278 Studies on the Ajople Canker Fungns 



the activity of the phellogen. The result of this growth is that cracks 

 sometimes occur in the bark in the vicinity of the infected portion (see 

 Fig. 9) ; but although mycelium is sometimes found in such places, its 

 occurrence is not frequent enough to suggest that infection by Nectria 

 is secured by this means. 



The cork layer, however, is not always developed quickly enough to 

 confine the canker fungus to the outside of the barrier. The scab fungus 

 is able to penetrate suberised tissue and its normal procedure is simply 

 to grow through any cork layer formed below it, especially at the edges 

 of the infected region. Nectria appears to follow the Venturia to some 

 extent. The antagonism which might be expected to be exerted by the 

 scab fungus appears to be quickly overcome and Nectria subsequently 

 dominates the situation. Its hyphae begin to grow inwards between the 

 cells of the cortical tissue, which towards the outside of the stem, includes 

 very few intercellular spaces. The penetration of the cortical cells, how- 

 ever, is not general, but confined at first to a few strands of mycehum, 

 which appear to be formed in the following manner. One hypha or a 

 strand of a few hyphae pushes its way somewhat deeply into the tissue, 

 travelhng almost invariably through the middle lamellae. Other hyphae 

 follow pushing their way alongside the original hypha which of course 

 continues its growth. In this way a whole strand of mycehum is built 

 up consisting of 20-30 or even more hyphae. Several such strands can 

 frequently be found radiating out from an infected scab pustule. In 

 Fig. 11 an excellent example of a young hyphal strand {A) penetrating 

 from the subepidermal stroma (C) can be seen; in the same figure a 

 more fully developed strand of many hyphae can be recognised at (B). 

 In Fig 9 also it is possible to follow the subepidermal mycelium along 

 from the place of the original scab infection towards the left, where a 

 strand of mycelium is seen penetrating inwards towards the wood. These 

 strands of mycelium are very characteristic of this type of Nectria in- 

 fection. If the cork layer is not in an advanced stage of development, 

 the Nectria hyphae are capable of penetrating between its cells through 

 the middle lamellae of the cell walls. Fig. 12, which is a photograph of 

 the section adjacent to that of Fig. 11 in the same series, shows such a 

 stage. At (D) the mycelial strand consisting only of very few hyphae is 

 penetrating the immature cork layer. The hyphae could be traced back 

 to a much larger strand which is shghtly out of focus in the photograph 

 but which can be seen at (B) and which originated from the subepidermal 

 mycehum (C). The young strand (A) (corresponding to (A) in Fig. 11) 

 which has not yet reached the phellogen can just be distinguished. Once 



