The Stigma of Dendrobium speciosum Sm. (Orchidaceae): A New Stigma Type Comprising Detached Cells Within a Mucilaginous Matrix

The stigma of Dendrobium speciosum does not correspond to anymorphological category previously described. The stigma lieson the lower surface of the column as an oval, bowl-shaped depressioncontaining a wet, jelly-like, viscid material, which comprisesa large number of elongated, detached cells suspended in a matrix.The stigmatic cells are thick walled and have a vacuolated cytoplasmcontaining amyloplasts. These cells give a positive reactionto fluorescein diacetate indicating that the plasma membraneis intact; this evidence points to the conclusion that the stigmaticcells are not detached, senescent or necrotic papillae. Histochemical staining of the matrix shows high levels of acidicpolysaccharides with low levels of lipids and proteins. Biochemicalanalysis shows this matrix to comprise around 80 per cent uronicacids. This highly specialized stigma may be a further evolutionarydevelopment of the wet papillate stigma resulting from the specialneeds of both pollination and the nutritional support of thelarge number of developing pollen tubes. Stigma, Dendrobium speciosum, Orchidaceae