8. P. Wiltshirp: 345 



grow apart from the host. The facultative parasite affords a line of 

 attack of this difficult problem, for after a determination has been made 

 of what can be seen to happen to the fungus within the host of both 

 susceptible and immune varieties it might be possible to submit the 

 fungus to culture apart from the host and in media which may have 

 been directly derived from the host. If we are to attempt to enquire 

 whether the non-infection of an immune form is due to a toxin, or 

 anything of a similar nature, we must follow some such line as this. 

 Venturia is quite a unique fungus in its normal habitat between the 

 cuticle and the host: and although the process of infection and subse- 

 quent manner of growth in the host may be peculiar to itself, yet it 

 may be well to discuss it in relation to what is known of other fungi. 

 It is not proposed to review the literature on this question in any 

 detail, since these results would not warrant such an exposition, but 

 a brief outline of the principal work has been done, especially that 

 associated with Venturia may be given. 



Biisgen^ found with Venturia that the appressoria are formed 

 frequently over the epidermal walls perpendicular to the surface, and 

 endeavoured to show that diffusion of some substance takes place at 

 this place and which acts chemotropically on the germ tube. 



Aderhold {loc. cit.) holds that no particular chemotactic stimulus is 

 necessary, but that there is simply a strong diffusion of the ingredients 

 of the cell contents; this, however, he was not able to ascertain in 

 demon.strable quantities in living tissues. He held that at the junctures 

 of the epidermal cells the external cell-wall must have other properties 

 than those of the flat surfaces of the remainder of the external walls. 

 He records experiments which are not very conclusive but from which 

 he gathers that the pectates (especially potassium pectate) act 

 chemotropically on the fungus and suggests that the middle lamella 

 may so act in nature. 



In addition there is the work of Myoshi, Xordhausen, Fulton and 

 others on general questions of chemotropism, but, so far as we know, 

 no other paper discusses the question in relation to Venturia. 



The question of how the infecting hyphae pass through the cuticle 

 must take into consideration the nature of the cuticle itself. The 

 work which has already been done does not give much information, 

 being chiefly concerned with the distinction of cuticle from cork, 

 whether cuticle contains any cellulose and similar chemical problems. 



^ Biisgen, M., Ueber einige Eigenschaften der keimlinge parasitische Pilzt. Bot. Zeit. 

 1893, p 53. 



