340 Apple and Pear Scab Fimgi 



increase in size and divide so as to form a pad of cells, sometimes 

 pushing the cuticle outwards, sometimes crushing the epidermal wall 

 inwards. This subcuticular mycelium continues to grow out in all 

 directions, sometimes long arms of the mycelium projecting from the 

 original point of infection. It may at first consist of one long hypha 

 which will divide to form the mycelium afterwards, but usually the 

 mycelium spreads in a plate-like manner. The cells always remain 

 hyaline and do not become brown as in the ordinary superficial mycelium. 

 The growth of the mycelium seems to be hindered to some extent at 

 the junctures of the epidermal cells, for it is not uncommon to find one 

 epidermal cell covered with mycelium and most of the neighbouring 

 ones quite free (Fig. 4). 



The exterior walls of the epidermal cells are usually convex, and 

 projecting portions of the cuticle fit in between adjacent cells. In 

 passing from above one epidermal cell to the next the fungus almost 

 always keeps between the cuticle and the epidermal walls without 

 dissolving away the former to any great extent. Therefore it seems 

 probable that for mechanical reasons it might be difficult for the fungus 

 to pass over these junctures, and especially would this difficulty be 

 felt by the older mycelium growing out laterally from the original 

 invading strand rather than by the young hyphae meeting the juncture 

 at right angles. The attack of one of these scab fungi at first has no 

 apparent influence on the epidermal cells, the nuclei remaining in their 

 normal position which is near the lower walls ; but later various changes 

 may take place, the cell contents being frequently conglomerated into 

 a granular gumming substance, especially with artificial infections made 

 in bell jars. A substance which gives the red colour characteristic of 

 wound gum, with phloroglucin and hydrochloric acid, has also been 

 found excreted into the intercellular spaces of the leaf of the pear when 

 its lower surface is attacked. The final effect usually produced on the 

 affected leaf is the extension of the palisade layers into a kind of 

 phellogen round the scabbed area, this development frequently causing 

 the leaf to be somewhat curled. 



Sometimes in the thickened region of the cuticle above the vertical 

 walls of the epidermal cells there appears a structure resembling a 

 hypha cut in transverse section and a similar structure can occasionally 

 be seen strictly following the junctions of the cells, if an inoculation be 

 examined in superficial view (Fig. 25). It does not seem that the 

 infecting hypha always behaves in this manner before it enters upon 

 its true subcuticular position : but when it is remembered that the 



