S. p. Wiltshire 277 



conditions usually prevailing in the autumn. The mycelium thus pro- 

 duced proceeds to establish itself on the stroma of the scab fungus. This 

 done, it begins to form typical conidia, which afford proof of the identity 

 of the canker fungus. Fig. 8 shows a drawing of a small pustule of the 

 canker fungus growing just over a young scab infection. The material 

 concerned was freshly collected from the plantation and the conidia 

 illustrated had developed there under natural conditions. A section 

 through a young scab infection attacked by Nectria gallifjena which has 

 reached a slightly later stage is shown in Fig. 9, the central portion of 

 which is enlarged in Fig. 10. Here the scab fungus has burst open the 

 bark and the canker fungus has attacked the mycelium of Venturia 

 inaequalis and produced numerous conidia. It is rather difficult to dis- 

 tinguish between the mycelia of the two fungi, but generally speaking in 

 a mixed stroma, the mycelium of the scab fungus appears dark and some- 

 what thick-walled whilst that of the canker fungus is hyaline, less dis- 

 tinct and not so robust. Its cells, too, are rather smaller in size than those 

 of the scab fungus. The two myceha are not absolutely intermingled but 

 are naturally divided into areas. In Fig. 10 the areas {a) and (6) are 

 probably Nectria mycelium whilst the areas (c) and {d) are probably scab. 



The penetration of Nectria galhgena from the scab stroma into the 

 cortex. As soon as the canker fungus has gained a firm hold on a scab 

 pustule, a struggle between host and parasite commences. It has been 

 shown (2) that if a cut is made in the cortex of an apple tree in such 

 a way that it does not extend to the wood, and if conidia or hyphae of 

 Nectria galligena are placed on such a wound, the host merely forms a 

 cork layer round the portion of cortex which becomes infected and the 

 fungus fails to estabhsh itself. The experiments were chiefly carried out 

 in the summer when the trees were active, and the results were so positive 

 that for some time it appeared very difficult to understand how the 

 canker fungus could infect from a shallow injury such as that effected 

 by the scab fungus. It is indeed probable that, in some cases at any rate, 

 such a cork layer is effective in preventing the development of canker 

 even in cases of Nectria infection of scab pustules. In Fig. 9 the whole 

 of the infected portion is surrounded by a cork layer formed some dis- 

 tance below the original seat of the trouble. If such a layer became 

 matured before the Nectria reached it, then it is extremely probable that 

 the latter could progress no further, unless assisted by a fresh develop- 

 ment of the scab fungus. 



With the formation of a phellogen, the cortical cells below frequently 

 begin to divide to produce new tissue in addition to that arising from 



