Walker—Nature of Disease Resistance in Plants. 
231 
substance by the parasite which killed the host cells by diffusing 
in advance of the hyphae. As to the nature of this action he was 
not sure, but he considered the cell wall solution enzymic in 
character, and suggested the possible effects of a soluble oxalate. 
Ward (87) in his study of the Botrytis disease of lily likewise 
demonstrated the production of a wall-dissolving enzyme at the 
tips of the hyphae. Nordhausen (58) working with Botrytis 
cinerea Pers. also described its enzymic activity but in addition 
a toxic action of the fungus due possibly to oxalic acid. Smith (68) 
took the extreme position of attributing all of the action of Botrytis 
upon the host to oxalic acid. Jones (43), working with the soft 
rot bacillus (B. carotovorus Jones), ascribed the action of the para* 
site to a secreted enzyme, pectinase, and gave little importance 
to oxalic acid. Brown (9, 10, 11) improved greatly on methods 
previously used. He distinguished a macerating and a lethal effect 
of Botrytis cinerea upon the host. The former precedes the latter 
and is due to a cytolytic enzyme produced largely at the actively 
growing tips of the fungus hyphae. The nature of the lethal 
principle was not determined. Brown points out that there are 
chemical differences between cell walls and that a more thorough 
study of hemicellulose and pectins is needed. The significance at¬ 
tributed by earlier writers to oxalic acid or a soluble oxalate is 
definitely refuted. 
In these early investigations the first stages of penetration were 
not made clear. Biisgen (15) did not consider that the parasite 
penetrated the cuticle by mechanical means alone. Miyoshi (56), 
however, showed that Botrytis cinerea was capable of penetrating 
paper, collodion, and other substances by mechanical pressure. 
More recent researches with the same fungus by Blackman and 
Welsford (8) and Brown (10) go to show that it does not secrete 
a cutin-dissolving enzyme but penetrates the cuticle by mechanical 
means alone. Further instances of apparent mechanical penetra¬ 
tion have since been demonstrated by Dey (27) in the case of bean 
anthracnose, by Waterhouse (90) in the case of Puccinia gra- 
minis on barberry and by Curtis (24) in the case of invasion of 
potato by the zoospores of the wart organism. 
Importance of Cell Membranes 
Hawkins and Harvey (39) give evidence to support their theory 
that in certain varieties of potato tubers, resistance to invasion 
by Pythium deharyanum Hesse is due to resistance of the cell 
