Trefoils belong to the Pea family - which is a large group with distinctive flowers. Each flower is 'built' from 5 petals. The broad, upper petal is known as the standard, the two narrower ones are the wings and the two lower / central ones (which enclose the stamens) form the keel. There is a labelled diagram on the gorse ID page of this arrangement.
The seeds form in a pod or legume - like that of the garden pea or bean. Members of this family include lupins, broom, gorse, clover, trefoils and vetches.
Leaves and stem
The common birds foot trefoil (Lotus corniculatus) is one of the commoner yellow flowers of the pea family. It is a perennial, with a prostrate habit (spreading along the ground).
Its leaves appear trifoliate but a pair of small oval leaflets may be seen lower down.
Flowers and Fruits
Flowers : there are up to 8 pea-like yellow flowers, clustered together to form a 'head' - which is supported on a longish stalk.
There are three sepal-like bracts (which may have an orange colour). The calyx (made up of the sepals) is 'tooth-like".
Ecology and other notes
The name of the plant derives from the appearance of the mature seed pods, which are like a bird's foot. There is a slender form of the birds foot trefoil (Lotus tenuis), which has narrow, spear like leaflets and narrower, smaller flowers.
The greater birds foot trefoil (Lotus uliginosus) is a deeper green and more luxuriant than the common form, with more flowers in the floral head. It can be quite hairy and may be found in moist grasslands.
This map gives the present distribution of the common trefoil