Plants of Hedysarum glomeratum F.G. Dietrich are self-compatible and fail to set seeds when protected from pollinators although its flowers are homogamous. Nevertheless, the stigma becomes receptive and self-pollination takes places if it is subjected to mechanical damage. In an ultraestructural study, we can appreciate that secretory products of the stigma are held beneath a cuticle even in very mature untouched stigmas. These products are not released to support pollen germination until the cuticle is broken. This mechanical barrier could prevent the spontaneous self-pollination whereas a pollinator does not visit the plant. Only when pollinators visit the flowers, both self and cross-pollination are possible. This mechanical system avoids to a large degree self-pollination in this annual herb.