SEXUALITY OF RUSTS 235 



its spores blow to the north in the spring from field to 

 field, and back to the south in the autumn. Probably 

 this is the more common mode of propagation upon the 

 Plains. Recently it has been found also that teliosporcs 

 occur on and in wheat kernels, and it is thought that 

 young plants may be infected directly from these. 



402. There are many kinds of rusts, distinguished 

 mainly by their teliospores, which are single (Uromyces 

 and Melampsora), in twos (Puccinia and Gymnospor- 

 angium), or several (Phragmidium). In many species 

 the round of life is similar to that in the Wheat rust 

 described above (heteroecious), the hosts, however, being 

 different, but in others there appears to be a constant 

 omission of certain stages. Moreover, in many species 

 all the stages develop upon the same host plant (autoe- 

 cious). 



403. Cell fusions which are now regarded as having 

 a sexual significance, and whose ultimate result is the 

 production of teUospores, have been observed in the 

 mycelium of some of the rusts. The simple sexual or- 

 gans (usually end cells of adjacent filaments) coalesce into 

 binucleate cells, which develop short hyphae of cells also 

 binucleate. In some cases these produce directly one 

 or more teliospores; in others one or two additional spore 

 forms are intercalated as aeciospores and uredospores. 

 Thus we may have either aecia or uredinia or both form- 

 ing as the first result of the sexual act, but in any event 

 the ultimate result is the production of teliospores. 

 Accordingly these several spore forms are all primarily 

 binuclcated, but the two nuclei unite early in the young 

 teliospore, and therefore the promycelial cells and sporids 

 are uninucleate. 



404. The Smuts (Order Ustilaginales). The i^lants 

 which compose this order are all parasites living in the 



