1903.] 



a Biologic Forms " of the Erysiphaceae. 



117 



sown on the internal tissues of the leaf exposed by the cut, and these 

 gave the same results. 



Using this method of culture, over fifty successful experiments, of 

 which details are given, were made. In these the conidia of certain 

 "biologic forms" were induced to infect " cut" leaves of host-species 

 which are normally immune against their attacks. 



The experiments proved that the range of infection of a " biologic 

 form " becomes increased when the vitality of a leaf is affected by 

 injury, and also that species of plants " immune " in nature can be 

 artificially rendered susceptible. 



Further experiments showed that the conidia of the fungus produced 

 on a " cut " leaf are able at once to infect fully uninjured leaves of the same 

 host-species. 



In other experiments, a method suggested by Professor H. Marshall 

 Ward, with the object of avoiding lesion of the leaf, was adopted. 

 Leaves were injured by touching the upper epidermis for a few seconds 

 with a red-hot knife, and conidia were sown on the injured place. It 

 was found that the cells immediately surrounding the place of injury 

 were rendered susceptible to the attacks of a " biologic form " which is 

 unable to attack uninjured leaves of the plant in question. 



In the third part of the paper, dealing with general considerations, 

 the following hypothesis is advanced as to the actual manner in which 

 the injury to a leaf causes it to become susceptible to a " biologic 

 form " otherwise unable to infect it. It is supposed that the leaf-cells 

 of each species of host-plant contain a substance or substances — 

 possibly an enzyme — peculiar to each species which, when the leaf is 

 uninjured and the cells are vigorous, are able to prevent the successful 

 attack of any mildew except the one " biologic form " which has 

 become specialised to overcome the resistance. When the vitality of 

 the leaf, however, becomes affected by injury, this substance is 

 destroyed, or becomes weakened, in the leaf-cells in the neighbourhood 

 of the injury, so that the conidia of other " biologic forms " are now 

 able to infect them. 



The author suggests that injuries to leaves, caused in nature by 

 hail, storms of wind, attacks of animals, etc., may produce the same 

 effect as the artificial injuries described above in rendering the injured 

 leaf susceptible to a fungus otherwise unable to infect it. Conidia 

 produced on these injured places would be able to infect uninjured 

 leaves, and would spread indefinitely. Such may be the explanation 

 of a common phenomenon — the sudden appearance of disease caused 

 by parasitic fungi on plants hitherto immune. 



A case is described which, it is believed, gives evidence that the 

 injuries produced by Aphides caused leaves previously " immune " to 

 become susceptible. 



In the concluding remarks, reference is made to the antagonistic 



