Aerial sclerotia originated from intercalary cells of aerial hyphae. Initially only a single cell was involved. Repeated branching from the initiation point formed the sclerotium initial which was a more or less spherical mass of undifferentiated, radially arranged cells. Accumulation of glycogen in cells towards the centre of the initial marked the commencement of maturation. Cells of the central (medullary) region went through a differentiation process which involved first a heavy accumulation of glycogen; the glycogen was then mobilized, its reduction in concentration being exactly correlated with the formation of a thick, hyaline, secondary wall. As the medulla developed, differentiation of localized areas of cells just within the margin of the initial occurred to form the protective rind layer. Wall thickening in rind cells was not preceded or accompanied by glycogen accumulation. Strains which failed to produce sclerotia behaved as stable variants, sclerotium non-production segregating as a single gene. Four such genes were recognized and some linkage information obtained.
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