WINTERING OF THE UREDO GENERATION. 51 



The wide variation in percentage of germination in collections 

 made at different times in these experiments is principally due to 

 the fact that spores at the same stage of development and equally 

 well protected can not be obtained twice in succession. It was 

 noticeable that those spores which were just mature and remained 

 well protected under the epidermis of the host were the most viable. 

 In a large number of cases such spores seemed to be as healthy in the 

 spring as they were in the fall. Those spores which broke through 

 the epidermis, dropped off from the old mycelium, and rested loosely 

 in the leaf sheath, seemed to lose their power of germination during 

 the winter and would not germinate in the spring. 



The winter of 1906-7 in Minnesota was not abnormal, and much 

 of the rust material collected was dug from under the snow and ice. 

 A thaw during a part of January incased much of the material in 

 frozen snow. About February 15 there was another thaw, and the 

 Agropyron repens material in particular became incased in ice, which 

 disappeared during the latter part of March. 



The tables show that a large per cent of the uredospores of Puccinia 

 graminis on Hordeum jubatum, on Agropyron repens, and A. tenerum, 

 collected from plants in the field, germinated throughout the winter, 

 such germinations having been made November 20, December 14 and 

 27, January 25, February 15, March 16, and April 15. After April 15 

 such spores were extremely hard to find in the locality under con- 

 sideration, as most of them had germinated in the warm, humid days 

 of early spring. Uredospores of the same rusts on Hordeum jubatum 

 and Agropyron repens, collected November 20 and 23, kept outside 

 until December 10 and then buried in snow, germinated on December 

 10, January 8, February 9, and March 20. After that date the snow 

 disappeared and the material could be kept no longer. The uredo- 

 spores on Agropyron tenerum were tested only on December 10, on 

 account of the scarcity of the material. 



Similar experiments with Puccinia rubigo-vera gave successful 

 germinations from material on Agropyron repens from the field 

 November 29, December 14 and 27, and January 25, while the small 

 amount collected on February 15 did not germinate; from material 

 on Triticum vulgare (winter wheat) successful germinations were made 

 November 20, December 14 and 27, and January 25, while after that 

 date no material could be obtained; Puccinia simplex on barley col- 

 lected in the field germinated November 20, December 14, and Decem- 

 ber 27. After that time no more could be found. 



Material of all three of these leaf rusts collected on their respective 

 hosts November 20 and 23, kept outside until December 10 and then 

 buried in snow, germinated December 10, January 8, February 9, and 

 March 20. After that date no trials were made, 



216 



