Stigma Development and the Stigmatic Cuticle of Alfalfa, Medicago sativa L.

Abstract

In Medicago sativa L., the stigma of young flowers is oval in face view and develops a fringe of nonsecreting hairs. A bilayered cuticle 120 nm thick covers the stigmatic tissue, which is composed of cylindrical cells heavily loaded with lipid droplets. During secretion of stigmatic exudate, lipid is lost from the cytoplasm, where it appears to be broken down through association with vacuoles. Vertical files of secreting cells become separated along lateral cell walls and form a copious stigmatic exudate composed chiefly of dense globules, apparently lipid. The cuticle remains attached to the outer, tangential wall of each cell file but stretches across spaces between files to confine exudate and act as a barrier between self-pollen and stigmatic fluid. In untripped flowers, the stigmatic cuticle is apparently not an absolute barrier to self-pollination and resultant autogamy. With artificial tripping, the stigma is mashed against the standard petal. The stigma face becomes concave, with self-pollen embed...