154 ARMILLARIA MELLEA, 



a thick wiiite flaky mass of mycelium ^^hich usually spreads 

 in the region occupied by the cambium. Compared with 

 the very thin layers of mycelium produced by Fomes annosus 

 the mycelium of Armillaria is almost leathery in consistency. 

 Like the rhizomorphs it is composed of a central loose 

 medulla and a denser cortex. The cortex is composed of 

 hyphae running longitudinally and cut up into almost 

 isodiametric segments by very numerous septa. Branches 

 are given oflf by these hyphae, and these branch hyphae 

 grow out into the surrounding tissues, attacking the phloem 

 on one side and the wood on the other, chiefly by way of 

 the medullary rays. It has been stated that the fclty 

 mycelial layer generally grows in the region of the cambium ; 

 sometimes, however, it is outside the phloem in the inner 

 cortex, or among the outer phloem cells. In either case, 

 further felty mycelial layers are subsequently made, so that 

 in pulling off the cortex and phloem of a root or stem in an 

 advanced state of attack, several layers of white mycelium 

 are found, one inside another. These layers are veined, and 

 at the upper limit present an irregular outline. 



The cambium and phloem are usually attacked in the 

 larch to a height of 3 to 5 ft. In rough-barked phies, such 

 as Scots and Corsican, mycelium is often found at a higher 

 level, whilst in Weymouth pine it is usually confined to the 

 lowermost 2 ft. On the death of the tree the mycelium 

 continues to grow as long as there is sufficient moisture. 

 When the bark contracts away from the wood, leaving an 

 air cavity, the mycelium takes the form of Rhizomorpha 

 subcorticalis, the structure of which has already been 

 described. 



In the wood the hyphae spread at first chiefly through 

 the medullary rays, and they often grow sufficiently fast to 

 reach to the centre of the tree from one side whilst the tree 

 is still living on the other. From the medullary rays the 

 hyphae grow into the tracheides and spread from tracheide 

 to tracheide through fine bore-holes made in the tracheide 

 walls. The hyphae naturally grow along the tracheides 

 much more rapidly than across them, so that the rot spreads 



