152 ARMILLARIA MELLEA, 
removal to very large culture dishes the cultures grew into 
more or less compact masses of rhizomorphs. So far, in 
development and form, the rhizomorphs agreed with the 
subcortical type. They were covered with hyaline hyphae, 
which stood out at right angles to the surface. The rind 
was white where submerged in the substratum, but brown 
or blackish where it rose above it. And each rhizomorph 
gave rise to frequent branches. But after a rest of some 
months a mass of the Rhiapmorpha subcorticalis made 
branches which grew out into the damp air and assumed 
the form of Rhizomorpha subterranea. 
All the authorities cited depict the apical growing points 
of the rhizomorphs. Three fairly distinct layers can be 
observed in this region. On the outside a fairly loose weft 
of thin hyphae which send branches out into the air. Next 
is a cortex of longitudinally running hyphae, which are so 
closely welded together that they have no intercellular 
spaces between them, and appear polygonal, not circular, 
in section. They have frequent septa, and near the apex 
the segments are not much longer than they are broad. 
The centre is occupied by the medulla, in which the hyphae 
are much broader (up to 20). These hyphae are thin 
walled and full of hyaline sap, and are arranged in marked 
longitudinal rows with numerous intercellular spaces between 
them. Farther from the apex the cortex increases its 
periphery through the branching of the hyphae that compose 
it, so that more room is left for the medulla. And since the 
medullary hyphae do not branch, they are drawn more 
apart and large air spaces are left between them. Usually 
a large central cavity is formed which becomes the main 
aerating system of the inner part of the rhizomorph. 
This type of structure only occurs near the growing 
point, and in tracing successive stages of maturation from 
the apex of the rhizomorph it is found that each of these 
layers undergoes modification. The outermost layer, which 
is at first slimy, owing to the secretion of gelatinous liquid 
from the hyphae, becomes firmer through desiccation. The 
outstanding branches disappear, and then the layer becomes 
