Christman—Wintering of Grain Busts. 107 
spores which inoculate the new leaves. Then follows a period 
of incubation which may be lengthened more or less according 
to the temperature and other conditions in the spring. 
The possibility that uredospores may be carried great dis¬ 
tances by the wind and in this way move northward to infect 
the crops in successive regions is very interesting as bearing on 
this question. Klebahn has recently attempted a quantitative 
determination of the number of rust spores occurring in the air. 
He collected spores on sheets of cotton exposed to the open air 
during the summer of 1901. His results show that in the course 
of the summer immense numbers of uredospores are to be found 
floating in the air. 'Whether sufficient floating uredospores are 
present at the first of the growing season to cause the abundant 
outbreaks of rust frequently observed, requires further proof. 
As noted above Carleton thinks that viable spores found dur¬ 
ing the winter are produced from time to time during the peri¬ 
ods of warmer weather. This, indeed, is very likely in a climate 
like that of Kansas where the temperature rises to about 60° F. 
us a maximum for each of the winter months. From the table 
it will be seen that the uredospores of P. rubigo-vera triticu 
which were gatheiod on February 6th, were either sixty-seven 
days old or had matured at a temperature not higher than 6' 
above the freezing point. It will also be seen that the tempera 
ture did not rise above 42° F. for a period of ninety-three days. 
I am inclined to think that at our winter temperature the uredo¬ 
spores may remain dormant for long periods without losing 
their vitality. Very likely, however, these spores play little 
part in producing infections in the spring, since with the first 
warm days the mycelium produces new pustules with a fresh 
crop of spores. 
As the severity of the weather must affect the amount of 
healthy host tissue that survives the winter, it must limit the 
amount of mycelium and so the number of uredospores at lianc 7 
in the spring, and is, in all probaoility, one of the chief factors 
in determining the violence of early outbreaks of rust. 
Madiso7i, Wisconsin, 
November i, 1901. 
