Oct. 28. 1918 Plasticity of Biologic Forms of Puccinia graminis 227 



generations, during which 25 leaves of wheat were inoculated, none of which became 

 infected. Usually this includes trials from most of the generations. The rust was 

 then kept on Elymus canadensis for five generations, during which 35 leaves of wheat 



"Ro — B4 



(wheat — )" would indi- 

 30 



were inoculated without producing infection 



cate that from the rust on barley 30 leaves of wheat had been inoctilated, but none had 

 been inoculated directly from rye. 



When "strains' ' of rust are spoken of, the word is used in the sense of a biologic form 

 with a certain history without any imputation that it is in any way different from a 

 normal form. 



The terms "intermediary host" and "bridging host" or "bridging species" are 

 used in the theoretical sense — that is, hosts which apparently ought to cause bridging 

 are spoken of as bridging hosts without any suggestion that they actually do cause 

 bridging. The term "common host" is sometimes used. This means a plant which 

 is a host for the biologic forms under discussion at that particular time. 



EXPERIMENTS WITH BARBERRY AS A BRIDGING HOST 



To test the ability of barberry to break down biologic forms and its 

 capacity for " reinvigo rating " the rust parasite, inoculation experiments 

 were carried on during four aecial seasons. The teliospores were obtained 

 from wheat, club wheat, oats and a few grasses. The common barberry 

 (Berberis vulgaris) was used as the aecial host, and the geciospores pro- 

 duced were used for inoculating wheat, oats, barley, and rye. Four 

 biologic forms of P. graminis — viz, tritici, ,.,;iici-compacti, avenae, and 

 secalis were studied and ^b.o. results obtained are given in Table I. 



Table I. — Results of inoculating cereals with ceciospores of biologic forms of Puccinia 



graminis 



o Puccinia graminis trilici-compacli; very weak infection on wheat; normal infection (yi) on club wheat. 

 6 Minute uredinium. 



