S. p. Wiltshire 341 



appressoria appear to be formed over the same points, it seems to afford 

 some significance, the importance of which is discussed below. 



The subcuticular mycelium having been formed in the leaf, the 

 uppermost cells of the stroma begin to enlarge, and push out the cuticle 

 until they burst it open and form the conidiophores from which conidia 

 begin at once to be cut off. 



The method by which the conidiophore passes through the thick 

 cuticle on the fruit may be of interest as affording a comparison with 

 the method of entrance. There are two ways in which it does this: 

 (a) by eating away the cuticle in exactly similar manner to which it 

 infects, (6) by growth of the subcuticular stroma at right angles to the 

 surface so that the cuticle is burst off. The fungus lives typically 

 between the cuticle and the outer wall of the epidermal cells, but on 

 the fruit it does not simply force its way between the layers. It also 

 partially eats away the cuticle as it is invariably seen that this layer is 

 thinner where it covers the mycelium. Sometimes at the edge of the 

 scabbed patch the actual hypha can be seen completely embedded in 

 the cuticle. Hence there is absolutely no doubt that there is some 

 cuticle dissolving excretion exuded by the fungus in its passage through 

 this layer. 



It is not purposed to continue a description of the life- history further, 

 since in a resistant variety the fungus never reaches a stage beyond 

 that already described. In the following account of the resistance and 

 susceptibility of the hosts, reference will be made again to various 

 stages in inoculation of other portions of the plant; but the above 

 account is typical of what happens in the normal infection of a young 

 leaf. 



The Question of Immunity. 



Perhaps no other important disease exhibits such irregular behaviour 

 as these fungi as regards their immunity. For example, it is well 

 known that one variety may be heavily attacked one year and remain 

 almost immune the next, although the organism may be quite near. 

 Wallace says "it is found that certain varieties may be resistant in one 

 year and susceptible in another year under conditions which for average 

 varieties are as favourable to the disease in the one case as in the other." 

 Salmon reports that at the Wye College Cox's orange pippin had almost 

 every shoot attacked by F. inaequalis but at this station extremely 

 little of this fungus was found on the variety at all during 1914. It is 

 considered that the suggestion which at once occurs, viz. that this 



2^—2 



