156 Transactions British Mycological Society. 



hyphae, however, can be distinguished, and they are of the 

 structure shown in Plate V, fig. 5. The cell-constrictions and 

 the positions of the first septa are characteristic, as already 

 mentioned, but the respective appearances differ inasmuch as 

 the hyphae of the soUd sheets possess the two typical charac- 

 teristics in what may be called an exaggerated form. Swollen 

 and triangular-shaped cells in which the basal constrictions are 

 less marked than usual invariably occupy the branching points 

 of the hyphae and give rise to new hyphae by a kind of dicho- 

 tomy, and anastomosis between adjacent hyphae causes a 

 pecuhar hammer-head appearance when the neighbouring cells 

 are not viewed in the same plane. A crust in the wood of a 

 root originates in the modification of hyphal cells which become 

 short and barrel-shaped, develop, numerous oil-droplets, darken 

 in colour, and blacken the walls and fill the cavities of adjacent 

 cells by expanding into a sclerotial-like mass. They assume their 

 characteristic appearance early, and so differ at once from the 

 vegetative hyphae which have given rise to them. The nature 

 of the cells points to their being able to pass through a resting 

 period, and support is lent to this view by the fact that myceli^ 

 growth is immediate and plentiful from fragments of lines taken 

 from dry pieces of diseased roots which have been lying in the 

 laboratory for over six months. The function of a crust, then, 

 is that of a rhizomorph, or, in view of the nature of the cells, 

 that of a piece of resting tissue capable of giving rise to creeping 

 and active mycelium, the equivalent of a mass of chlamydo- 

 spores or sclerotia. This function is clearly indicated, and, when 

 considered in conjunction with the structure and nature of the 

 tissue, may justify the substitution of the term "sclerotial 

 plate" for that of "crust." In fact, it is difficult at times to 

 distinguish readily between the structure of a sclerotium exposed 

 by a section and that of a piece of sclerotial plate when cut 

 across. A comparison of the material of an early case of Rhizoc- 

 tonia disease with that of one of long-standing seems to point to 

 the formation of sclerotial plates preceding the production of 

 sclerotia, but, on the other hand, it has been noted that sclerotia 

 are present in tissues from which the plates are absent. The two 

 structures are, therefore, independent of each other, although, 

 as is mentioned later, a sclerotium may arise indirectly from a 

 sclerotial plate. 



(c) Sclerotia. 



The sclerotia are found in great numbers in parts of diseased 

 roots. They may protrude from the bark and be numerous in 

 the cortex. They also occur throughout the wood, but their 

 appearance is most striking when they are seen as a mass of 



