ALTERNATION OF GENERATIONS 



357 



common phenomenon of alternation of generations. This 

 occurs in the Hydroid colony when the free-swimming sexual 

 medusa originates by budding from a colony, and itself gives 

 rise to a new colony by a sexual method. This type of polymor- 

 phism is well exemplified by many of the trematodes (see p. 368), 

 and is particularly widespread among plants. Indeed, all plants 

 above the thallophytes undergo a regular alternation of genera- 

 tions. In the higher forms it is rather obscure and not easily 



FIG. 223. Polymorphism in Termes lucifugus. A, Adult worker; B, soldier. 

 Both A and B are undeveloped males or females. C, Perfect insect (male or 

 female); D, same after shedding the wings; E, young complementary queen; F, 

 older complementary queen. Enlarged. (From Folsom after Grassi and 

 Sandias.) 



described. In Mosses it is most conspicuous. The leafy 

 moss plant develops from a spore and is itself sexual and de- 

 velops eggs and spermatozoids. From these are developed the 

 spore capsule with its stalk. These remain connected with the 

 sexual plant, but are themselves the asexual generation by 

 which the spores are produced. 



742. There are still other types of polymorphism of less com- 

 mon occurrence. Seasonal dimorphism occurs, for example, 

 among some butterflies. In this the broods produced at diff- 

 erent seasons are often very differently colored, so that there 

 are summer andjall types or wet'andjiry season types. 



