y6 Journal of Agricultural Research voi. xvi. No. a 



SUMMARY 



(i) The amount of spore material used for inoculation has no percepti- 

 ble effect on the result of infection or size of spores, except in so far as a 

 more extensive area of and a greater certainty for successful infection 

 may be secured. 



(2) The optimum length of incubation period in the moist chamber is 

 48 hours, thereby securing all certainty of infection without causing a 

 tendency to supersensibility. 



(3) The superficial layer of each uredinium contains larger spores, and 

 when this layer is removed the remaining spores are considerably smaller. 

 But if the uredinium is allowed to produce a new crop of spores those 

 on the surface again attain the same dimensions as the original ones. 



(4) In spore measurements 100 spores, obtained from a number of 

 uredinia, are representative of their group. Modes are a practical basis 

 for comparison. 



(5) Biologic forms are constant not only parasitically but also morpho- 

 logically. As a general rule the morphologic differences between the 

 various biologic forms are fully as great and distinct as between many 

 established species of fungi. The morphologic stability of a biologic 

 form is exhibited in the constancy of size, shape, and color of the uredinio- 

 spores of the particular form. The stemrust of oats (caused by P. 

 graminis avenae) is an exception to this rule in so far as the shape and 

 size of urediniospores are concerned, these being very plastic. 



(6) Common hosts which are congenial to different biologic forms 

 lack the ability to unify them, as they are unable to exert any influence 

 on the spore morphology. Uncongenial hosts, on the other hand, 

 almost invariably tend to decrease the size of uredinia and spores. 



(7) In computing data and comparing results it is necessary to take 

 into consideration the ecological conditions under which the rust had 

 been cultured — that is, cultural conditions should be kept as far as pos- 

 sible uniform; or proper allowances should be made for any variation 

 before final conclusions are drawn. 



(8) Adverse environmental conditions unfavorable for the host are 

 also unfavorable for the parasite, affecting the virulence and spore size of 

 the latter. 



(9) The optimum atmospheric temperature for the development of the 

 rusts studied appears to range between 66.5° and 70° F. Sufficiency of 

 water and plentiful light are indispensable for the best growth of the rust. 



(10) The age of the host seedlings, provided they are healthy at the 

 time of inoculation, has no determining affect on the virulence of infec- 

 tion or size of the urediniospores. 



(11) The length of association of a rust with its host, after the first 

 uredinia have burst the epidermis until teliospores are formed, does not 

 impair the viability of the urediniospores, nor does it exhibit any marked 

 and consistent effect on their size. 



