DEVELOPMENT OF PLANTS 



239 



of the seasons, assisted possibly by the different plants to which 

 the parasite became adapted. It is evident that the formation 

 of aecio- and uredinio-spores which occur in such large numbers 

 and are easily transported by the wind and germinate quickly 

 are all types of variation that would naturally be preserved since 

 they are highly advantageous to the parasite. 



95. Order b. Ustilaginales or the Smuts. — These parasites are 

 more primitive than the rusts to which they are doubtless re- 

 lated. They produce very generally but one form of spore that 

 may possibly be compared to the teliospore. They are com- 



ix 



^Ia-V*- 





Fig. 163. 



Fig. 164. 



Fig. 163. A common smut, Ustilago, transforming the kernels of corn 

 into sooty black pustules. 



Fig. 164. The formation and germination of the spore of a smut: A, 

 the formation of the spores from the mycelium in the kernel of corn. B, 

 germination of a spore and the appearance of the basidiospore. 



mon and exceedingly destructive parasites, affecting especially 

 the flowers and fruits of corn and other cereals as wheat, oats 

 and barley (Fig. 163). The damage by smuts to the corn crop 

 of the United States exceeds $2,000,000 annually. In the case 

 of corn, the parasite keeps pace with the growth of the plant 



