70 BROWN RUST 



The hosts of these are indicated by their names, but there 

 is a very high probability thai their distinction depends entirely 

 .■I! accidents of weather or manipulation at the time when the 

 inoculation was made. 



Finally, in regard to another case, P. dispersa (sens, la t.) 

 the Brown 1 Just of Corn, it will be seen by referring to the 

 systematic pari thai it is divided into a number of subordinate 

 tonus or sub-species, which are for the most part only dis- 

 tinguishable biologically: though here the amount of difference 

 i- much greater than in P. graminis, and there is more to be 

 said in favour of calling these forms distinct species, as is often 

 done. For, as will be seen by the descriptions, in some of them 

 an a-cidium stage is known, in others not, though Klebahn 

 remarks that in the latter cases we might possibly find the 

 secidium, if we could trace each form to its ancestral home. 

 Moreover, in some of these cases, the teleutospores germinate 

 in the spring, in others in the autumn. 



One of the most remarkable of these forms is P. bromina, 

 on species of Bromus, from which Ward (1902) obtained such 

 important results. For instance, he showed that uredospores 

 taken from B. mollis always infected B. mollis and B. seculiims 

 and their close allies, but not B. sterilis and its allies : while 

 on the other hand those on B. sterilis would infect B. sterilis 

 and its ally B. madritensis, but rarely the other Bromes. We 

 can reason, as Ward says, that uredospores from B. mollis infect 

 that species readily " because their food-supplies and previous 

 environment have affected their protoplasm in some way which 

 makes it easier for their germ-tubes and mycelium to grow in 

 tissues which afford them the same nutriment and present the 

 same obstacles, as they have hitherto enjoyed or been con- 

 fronted with" (p. 299). They can flourish in B. secalinus 

 because here also the food-supplies etc. offered are nearly the 

 same. But in B. sterilis the resistance of the plant to infection 

 is sufficiently great to present a barrier which is incapable of 

 being overcome except by an odd spore, here and there, varying 

 from the normal. In 4, out of 148, trials, uredospores from B. 

 mollis infected B. sterilis and these might then produce spores 

 which could pass on to B. madritensis, although in no single 



