158 ARM ILL ARIA MELLEA, 



to indicate a stage of excessive vigour in the mycelium, and 

 though it does not directly cause marked changes in the 

 character of the ,wood, it appears in some way to transform 

 the wood into a state in which it is easily acted on by the 

 hyphae behind. 



The dark-brown pigment differs from that found in the 

 black specks of wood rotted by Fomes annosus in being 

 unaffected by hydrochloric acid, though it is bleached by 

 concentrated nitric acid. 



When two black lines approach each other they seldom 

 unite, but cease to move when about 1 to 5 mm. apart. 

 This causes the frequent phenomenon of a pair of black 

 lines running parallel to each other. In some parts a black 

 line is replaced by a much broader band of colourless, 

 bladder-like hyphae. These resemble the black-line hyphae 

 in all respects except their colour and the thickness of their 

 walls, and apparently are much more slowly decomposed. 

 They may cover a width of as much as 6 to 10 tracheides. 



Resin flow. The disease is generally accompanied by an 

 external flow of resin, and sometimes the quantity of this 

 substance excreted is so great that the fallen needles, twigs, 

 and soil round the base of the trunk become compacted 

 into a hard adherent crust. The resin arises in two ways. 

 Firstly, the living cells lining the resin ducts in the cortex 

 and wood are killed so that the contained resin escapes, 

 and, taking advantage of the cracks formed by the drying 

 of the bark, runs down the outside of the trunk. Secondly, 

 the living tissues in 'the neighbourhood of the fungus excrete 

 an abnormal amount of resin, and wood which is formed 

 after some part of the tree has been attacked is characterized 

 by containing a large number of irregular resin ducts, so 

 that in this respect the wood resembles the abnormal wood 

 made in the region of a canker. On account of this escape 

 of resin the disease is sometimes known under the name of 

 ' resin flow ' or ' resin glut ', and though the flow of resin 

 is commonly not so great in the larch as in the Scots or 

 Austrian pine, it is often the first external symptom of the 

 disease. 



