DIRECT PENETRATION 243 



the organism could be isolated from the interior of these host 

 species several days after inoculation. Similarly Young (1926), 

 using Diplodia zeae y Cephalosporin?;? acremormum, Colletotrich- 

 ium nigrum, Hefomnthosporium gra???i??eu???, and other fungi, was 

 able to produce lesions or callosities on many kinds of plants not 

 normally infected by these fungi. Since the spores of a multitude 

 of different fungi must find lodgment at the surface of every 

 green plant, it is reasonable to expect that their hyphae may gain 

 entrance yet be unable to establish a pathogenic relationship. 

 That this occurs is, moreover, attested by the experiences of 

 everyone who has attempted to isolate fungi by using bits of host 

 tissues as inocula. It is not surprising, therefore, to find many ad- 

 herents to the viewpoint voiced by Brown (1934) that neither 

 positive chemotropism nor negative chemotropism plays any sig- 

 nificant role in penetration. 



Further examination of the perforated mica-plate method as 

 a working model to represent host tissue with its natural openings 

 appears pertinent. Without the arguments being followed out, 

 this analogy would appear to be in the same category as the sub- 

 stitution of glass slides sprayed with fungicides for the surface of 

 leaves and fruits in tests to determine the value of fungicides. 

 Studies on the toxicity of Bordeaux by Yarwood (1943) show 

 that it is more active on bean leaves against the urediniospores of 

 Uromyces phase oli than it is on glass slides. These results serve 

 to bring into sharp perspective the differences between in vitro 

 and i?i vivo tests for the toxicity of fungicides. 



Brown and Harvey (1927) got ready penetration by germ tubes 

 of Botrytis cinerea of epidermis from onion scales and of Eucharis 

 leaves, either inward or outward, if the membranes were washed 

 to remove diffusible substances. Similar results followed the sub- 

 stitution of membranes made of paraffin. These results and a 

 mass of similar ones by other workers show that penetration may 

 be independent of any chemotropic stimulus. 



Failure to establish response to chemotropic stimuli as a satis- 

 factory explanation for penetration has focussed attention on the 

 role of the stimulus of contact, haptotropism. It may be recalled 

 that certain species of Botrytis, Sclerotinia, Colletotrichum, Gloeo- 

 sporium, and Marssonia show their reaction to contact by form- 

 ing attachment organs. The best evidence in favor of hapto- 

 tropism as a factor in penetration comes from experiments with 



