S. p. Wiltshire 339 



and there is apparently a tendency for appressoria to be formed 

 at the place recorded. Further details on this point are referred to 

 later. 



Penetration. A point of great importance in infection is that only 

 young leaves or fruits are capable of being infected. This fact has 

 been firmly established both by observations in the field and by direct 

 experiments. In July the ordinary summer pruning was carried out 

 in the nursery at Long Ashton, and at the end of September a good 

 number of young shoots had developed from the dormant buds which 

 had thus been roused into activity. On the susceptible varieties this 

 young foliage was heavily infected while the mature foliage showed no 

 recent infection. Aderhold has noted the liability of the young organs 

 to be attacked, but recently Wallace cited instances where V . inaequalis 

 spread rapidly amongst apples in storage. Whilst under certain 

 conditions therefore mature organs may be successfully infected, it 

 appeared certain in the case of the trees under inspection at Long 

 Ashton that little late infection took place. 



To avoid possible misunderstanding it should be explained that the 

 use of the term "infection" in the previous paragraph includes not 

 merely the penetration of the fungus into the tissues of the host, but 

 also its development afterwards to such a degree that the infected area 

 carrying a crop of conidia can be seen with the naked eye. It is 

 possible that penetration may occur on older foliage and fruit: but if 

 so, the fungus appears unable to grow to any appreciable extent in the 

 tissues of the host. 



In view of the above it is clear that normal penetration of the cuticle 

 and further growth of the fungus must be followed on very young 

 leaves. When the appressorium is firmly established a growth of the 

 hypha into the cuticle takes place, beginning as a small bulge from the 

 centre of the attached disc (Fig. 5 and compare Figs. 11, 12). The 

 hypha usually grows directly into the cuticle at right angles to the 

 surface of the leaf, but occasionally it is inclined obhquely (Figs. 6 and 

 7). As it penetrates inwards it very often becomes enlarged, possibly 

 in order to secure the further attachment of infecting hypha. The 

 cuticle being extremely thin in the case of young leaves, the wall of 

 the epidermal cells is reached almost at once by the hypha which then 

 begins to grow between the epidermal wall and this cuticle, forming a 

 plate-like mycelium of very characteristic appearance (Figs. 2 and 3). 

 The cells at first are very thin and the plate is on y one cell in thickness, 

 but soon the fungus gets food from its host and the hyphae begin to 

 Ann. Biol, i 23 



