Correlative Studies on Seed Coat Structure, Chemical Composition, and Impermeability in the Legume Rhynchosia minima

The seed coat of Rhynchosia minima is composed of (1) a surface deposit of waxy material interfused with a lipoidal substance, β-sitosterol; (2) a subjacent 3-μm adcrustation of hemicellulose-cellulose complex, which is shown to be a separate entity; and (3) a layer of palisade cells in which the secondary walls are impregnated with arabinan and the lumen contains tannin and phenolic compounds. These features constitute a gradative barrier to water uptake by seed. Seeds failed to germinate unless the impermeable seed coat surface or hilum was damaged. We have demonstrated that the micropyle and hilum function as hygroscopic valves.