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the mycelium has penetrated beyond the cork layer, the host sometimes 

 makes a half-hearted attempt to stop its progress by the formation of a 

 second phellogen as shown in Fig. 13. The Nectria, however, is powerful 

 enough to penetrate the new phellogen in the same way as it did the old 

 one, provided that cork formation has not yet taken place. 



From the appearance of sections of old infections, the fungus seems 

 to be capable of secreting some substance, probably of the nature of 

 an enzyme, which is able to attack the cell walls of the cortical tissue. 

 These are not totally destroyed but they lose their power of staining 

 with Fuchsin and the tissue so affected appears indistinct and dis- 

 organised. Whether the secretion acts in advance of the Nectria 

 mycelium is rather difficult to determine exactly but it appears to be 

 probable, for cells on the outside of the infected area frequently have 

 their contents coagulated and the cell walls rather heavily stained, 

 before mycelium can be found among them. This stage, the first step 

 in the disintegration of the tissue, is followed by the loss of the 

 staining powers and by the advance of the mycelium in the middle 

 lamellae between the cells in the intercellular spaces and in the cells 

 themselves. The nature of the secretory substance has not been in- 

 vestigated. If present, it probably begins to take effect from the very 

 early stages of infection and is perhaps a potent factor in overcoming 

 the resistance of the host. 



In this connection, it might be well to refer to some infection experi- 

 ments carried out some years ago. In these it was sometimes found that 

 if Nectria conidia were placed on superficial wounds on cut shoots under 

 a bell-jar, the fungus penetrated to the cortex even when no inter- 

 cellular spaces were exposed and before any cork layer was developed. 

 The delay in forming a wound cork was considered to be due to the 

 dormant and unhealthy condition of the host, as most of the experiments 

 were made in winter and all of them on cut shoots which could not be 

 expected to have the same vigour as the living tree. Normally Nectria 

 mycelium grows in the intercellular spaces of the cortex and as these 

 do not extend to the outside layers of the cortex except at lenticels it 

 was obvious that in infecting through superficial cuts not over lenticels 

 the fungus would have to pass through the cell tissue. The way this was 

 brought about was by the solution of the middle lamellae of the cell 

 walls. Repetition of the experiments during the summer, on growing 

 trees, invariably resulted in a cork layer being formed round the infected 

 portion and the latter completely excluded as mentioned above. It 

 seems clear, therefore, that the secretions of the canker fungus cannot 



