THE LARCH CANKER 



27 



The most marked development of the mycelium is located in 

 the inner cortex and outer phloem. The hyphae grow freely 

 in the intercellular spaces (which, as shown in Chapter I, 

 are especially large and frequent in the outer phloem) and 

 the resin cysts, and send branches through the walls into 

 the cells, and these branches ramify and pass from cell to 

 cell. The hyphae are usually rather small in the inter- 

 cellular spaces, and the branches 

 which enter cells are broader. 



These larger hyphae have often a 

 wavy outline, and contain numerous 

 small oil drops. As the hyphae use 

 up the food contained in the cells of 

 the cortex and phloem, these cells 

 contract and leave large spaces which 

 become tightly packed with the 

 mycelium of the fungus, and it is 

 from the more superficial of these 

 close masses of hyphae that the 

 fructifications arise. 



Except in cases of advanced 

 disease, the hyphae do not grow 

 luxuriantly in the inner phloem, 

 cambium, or wood ; this is probably FIG. 10. Mycelium of 

 due to there being few intercellular Dasyscypha in the wood 

 ,, . ,Y> . .of the larch. Radial 

 spaces, and consequently insufficient sec tion ( x 630). 



aeration. When they do enter the 



wood they are at first nearly confined to the medullary 

 ray parenchyma ; only a few branches enter the tracheides, 

 and these do not grow freely. But as the disease advances 

 the air-content of the wood becomes relatively much higher, 

 and the mycelium enters the tracheides and changes the 

 wood from the normal yellow to a reddish-brown colour. 

 Gradually in this way the mycelium may penetrate to the 

 centre of the stem and attack the heart-wood, which, having 

 a higher air-content, provides a more suitable substratum 

 than the sap-wood ; and, as the heart- wood increases, the 

 fungus can flourish in it and fill many of the tracheides with 



