Stigma development and the stigmatic cuticle of Medicago scutellata
The self-fertile annual Medicago species evolved from the cross-pollinated perennial species. We used light and electron microscopy to study the development and structure of the stigma in annual tetraploid Medicago scutellata (L.) Mill to help elucidate the mechanism of self-pollination. Immature stigmatic cells have extensive lipid deposits. During development, stigmatic cells become separated and cellular lipid is transferred to intercellular spaces as part of the copious stigmatic secretion. The cuticle of the stigma is lifted away from underlying cell walls and confines secretion around stigmatic cells. The cuticle is thin, about 75 nm, and is composed mainly of a single layer traversed by dense strands. The cuticle is virtually always disrupted during late stages of flower maturation, as evidenced by penetration of stain into the stigma. Self-pollination may occur without flower tripping.