;Ut> Af)pl< ami Prtn- Scab FfOHji 



The outiclo is a protective covering layer to all young part*^ of plant* 

 and hence must be capable of very rapid extension. It is not known 

 how such growth takes place, whether the outer walls of the epidermal 

 cells become gradually converted into cutin or whether the cutin itself 

 is a secretion of the cell. The cuticle, however, must not be regarded 

 as being a simple homogeneous body. Von HohneU in 1878 stated that 

 the external layers of the cuticle are free from cellulose, whereas those 

 layers bordering on the external walls of the epidernuU cells represent 

 cellulose saturated with cutin. More recently Geneau de Lamartiere 

 in 190ti has seen tit to distinguish the former as the epicuticle. With 

 all the common stains it is found that the portion of the cuticle next 

 the epidermal cells is coloured nu"»re deeply than the rest of the cuticle, 

 particularly in the projections which tit between the adjacent epidermal 

 cells: and moreover, the general staining of the cuticle suggests that 

 it has been formed in layers. The outer layers, being continually 

 exposed to complete desiccation, must be under greatly different 

 conditions from those next to the epidermis. Heuce it appears probable 

 that the cuticle has a structure of its own: and this may be an 

 iniportant factor in the penetration of the infection hyphae through it. 



The permeability of the cuticle is of interest, as regards the 

 chemotropic attraction which certain substances have on fimgal hyphae. 

 That substances can diffuse through the cuticle in some cases cannot 

 be doubted, since the copper of Bordeaux mixture slowly passes into 

 leaves of certain plants, such as potatoes, which have been sprayed 

 with it (Barker and Gimingham-). Whether any substance which may 

 render the copper in such cases soluble diffuses out through the cuticle 

 is not known, and the position is similar with regard to the diffusion 

 of substances possibly chemotropic to fungal hyphae. Nevertheless it 

 must always be remembered that such a thing may happen in nature. 



The observation tJiat the appressoria are formed frequently over the 

 juncture of epidermal cells, seems to be justified from a careful 

 examination of inoculations in surface view. But it is by no means 

 the case that they are formed only in these positions. On examining 

 300 appressoria. 26 '^^, were found to be over the centre of the cells 

 and between 14-19 % did not touch the jimctures although they were 

 perhaps near them. If we take into account the relative sizes of the 

 appressoria and the cells, this result may not appear to be particularly 

 imexpected. The area within which an appressorium if it fell would 



• See Ctapek, Biixhcmit der Pjianzen, p. 702, 1912 Ed. 



* Barker and Giminjrli.*»ni. Ann. Applied Biolotiy, To!, i. p. 19. 



