242 HOST PEXETRATION 



A different point of view, however, results from the experi- 

 ments of Fulton (1906). He followed the same techniques as 

 Mivoshi, using among others the following fungi: Botrytis vul- 

 garis, Penicillium glaucum, Sterigmatocystis nigra, Mucor vm- 

 cedo, Monilia sitophila, M. fructigena, and Sphaeropsis malorwn. 

 Fulton postulated a negative chemotropism, resulting from meta- 

 bolic staling products produced by the fungus itself. The germ 

 tubes showed quite as much turning toward pure water and non- 

 nutrient solutions as toward substances that were presumed to 

 act as attractants. 



Graves (1916) reinvestigated the problem of chemotropism, 

 usin^ reactions of germinating spores of Rhizopns nigricans and 

 Botrytis cinerea. Me too employed the perforated mica-plate 

 technique. His evidence inclined him toward the negative- 

 chemotropism hvpothesis of Fulton for these reasons: (a) the 

 germ tubes and hvphae turn away from the layer on the opposite 

 side of the mica plate if it is already well occupied by hyphae 

 or already contains their own staling products; (b) the germ tubes 

 and hyphae turn toward the layer on the opposite side of the mica 

 plate if it is free of hvphae and staling products, unless it con- 

 tains some other substance capable of evoking a negative chemo- 

 tropic reaction; (c) the germ tubes and hyphae, when present in 

 equal amounts on both sides of the mica plate, exhibit no turning 

 from one side to the other. Nevertheless, Graves found justifica- 

 tion also for the views of Miyoshi. In his general conclusion he 

 took the position that positive chemotropism is to be regarded 

 as one of the factors that govern penetration, but that negative 

 chemotropism is the major factor. 



It becomes of interest to follow the implications that logically 

 follow the acceptance of these conclusions. Susceptibility could 

 be attributed to the possession by the host tissues of substances 

 that attract and, conversely, resistance to substances that repel. 

 A specialized pathogen, then, is one which would react to one 

 particular substance onlv, whereas a generalized pathogen would 

 react to a variety of substances. That such is not the situation is 

 shown by the work of Johnson (1932) in his studies with Col- 

 letotrichum circinans. He found that this organism is capable of 

 penetrating such widely unrelated species as buckwheat, bean, 

 cotton, tomato, cucumber, tobacco, cabbage, castor bean, and 

 morning glory. It was unable to produce lesions, to be sure, but 



