THALLOPHYTES 85 



tinues in the- uredospore-producing mycelium, in the uredospore, and in the young 

 teleutospore. In the maturing teleutospore, however, the nuclei fuse, so that 

 the cells of the mature teleutospore are uninucleate. This uninucleate condition 

 continues in the cells of the basidium (promycelium), in the basidiospores (sporidia), 

 and in the mycelium on the barberry. Some investigators see in this nuclear 

 history an alternation of generations, the double number of chromosomes (2x) 

 being represented by the two nuclei, and the reduction division (resulting in the 

 X number) occurring in the formation of the four cells of the basidium. If this> 

 view is correct, the mycelium on wheat is a sporophy te, and the mycelium on , 

 barberry is a gametophyte. 



Other rusts. — Owing to its infrequency in those regions, it is evident 



that barberry cannot be a general host in the chief wheat-producing 



areas of North America. Much of the rust attacking the wheat in these 



fields is not P. graminis, but P. coronata, whose aecidium develops on 



buckthorn (Rhamnus), and P. rubigo-vera, whose aecidium develops 



on Echium. It has been discovered also that uredospores may. retain 



their vitality throughout the winter and attack directly the young wheat 



in the spring, thus eliminating the need of an aecidium host. It is also " 



" Tound that the hasidiospores may germinate upon very young whea t 



- plants and infect them, eliminating the aecidium stage in another way ^ 



The common species of wheat rust mentioned above have now been 



broken up into numerous species and varieties upon what are called 



physiological characters. This means that althoughthey may be alike 



in their appearance, they can be distinguished by their behavior in 



the selection of hosts. 



As might be expected, the complete life histories of comparatively 

 few rusts having different hosts are known. The two hosts do not sug- 

 gest one another, and therefore numerous rusts in their various stages 

 are described as Uredo, Puccinia, and Aecidium, without any knowledge 

 as to the forms that belong together in a single life history. Recentiy 

 the work of linking these forms together has gone forward with consid- 

 erable rapidity. The following list will serve as an illustration of a few 

 of the results, showing also the unrelated character of hosts : 



Uredo- Puccinia host Aecidium host 



Cereals Barberry, buckthorn, etc. 



Poa . . Buttercup 



Pea . Euphorbia* 



Senecio Pine 



Heaths Spruce 



Juniper Apple, haw, etc. 



