Uredospores. 35 
as it does in September and October, the frosts of winter 
soon destroy their foliage, so that the uredospores have to 
reproduce themselves throughout the winter and spring 
months on the wheat plant. Rostrup* has remarked the 
same fact with regard to Coleosporium senecionzs, that when 
it occurs in localities from which fir-trees are absent it con- 
sists almost wholly of uredospores. Independently of this, 
however, the uredospores of some species are much more 
abundant than of others; in P. oblongata, for instance, 
they occur in great quantities, while in P. Aydrocotyles the 
teleutospores are very few in number, and occur in the same 
spore-beds as the far more numerous uredospores. 
Although uredospores have their vitality so easily 
destroyed by heat and dryness, they can withstand a con- 
siderable amount of cold, for freshly developed spore-beds 
of P._rubigo-vera can be found almost at any time, on 
wheat, during the winter months. Of course, it may be 
that these have been developed by mycelium produced 
from a spore-infection effected at an earlier date. I found 
that the uredospores of this species which had been ex- 
posed to several nights of frost, when the thermometer 
registered 23° F. (— 5° C.), germinated with the greatest 
freedom. Dietel found the uredospores of Phragmidium 
obtusum, which had been covered by snow from December 
18 to January 28, germinated in a day and a half in a 
warm room. 
* Rostrup, ‘‘Heterceciske Uredineen” (1884), p. 6. 
+ P. Dietel, ‘‘Beitrige zur Morphol. und Biol. der Uredineen ” (1887), 
Pp. 9 
