.-1 Rhizoctonia causing roof disease in Uganda. W. Small. 157 



black dots on the surface of the exposed wood. They appear to 

 be smooth, but under high magnification the smoothness is seen 

 to be modified sometimes by the protruding ends of h\'phae 

 which are hyaline or pale-bro%\"n and often conical in shape. 

 Their outhne is conditioned to a certain extent bv the sur- 

 rounding tissue, but is. for the most part, circular, oval or pear- 

 shaped. The majority of the sclerotia are flattened and closely 

 attached to the substratum; when they are old, they can be 

 readily detached by mechanical means. In the wood they occupy 

 conjunctive tissue spaces, and they also occur on the vascular 

 bundles and along the medullar^' rays. Their greatest length is 

 about I mm. and greatest breadth about -S mm., but the end- 

 wise coalescence of two adjacent sclerotia may present a dumb- 

 bell shape and an apparently greater length than the figure 

 given. The sclerotia have not been ob5er\-ed to unite to form 

 coriaceous masses. 



In the tissues, a sclerotium originates in a short hne of brown, 

 thick-walled, barrel-shaped and guttulate cells occurring in the 

 course of a hypha which may or may not be in direct connection 

 with a black hne (Plate V, fig. 6). The young sclerotium is 

 comparatively long and narrow (Plate VI, fig. 21) and bounded 

 by the ceUs of the tissue in which it is formed. In structure, a 

 sclerotium consists of a spongs* core of irregularly-shaped, more 

 or less thin-walled parench^Ttiatous ceUs. the average diameter 

 of which varies from lo/x in some to 5 ^ in others. The sclerotia 

 are of one t^'pe only. They are remarkable for their large oil- 

 content and are apparently weU-fitted for a resting function. 

 Plate V, fig. 7 shows part of a sclerotium with connected h\-phae 

 penetrating the adjacent tissues. 



(d) Hymenophore. 



Diuing the study of the Grevillea material, the possible 

 existence of a fruiting or Corticium stage of the Rhizoctonia was 

 kept in mind, and, after much search, a supposed h\Tnenophore 

 was encountered in small amount on one lot of material and in 

 association with a sclerotia! plate. Little is known about it and 

 further study of its pecuhar formation is not only desirable but 

 essential to throw hght on its true function and significance. 

 Under a lens, the usual appearance of a sclerotial plate in either 

 natural or artificially induced growth is that of a smooth and 

 apparently hard carbonaceous surface, but, in the case of the 

 material under discussion, there is to be noted a minutely pilose 

 appearance which on analysis is found to be due to the presence 

 of basidia-like structures and numerous processes arising from 

 the mycehum of the plate. The processes are simple and finger- 

 like or branched and swollen (Plate V, fig. 8). They measure 



