i6o Transactions British Mycological Society. 



regarded as starved spores, but, in other cases, they seem to be 

 the remains of abortive attempts to form lateral branches of 

 the hyphae. In all probability they would have developed into 

 cell-masses or hyphae if growth had continued. The sclerotia 

 of a dr3dng culture measure only '^S by -27 mm. Sclerotia form 

 profusely in solid media and also on the surface of moistened 

 filter-paper cultures. Their development is seen more easily in 

 the case of the latter, and the drawings and notes therefore 

 refer to filter-paper growths, although the process is the same 

 in all media. A sclerotium originates in a small group of hyphal 

 cells which become darker than the adjacent cells in colour, 

 rounded, thick- walled and guttulate, by continued segmenta- 

 tion not only of the original cells but also the laterally-placed 

 units to which the core has given rise. The original cells are soon 

 lost to view, and certain of the peripheral cells grow outwards 

 to form hyphae (Plate VI, figs. 17-20). The whole structure 

 becomes a mass of pseudoparenchyma which expels a large 

 quantity of oil on being crushed under a cover-glass. In the 

 mature sclerotium, the central cells have thinner walls and larger 

 lumina than the peripheral cells. During their growth, the 

 sclerotia can be seen in the medium as dark masses which 

 become black in eight days and mature in about one month. 

 They can then be detached easily from the medium, especially 

 if the latter is in process of drying up. In culture they are of a 

 more regularly round-oval shape than in nature, but some are 

 elongated and narrow while others are almost spherical. An 

 average measurement is -5 mm. long by '35 mm. broad, but 

 very many of the sclerotia are smaller and others are larger 

 than this. The mycelium in the body of the medium has not 

 been observed to assume the colour and form of that of the 

 black plates in the tissues of diseased roots, and the sclerotia, 

 like those formed naturally, have refused to give growth of any 

 kind even after a nine months' rest. No Corticium stage of the 

 Rhizoctonia has been found in any of the cultures. 



As alread}'' mentioned, the fungus in Grevillea tissues can be 

 induced to grow by keeping pieces of infected roots in damp 

 sand. The exposed surfaces of the pieces become covered with 

 mycelium which eventually assumes the black sheet or crust 

 form, and sclerotia develop in great numbers independently or 

 in direct connection with the sclerotial-plate tissue. In such 

 cases, the sclerotia are larger and more swollen than those 

 developed naturally, and the black sheet assumes the form of 

 a detachable crust. Although it might be expected to appear, 

 no Corticium stage of the Rhizoctonia has been found in these 

 root-fragment cultures. 



