This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
This plant is found at the present day (not in any earlier beds) in Western and Southern Europe and North Asia in the North Temperate Zone. In Great Britain it is found in South Devon and Somerset in the Peninsula province; in the whole of the Channel province, except in S. Hants; and in the Thames province, except in Middlesex; in Anglia, except in Hunts; in the Severn province, except Monmouth; only in Glamorgan and Carnarvon in Wales; and in the Trent province; in Mid Lancs; in the Humber province, not in N.E. York; elsewhere northward in Western Ireland, Ayr, and Kincardine, and it ascends in Yorkshire to 1800 ft.
The Horseshoe Vetch is especially prevalent in the chalk districts, where it is a conspicuous hill-side plant, growing on stony and rocky pastures. It is accompanied by Mountain Flax, Rock Rose, Man Orchis, Pyramidal Orchis, Viola calcarea, Polygala serpyllacea and P. calcarea, Field Mouse-ear, Snapdragon, etc.
This plant has the Lotus habit, not growing very tall, but sub-erect or prostrate, then ascending, with inversely egg-shaped, blunt leaflets, 4-13, the leaves with lobes each side of a common stalk, with an odd one.
The flowers are yellow on short stalks, and the slender flower-stalks exceed the leaves. The 2 upper teeth of the calyx are united below, and the petals have a long claw or stalk. The pods are stalked in clusters, bent in a circle, and in shape like a horseshoe (hence the names, Hippocrepis and English name above), the hollow margin continuous, the convex margin wavy. The pod does not break up at the narrow part, but across the middle of the broader part. Each segment contains two seeds, but one is sterile.
The plant is usually about 6 in. high. The flowers are in bloom from April to August. Horseshoe Vetch is a perennial, deciduous, herbaceous plant.
The Horseshoe Vetch has a flower in general like that of Lotus, in so far as the method of pumping out the pollen is concerned. The vexillum or standard has a narrow stalk or claw, and is curved, enabling one to distinguish it between the vexillum and the stamens. Underneath it bears a flat, irregular process which fits exactly upon the nectaries, and serves to close them very effectually. The vexillum is employed by a bee visitor as a long lever by which to raise this lid and then insert its head beneath the vexillum, a procedure which leads to effectual cross-pollination. It is visited by bees and Lepidoptera.
The pod breaks up into several joints, which are dispersed with the contained seeds to a short distance, aided by the wind, as the plant is semi-erect in habit.
Photo. A. R. Horwood - Horseshoe Vetch (Hippocrepis comosa, L.)
Hippocrepis, Linnaeus, is from the Greek hippos, horse, and crepis, shoe, from the form of the joint, and comosa means hairy.
The only common name is Horseshoe. It is called also "Unshoe the Horse", because its seeds resemble a horseshoe, and by Doctrine of Signatures (!) it was therefore said to unshoe horses.
Essential Specific Characters: 87. Hippocrepis comosa, L. - Stem branched, woody below, procumbent, leaves pinnate, leaflets narrow, obovate, 6-12, flowers yellow, in umbels, 5-8, pod jointed, curved, like a horseshoe.