This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Flowers - Purple, butterfly-shaped, consisting of standard petal, wings, and keel; 1 in. long or less, clustered in short raceme at end of slender footstalk from leaf axils; calyx 5-toothed; stamens 10 (9 and 1); style curved, flattened, bearded on inner side. Stem: 1 to 2 ft. long, stout, reclining, spreading, leafy. Leaves: Compounded of 3 to 6 pairs of oblong leaflets somewhat larger than halbert-shaped stipules at base of leaf; branched tendrils at end of it. Fruit: A flat, 2-valved, veiny pod, continuous between the seeds.
Preferred Habitat - Beaches of Atlantic and Pacific oceans, also of Great Lakes.
Flowering Season - May - August. Sometimes blooming again in autumn.
Distribution - New Jersey to Arctic Circle; also Northern Europe and Asia.
Sturdy clumps of the beach pea, growing beyond reach of the tide in the dunes and sandy wastelands back of the beach, afford the bee the last restaurant where he may regale himself without fear of drowning. From some members of the pea family, as from the wild lupine, for example, his weight, as he moves about, actually pumps the pollen that has fallen into the forward part of the blossom's keel onto his body, that he may transfer it to another flower. In some other members his weight so depresses the keel that the stamens are forced out to dust him over, the flower resuming its original position to protect its nectar and the remaining pollen just as soon as the pressure is removed. Other peas, again, burst at his pressure, and discharge their pollen on him. Now, in the beach pea, and similarly in the vetches, the style is hairy on its inner side, to brush out the pollen on the visitor who sets the automatic sweeper in motion as he alights and moves about. So perfectly have many members of this interesting family adapted their structure to the requirements of insects, and so implicitly do they rely on their automatic mechanism, that they have actually lost the power to fertilize themselves.
In moist or wet ground throughout a northern range from ocean to ocean, the Marsh Vetchling (Lathyrus palustris) bears its purple, butterfly-shaped flowers, that are the merest trifle over half the size of those of the beach pea. From two to six of these little blossoms are alternately set along the end of the stalk. The leaflets, which are narrowly oblong, and acute at the apex, stand up opposite each other in pairs (from two to four) along the main leafstalk, that splits at the end to form hooked tendrils.