DIRECT PENETRATION 239 



were being pierced by the infection hypha. Biisgen (1893) veri- 

 fied Frank's observations and concluded that appressorium forma- 

 tion results in response to contact of germ tubes or hvphac with 

 a solid body. In hanging drops or in drops of liquid on glass 

 slides, appressoria generally form as soon as the tube emerges. 



Hasselbring (1906) noted the existence of appressoria 12 to 18 

 hours after inoculation in Gloeosporhim fructigenum, Allen 

 (1923) on the day after inoculation with wheat rust, and Aro- 

 nescu (1934) as early as the ninth hour after inoculation with 

 Diplocarpon rosae. Appressoria become separated from the tube 

 by a septum, their wall thickens, and eventually they become cir- 

 cular in outline, being flattened, however, on the side in contact 

 with the solid body. These characteristics led various American 

 workers who early studied the organisms causing cotton anthrac- 

 nose, apple bitter-rot, ripe-rot of grapes, and pepper anthracnose 

 to regard appressoria as secondary spores. Attention was directed 

 to this error and to their true function among anthracnose-pro- 

 ducing species by Hasselbring (1906). He also observed that 

 lack of food is a factor in their formation, since they may not 

 develop in the presence of a supply of nutrients. Similar studies 

 involving appressoria of Collet otrichum lindemathiamim and C. 

 gloeosporioides were made by Dey (1919, 1933). His evidence 

 indicated that appressoria can withstand drying and that they give 

 rise to the penetration tube only when nutrients are available. 

 It is indicated that substances diffuse out through the cuticle to 

 stimulate germination. 



In 1886 de Bary described organs of attachment that facilitated 

 penetration among species of Sclerotinia. Details of the penetra- 

 tion by 5. libertiana were presented by Boyle (1921). When this 

 fungus was placed on bean leaves, appressoria formed near the 

 hypha tips. Thev became fixed to the leaf surface by means of a 

 mucilaginous sheath. From beneath the appressoria a very slender 

 "infection hypha" then developed, which indented the cuticle at 

 the point of contact. There was no evidence of dissolution of 

 the cuticle, but eventually the infection hypha penetrated this 

 membrane by mechanical pressure. 



Diplocarpon rosae pierces rose cuticle wholly by mechanical 

 pressure, but the enlargement of the infection peg into an infec- 

 tion hyphae is interpreted by Aronescu (1934) to indicate that 

 further penetration occurs in a different manner. 



