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 Coleosporiinn senecionis, 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. hydrocotyles 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 Phragmidiiim 

 obtics7iin, Avhich had been covered by snow from December 

 18 to January 28, germinated in a day and a half in a 

 warm room.j 



* Rostrup, " Heteroeciske Uredineen " (1884), p. 6. 



t P. Dietel, "Beitrage zur Morphol. und Biol, der Uredineen" (1S87), 

 p. 9. 



