SUMMARY.The black lines ofArmillaria melleuhave been found in the tissues of a number of plants. Isolations were made from the lines and an account of the development ofArmillaria melleain culture is given. Particular mention is) made of the formation of sclerotium‐1il.e bodies in the culture medium and of the development of the rhizomorphs. Pure cultures of the fungus on wood blocks have produced black lines in the substratum similar to those occurring in nature. A short review is given of the nature of the sclerotium, and from a consideration of the formation of the black plates, or lines, as they appear in section, the suggestion is made that they form the limiting layer of sclerotium‐like bodies, which are partly immersed in the substratum. The black lines in the wood are continued at the surface as the black rind of an effused mycelial mass named the black xylostroma. Thus there is formed a body comparable to a sclerotium in that it is completely surrounded by a black rind of bladder hyphae and has a medulla of hyaline hyphae. It is, however, partly immersed in the wood substratum and, in consequence, contains a considerable proportion of host tissues. Further, this body, probably as a result of its immersion in a hard woody substratum, has developed the power of apical growth resulting in