As I was writing a post on Spanish broom (Spartium junceum) I referred to its pea-like flower. We often refer to a flower as being “pea-like” and give no further description. What is a “pea-like flower”?
Members of the Pea Family (Fabaceae) have flowers with five fused sepals (outermost whorl of a flower) and five petals. The upper petal is larger and is called the banner. The two lower petals are fused at the apex and remain free at the base. These petals resemble the keel of a boat, thus their name, keel. The final two petals resemble wings, one on either side of the keel, and often almost completely conceal the keel. These petals and their arrangement are shown in the flower schematic. (Please excuse my poor drawing skills.)
Pea Family flowers have ten stamens (male pollen producing structure) and one superior (lies above the attachment of the sepals and petals) ovary, a single curved style and one stigma. The ovary, style and stigma together form the pistil. Nine of the stamens are partially fused into a tube and one stamen remains free. The ovary is inside the tube created by the fused stamens.
Pea flowers have an interesting fertilization mechanism. The stamens and style within the keel are “spring loaded”. When the keel is depressed by a visiting insect the stamens with their pollen-bearing anthers and the style are released and strike the insect on the abdomen. As the insect moves about the flower the pollen is transferred to the style resulting in fertilization.
Lupines, vetches, locoweeds, milk vetches, brooms and clovers are a few of the plants that have pea-like flowers. The two pictured examples are Spanish broom and sickle milkvetch (Astragalus curvicarpus), which I will feature in my next two posts.