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 (2*) 
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 
fev, 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. 
