A Rhizoctonia causing root disease in Uganda. W . Small. 155 



in tracheids than in the parench\Tna. In penetrating a cell- 

 wall or pit, a h^-pha may be constricted during passage and 

 swollen immediately afterwards. Clamp connections are scarce, 

 but are to be found on the larger threads. The basal constrictions 

 of the cells and the position of the first septum of a branch 

 hypha, although known to occur in other forms, are nevertheless 

 characteristic of Rhizoctonia mycehum. Chlamvdospores occur 

 occasionally in the course of the h^-phae. Anastomosis of neigh- 

 bouring h\^hae is frequent, and the branching is free. (Plate V, 

 figs. I and 2 show two t\'pical portions of the older mycelium 

 of Rhizoctonia in the wood and cortex of a Grevillea root, and 

 fig. 3 gives an example of the extent to which the hyphae may 

 fill the cell-caxities of elements of the medullary rays.) A certain 

 amount of choking of the cell-ca\'ities by accumulations of gum 

 always occurs. 



(b) Sclerotial plates. 



The so-called black crusts which are found in affected roots 

 are seen in section to consist of plates of cells which varv in 

 thickness and which extend laterally and irregularly by ad- 

 vancing into and filling up adjacent cell-ca\ities. The cell- walls 

 are dark-bro\Mi in colour and comparative!}' thick. The cells 

 contain oil-globules and frequently appear to telescope into each 

 other (Plate V, fig. 4) . A crust however is seldom of so simple 

 a structure as sho\Mi in the figure or of so uniform a thickness. 

 It consists rather of a set of two or more plates of cells more 

 or less parallel connected by short hyphae, and becoming more 

 or less fused together. The black crusts in the wood of the roots 

 of long-standing cases of disease (for example, in those of a 

 tree which was not dug up until four months after death), are 

 frequently* found to have coalesced with the sclerotia already 

 present, to form continuous sheets which often take the place 

 of connective tissue and grow upon and over the vascular 

 bundles in the form of a crust; they can be made to grow in 

 this manner in the laboratory by keeping a piece of root \\ith 

 Knes and sclerotia in damp sterihzed sand. Growth of this 

 nature is easiest along the medullary rays, but it is slow, and 

 the time required for it to cover the exposed surface of a spHt 

 block of root two inches in length and one inch in diameter is 

 from four to six months. 



When the material is bleached and mounted, the surface \iew 

 of a piece of such a sheet presents a striking appearance. The 

 black tissue is seen to consist of a closely-knit mosaic of dark- 

 bro^^^l cells which at first sight and in most cases, since the 

 sheet is invariabh' more than one cell thick, seem to be triangular 

 in shape and remarkably similar to each other. Individual 



