THE LARCH CANKER 31 



them. The hyphal development in the wood never seems 

 to be of a nature that can cause reinfection of the phloem, 

 so that probably the hyphae get round the cork layer in 

 the cambium just outside the wood, and spread from that 

 point. Cork formed in the vicinity of the fungus is apt to 

 become red. This is due to infiltration with resin, and cork 

 cut fresh from a healthy tree may be made to assume this 

 red colour by boiling in resin. 



(iii) The character of the secondary wood which is made 

 in the neighbourhood of the canker is also profoundly 

 affected. 



Where the cambium is killed, of course no more wood 

 can be formed ; but the cambium which is still living on the 

 flanks of this dead patch cuts off wood elements which are 

 essentially different from normal xylem (fig. 4 and fig. 11). 

 The cells which are first differentiated in the spring wood 

 remain thin walled and show no tendency towards sliding 

 growth. Their walls become lignified and have numerous 

 simple pits. Sometimes the living contents remain, but not 

 infrequently they become disorganized and are replaced by 

 water. Thus in many ways they resemble medullary ray 

 parenchyma, except that they are extended longitudinally 

 instead of radially. Irregular intercellular spaces are formed 

 between them, and these often become filled "with resin. 

 Normal resin ducts are also formed, besides such irregular 

 abnormal ones (fig. 4, p. 8). 



As the summer advances tracheides are formed. These, 

 however, have an irregular wavy outline, which gives 

 a truly longitudinal section the appearance of being oblique, 

 although entire tracheides may be included in it. Subse- 

 quently formed tracheides are normal except for the fre- 

 quency of a tertiary spiral thickening of the walls. Tannin 

 cells are frequent throughout this portion. 



The significance of this abnormal wood is difficult to 

 define. In many respects it resembles the wound- wood 

 described by Vochting (1892) and Kuster (1903); but 

 Hartig (1892) figures a very similar development in spruce 

 after defoliation by the mm moth, and Harper (1913) \has 



