Structure of the Keel‐Locking Mechanism in Insect‐Pollinated and Self‐Pollinated Alfalfa Species 1

Keel petals of the papilionaceous legume Medicago were studied by scanning and transmission electron microscopy to further elucidate the flower‐tripping mechanism. The structures responsible for closure of keel petals in untripped flowers were compared for insect‐pollinated, perennial alfalfa ( M. sativa L.), and self‐pollinated, annual M. scutellata (L.) Mill. In both species, the surfaces where the keel petals are locked together are characterized by parallel ridges and grooves. In the closed condition, the ridges on one petal are sunken into the grooves of the opposing petal. The large cell‐wall ridges of M. sativa intermesh in a tongue and groove arrangement and in microscopic sections resemble interlocked jigsaw puzzle pieces. Keel petal ridges of M. scutellata are small parallel folds of the epidermal cuticle, so that strength of attachment between keel petals is weak. The tongue and groove closu?e of the M. sativa keel is consistent with the view that moisture tension facilitates tripping of alfalfa flowers by pollinators. We speculate that intermeshed cell‐wall ridges partially collapse with a loss of turgidity and are more easily disengaged by pollinator activity.