28 DISEASESiOF CROP-PLANTS 



" The externa] hyphge are somewhat coloured, usually yellowish brown, 

 and they are generally of two types. One type may be designated as 

 purely vegetative and another as constituting the external tufts or masses 

 when these occur. All hyphae are practically colourless when young, 

 vacuolate, more or less irregular, septate with the septa at intervals of 

 100-200 microns. The diameter of vegetative hyphae is 8-12 microns. 

 Branches arise, and when young these are inclined in the direction of growth 

 and are invariably somewhat constricted at the point of union with the main 

 hyphae. As the hyphae mature and become more deeply coloured they are 

 more imiform and rigid, the distances between cross walls are greater, the 

 constrictions where branches arise less marked, and the branches are 

 approximately at right angles to the main hyphae. On certain affected 

 plants a short tufted or mealy growth occurs and this is made up of hyphae 

 of very different characteristics. In the young condition threads are 

 profusely branched and lobed, sometimes botryoid, and they are ultimately 

 divided into short ovate cells, arranged in short chains, or elbowed, and 

 producing branches in a more or less dichotomous fashion. In culture the 

 denser masses give rise to sclerotia. With maturity these hyphae become 

 light brown in colour ; they break up into short hyphal lengths or single 

 cells, the indiA'iduals of which bear some resemblance to conidia. 



"The sclerotia vary in size from those so minute as to be scarcely 

 visible to others which may be a centimetre or two in diameter. They are 

 generally more or less flattened, irregular, deep chestnut brown, and 

 generally smooth on the surface.',' 



The Corticium fructification is formed as a collar around the 

 base of living stems, or on objects lying on the soil, and consists 

 of a thin web or membrane, pale olive-buff to cream colour, on 

 which the scattered basidia are borne. 



In the examples of damping-off which have come to the 

 writer's notice in these islands a mycelium with the characters of 

 Rhizoctonia has frequently been found, and it is probable that 

 this fungus is much more common than the classic parasite of 

 this affection in Europe, Pythium de Baryanum, if indeed the 

 latter occurs at all. A Rhizoctonia collar-rot of bean plants 

 {Phaseolus vulgaris) is not uncommon and the fungus has been 

 found on the roots of diseased sugar-cane seedlings. Whether 

 the collar or crown rot which is reported to prevent the cultiva- 

 tion of alfalfa is also due to Rhizoctonia cannot be said with 

 certainty, but specimens received from Montserrat suggest the 

 possibility. Other occasional root diseases of herbaceous plants 

 occur in which a sterile mycelium is concerned, and require to be 

 examined from this point of view. 



The genus Marasmius. 



The genus Marasmius contains several hundred known 

 species, most of which are woodland saprophytes on dead 

 vegetable matter. The fructification is of the toadstool form, 

 and is distinguished by the papery, not fleshy, consistency of its 

 substance, which when shrivelled by dr3/ing is capable of restora- 

 tion to something like its original form on being re-moistened. 

 Several species occur in the West Indies about the bases of the 

 shoots of grasses, including sugar-cane, and are well-known in 

 connection with root disease of that plant. Others occur on the 



