374 PHYSIOLOGY OF THE FUNGI 



Sclcrotinia sclerotiorum, and Collctoirichum lindemuthianum until after the 

 fungus penetrates the cuticle of the host. In other words, there is little 

 or no diffusion of the toxic materials through the cuticle. Direct penetra- 

 tion through cutinized walls is believed to be entirely by mechanical 

 pressure, since no cutin-dissolving enzyme has been demonstrated in the 

 fungi. 



The rhizomorphs of Armillaria mellea usually gain entrance directly 

 through the sound cork layer of comparatively old roots (Thomas, 1934). 

 Penetration is believed to be accomplished partly by mechanical pressure 

 and partly by chemical means. There is evidence that a suberin-dis- 

 solving enzyme aids in the destruction of some of the cork cells. Some 

 fungi may enter the same host by more than one method. Fusarium 

 lint may enter through young epidermal cells of the root, root hairs, 

 stomata of seedlings, and perhaps through wounds. 



The penetration of noncutinized cell walls may be either by mechanical 

 pressure or by the dissolving action of enzymes secreted by the fungus. 

 Hawkins and Harvey (1919) concluded that the hyphae of Pythium 

 debaryanum penetrated the cell walls of susceptible potato tubers by 

 mechanical pressure, and that the resistant varieties in general showed 

 greater resistance to mechanical puncture. They found no evidence of 

 cellulases which might aid in penetration by dissolving the cellulose cell 

 wall. Using cane sugar as the plasmolyzing solution, they found that 

 the hyphae of P. debaryanum were capable of exerting as high as 54 atm. 

 osmotic pressure. These hyphae would have a strong tendency to absorb 

 water, and as a result greater internal pressure would be exerted against 

 the hyphal wall. Apparently the hyphal wall is capable of withstanding 

 this pressure at all points except its tip, where growth occurs. The 

 pressure exerted by the growing tip is believed to be sufficient to cause 

 penetration of the host cell wall. By direct microscopic examination 

 Hawkins and Harvey observed that, just after the hyphal tip came in 

 contact with the host cell wall, it formed a swelling, back of which a bend 

 developed. This was followed by penetration of the wall by a small tube. 



Penetration through noncutinized cell walls by chemical means has 

 been described for Spongospora suhterranea by Kunkel (1915). It seems 

 likely that other nonfilamentous fungi penetrate cell walls in the same 

 way. Likewise, wood rot fungi penetrate the cellulose and lignified cell 

 walls by enzymatic action, as evidenced by the boreholes in decaying 

 wood. It may be significant that the hyphal walls of Pythium, as well 

 as of other Oomycetes, contain cellulose, while the hyphal walls of other 

 fungi are composed principally of chitin, which would not be acted upon 

 by cellulases. 



It must be emphasized that penetration of the host in itself does not 

 necessarily lead to the establishment of the fungus in the host and the 



