* THE HONEY FUNGUS 158 
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 rhizomorph, 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, }-1 mm. 
in diameter, which appears on the surface. The hyphae 
forming this tuft arise partly from the surface of the rhizo- 
morph and partly fram 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 
