28 THE LARCH CANKER 



an anastomosing mass of hyphae. In this case a red-brown 

 stain is formed in the centre of the stem, irregularly star- 

 shaped in section and continuous on one side with the 

 canker, but spreading for a short distance above and below 

 it in the heart-wood. 



The hyphae are colourless arid of markedly varying 

 width ; they pass through the tracheide walls by very fine 

 bore-holes, which are usually accurately transverse, but 

 may be somewhat oblique or may even be transverse on 

 one side of the middle lamella and oblique 011 the other. 

 The whole mycelium in this region is evanescent and does 

 little damage to the wood. 



An interesting phenomenon in connexion with this attack 

 is the formation of a gum by the infected wood. This gum 

 has been worked out more in detail in connexion with the 

 larch heart-rot which also stimulates its formation. 



Under the influence of fungal attack the tissues show 

 certain changes which must be described in detail. 



(i) The walls of all the cells in the neighbourhood of the 

 mycelium turn brownish yellow in colour ; frequently their 

 contents are modified ; the protoplasm becomes markedly 

 vacuolated, and the nuclei lose their rounded outline. 

 Large accumulations of crystals of calcium oxalate, both of 

 the mono- and tri-hydrated form, appear in the cortex, 

 and crystals of the trihydrated form become much more 

 frequent in the phloem parenchyma, and may also occur 

 in the medullary ray cells. Large quantities of resin are 

 formed in all cells, much more than can be contained in 

 the resin cysts, so that all the tissues become saturated 

 with it. The tannin content is also increased. Tannin and 

 resin are probably products of excessive katabolism, show- 

 ing that the protoplasm of the cells has respired to an 

 abnormal extent, only ending in death. The calcium 

 oxalate may arise partly in this way, though as shown 

 below it is also excreted from the fungus. 



These phenomena are not confined to the cells which 

 either contain hyphae or have hyphae touching them ; but 

 all the cortical and phloem tissue, for a thickness of about 



