DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 56 1 



extent by chilly nights with alternating warm days. Cluster cups that 

 originate from spores produced on the wheat plant, develop aecio- 

 spores, which will infect only wheat plants. If it should happen that 

 these aeciospores are blown to rye, oats, barley and rye, no infection 

 takes place, so that the same specialization of spores form is noticeable 

 here as with the uredospores. 



In America, the barberry shrubs are extremely rare and to account 

 for the completion of the life cycle on this side of the Atlantic Ocean, 



4 



Fig. 204. — Germination of the chlamydospores of Tillelia falens several days 

 after being placed on moist plaster of Paris slabs, c', showing conjugating basidio- 

 spores. (^After Bull. 57, Univ. III. Agric. Exper. Slat., March, 1909.) 



recourse has been had to amphispores, which are thick-walled stalked 

 urediniospores produced in the western states under more or less arid 

 conditions, but Arthur thinks that the perennation of urediniospores 

 alone is sufl&cient to explain the recurrence of the disease on the wheat 

 plant in succeeding years. 



It should be emphasized also that within the species of black rust, 

 there exist several specialized forms, more or less adopted to their own 

 36 



