154 Transactions British Mycological Society. 



to be in a normal condition, and only a histological examination 

 of the internal tissues will prove the presence of the Rhizoctonia. 

 In older cases, again, but only in the larger roots, careful 

 washing may reveal on the surface of the bark thin black sheets 

 of tissue which are often slightly raised or ruptured, and which 

 are in evident connection with the internal mycelium of the 

 fungus. Their position and the ease with which they can be 

 detached may enable them to function as rhizomorphs, for it 

 has been shown by experiment that a rapid and profuse growth 

 of mycelium can be induced from a small piece of black sheet 

 by placing it in a suitable medium, and it is therefore probable 

 that they play a part in the dissemination of the Rhizoctonia 

 under natural conditions. They, however, are not of regular 

 occurrence in an exposed position like the surface of a root. 

 Owing to the dry condition of the wood, the roots are usually 

 firm and brittle, and a closer examination of the bark and of 

 the surface of the wood after removed of the external tissues 

 may disclose the presence of numerous small black sclerotia in 

 the wood, cortex and bark. The sclerotia may be numerous 

 enough to show through the bark, and cause a distinct blackening 

 of the surface of the roots. Sheets or crusts of black tissue also 

 occur in the roots and pursue an irregular course and thus 

 appear in a rough longitudinal section as thin wavy lines which 

 in places double upon themselves and form "islands." They 

 are seen to most advantage in the wood, and their presence is 

 sometimes denoted by a distinct yellowing of the tissues, a 

 discoloration which is more or less bounded by the extent of 

 the lines. The black sheets or crusts are not consistently present 

 throughout the diseased tissues, but they can be found at times 

 in the bark and cortex as well as in the wood. In the former 

 they are connected with and give rise to the external sheets 

 of black tissue mentioned above ; in the latter they do not define 

 necessarily the limits of penetration of the hyphae. The small 

 black sclerotia, on the other hand, are to be found in advanced 

 cases of infection throughout the tissues of the roots and 

 extending into the wood of the collar of the tree. 



The Rhizoctonia in the Tissues of Grevillea robusta. 



(a) Hyphae. 



The hyphae of the fungus occur in quantity throughout 

 cdl the tissues of the diseased roots. In the wood, they vary 

 from very pale- to dark-brown in colour according to age, are 

 septate, usually granular and with oil-globules, and measure 

 up to 10 fi or more in thickness. They may be irregularly swollen 

 and angled or barrel-shaped, and are often more gnarled 



