, showing a vegeta- 

 tive leaf (left) and a spore- 

 bearing leaf (right). (From 

 Woodruff, after Bergen and 

 Davis.) 



Fig. 10.. — Gametophyte or 

 prothallus of a fern, 

 Aspidi\imftlix mas, from 

 below. (From Stras- 

 burger, after Schenck.) 



tion. Holometabolous insects show a maximum degree of meta- 

 morphosis in their transition from egg to larva, to pupa and thence 

 to adult (Fig. 6). The larva is essentially a growing stage, the 



