THE HONEY FUNGUS 153 



nothing but a firm gelatinous covering to the rhizomorph. 

 Eventually even this becomes indistinguishable. The most 

 noticeable change in the cortex, which becomes the rind of 

 the mature rhjzomorph, is the coloration of its outer 

 layers, which begins a few millimetres from the apex. The 

 coloration, which is associated with a hardening and 

 thickening of the outer walls, gradually spreads inwards 

 until it involves all the cortex and also a part, if not the 

 whole, of the primary medulla. By the formation of a central 

 cavity the primary medulla has been restricted to half 

 a dozen or more layers of hyphae lining the inner side of 

 the cortex. But a secondary medulla is formed by branch 

 hyphae, which grow from the primary medulla or inner 

 cortex and finally fill up the central cavity. 



This description applies to the subterranean form. In 

 the subcortical rhizomorph the outermost gelatinous layer 

 does not dry up in the same way, and the branch hyphae, 

 which arise not only from the superficial hyphae but also 

 from those more deeply rooted, penetrate the surrounding 

 tissue of the host, especially through the medullary rays, 

 and, while destroying the tissues, serve to nourish the 

 rhizomorphs. Subcortical strands usually remain colour- 

 less until the severance of the bark from the wood lets in 

 air, when the cortical walls of the rhizomorphs become 

 thickened and pigmented, though not so intensely as in the 

 subterranean form. When bark, already loose, is first 

 pulled off them the rhizomorphs are frequently red, but this 

 colour gives place to a dull black within a few hours. 



Branches arise through a new growing apex being formed 

 in the inner layer of the cortex, which, in a few days, breaks 

 through the rind and emerges from the parent strand. The 

 birthplace of every new branch is indicated, some days 

 prior to its emergence, by a floccose tuft of hyphae, J-l mm. 

 in diameter, which appears on the surface. The hyphae 

 forming this tuft arise partly from the surface of the rhizo- 

 morph and partly from the hyphae more deeply seated. 



The effects of the honey fungus on its host. As soon as 

 a rhizomorph of Armillaria has entered a larch tree, it forms 



