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 teliospores 
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 teliospores, 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 
binucleated, but the two nuclei unite early in the young 
teliospore, and therefore the promycelial ce!ls and sporids 
are uninucleate. 
404. The Smuts (Order UstinaGiInaLes). The plants 
which compose this order are all parasites living in the 
