1902.] Starvation on the Parasitism of Puccinia. 139 



species which are most nearly related to B. mollis, less and less readily 

 species more remote from B. mollis, but in the same sub-genus, and 

 least readily, or not at all, species in other sub-genera. 



In the attempt to obtain some insight into what causes are at the 

 bottom of this remarkable phenomenon — -the predisposition to, or 

 immunity from, infection of the host by the parasite — it was shown 

 that anatomical differences on the part of the host-plants, such as the 

 sizes and numbers of stomata, hairs, and so forth, do not suffice to 

 explain it, since no relation could be detected between the curves 

 expressing the percentage of infection and those expressing the sizes, 

 numbers, &c, of the hairs, stomata, &c. 



On the other hand, the evidence suggested some such assumption 

 as the following. The fungus, when growing on a species of Bromus A, 

 may refuse to infect another species B, either because B secretes some 

 body of the nature of an enzyme or anti-toxin which effectually 

 keeps the mycelium of the fungus at bay, or because the fungus 

 habituated to the peculiar nutritive or other conditions afforded it by 

 the host-plant A, cannot immediately adapt itself to the very different 

 conditions offered by the species B. 



Although the attempts to isolate any such anti-toxin failed, and 

 experiments of a preliminary character to test the effect of differences 

 of nutrition yielded little or nothing of a positive nature, I showed in 

 the discussion of the probable factors concerned that some subtle 

 relations between host and parasite must be assumed to account for 

 the curious facts of immunity and predisposition on the part of the 

 former, and of capacity and incapacity for infection on the part of the 

 latter, in each case in various degrees according to the species of host 

 offered for infection, or on which the fungus has hitherto been 

 reared. 



During the past year I have attempted to pursue this subject 

 further, and limit myself for the present to the following theme. If 

 the varying infective power of the fungus towards different species of 

 host-plant is derived solely from the " nutritive conditions " afforded 

 it by the host-plant it has hitherto been growing upon, two cases are 

 possible — (1) these " nutritive conditions " may be simply the expression 

 of the power of the tissues to yield certain food-substances to the 

 parasite in proper proportions and in sufficient quantity, or (2) they 

 may imply some more subtle relations between the mycelium of the 

 fungus and the living contents of the host-cells. For instance, it may 

 be not sufficient that the food-substances suitable to the fungus should 

 exist in the cells of the host, but they must be there in a certain 

 superabundance, or presented in a certain manner, and so on ; or, it 

 may be that the fungus must be vigorous up to a certain standard 

 before it can obtain a hold on such food, and so on. 



In order to test some of the possibilities here referred to I planned 



