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Abstract
SUMMARY: A new genus and species of the Acrasieae is described, and for this the binomial Acytostelium leptosomum is applied in recognition of its acellular soro-phores and the diminutive proportions of its sorocarps. This slime mould has been isolated upon four occasions from soil and forest litter collected from deciduous forests within the United States. The vegetative stage of A. leptosomum, as that of the other cellular slime moulds of this Class, consists of the independent growth of myxamoebae which feed upon bacterial cells. The fruiting stage likewise duplicates that of other members of the Acrasieae in its early manifestations; but it differs markedly from these in its terminal phase, i.e. the construction of the mature fructification, or sorocarp. A. leptosomum is at present unique among these slime moulds in its capacity to produce a sorocarp with a non-cellular stalk. The morphogenetic process whereby this is accomplished is not thoroughly understood; nevertheless, a possible explanation of how the stalk may be formed without the expenditure of any cells is offered.
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