BIOLOGIC FORMS 155 



mycelium producing conidia were obtained on the barley 

 leaves. 



We may mention here the hypothesis, advanced by 

 Salmon, as to the factors determining infection, and the 

 explanation given of the susceptibility shown by injured 

 leaves to the attacks of biologic forms which are unable to 

 infect normal leaves of the species in question. It seems 

 clear from the result obtained in the following experiment 

 that the immunity or susceptibility of a given species in no 

 way depends on any structural or anatomical peculiarity of 

 its leaves (nature of the cell-wall, the presence of hairs, 

 ribs, etc.). If a leaf which has proved persistently immune 

 to the attacks of a certain biologic form is cut with a razor 

 in such a way that the epidermal cells on one surface, and 

 all or most of the mesophyll tissue are removed at the cut 

 place, but the epidermal cells on the other surface (oppo- 

 site the cut) are left uninjured, and then conidia or asco- 

 spores of the same biologic form are sown on the cuticular 

 surface of the uninjured epidermal cells over the wound, 

 infection will now result. It is necessary to assume, there- 

 fore, that the physiological characters possessed by the 

 leaf-cells are the determining causes governing infection 

 and immunity. 



On general grounds we may assume the existence of 

 enzymes, or toxins, in the cells of the fungus, and of anti- 

 toxins or similar substances in the cells of the host-plant. 

 We may suppose further that the leaf-cells of the various 

 host-species wheat, barley, etc. contain, or are able to 

 produce, substances peculiar to each species, which, when 

 the leaf is uninjured and the cells are vigorous, are able tp 

 prevent the successful attack of any mildew except the one 

 biologic form which has become specialised to overcome 



