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 sporophyte, 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 one or more other species whose aecidia 

 develop upon other intermediate hosts. It has been discovered also 

 that uredospores may retain their vitality throughout the winter and at- 

 tack directly the young wheat in the spring, thus eliminating the need 

 of an aecidium host. It has also been claimed that the basidiospores 

 may germinate upon very young wheat plants and infect them, but the 

 claim is very doubtful. 



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 although they 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 

 fevv 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. Recently 

 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. 



