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 EhizomorpJia subcorticalis made 

 branches which grew out into the damp air and assumed 

 the form of Ehizomorpha 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 p.). 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 



