THE LARCH CANKER 27 
The most marked development of the mycelium is locatedin 
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 | 
a 
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 thesc 
close masses of hyphae that the HL | 
fructifications arise. | (0) 
Except in cases of advanced Vy 
diseaye, the hyphae do not grow Hh 
luxuriantly in the inner phloem, 
cambium, or wood; this is probably — yc. 10,—Mycelium of 
due to there being few intercellular Dasyscypha in the wood 
3 ‘ of the larch. Radial 
spaces, and consequently insufficient section (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 
