The arrangement of the petals and the number and cohesion of the stamens vary in the three suborders. In Mimosoideae, the smallest of the three, the flower is regular, and the sepals and petals have a valuate aestivation, and are generally pentamerous, but 3-6-merous flowers also occur. The sepals are more or less united into a cup, and the petals sometimes cohere at the base. The stamens vary widely in number and cohesion; in Acacia they are indefinite and free, in the tribe Ingeae, indefinite and mona delphous, in other tribes as many or twice as many as the petals. Frequently, as in Mimosa, the long yellow stamens are the most conspicuous feature of the flower. In Caesalpinioideae the flowers are zygomorphic in a median plane and generally pentamerous. The sepals are free, or the two upper ones united as in tamarind, and imbricate in aestivation, rarely as in the Judas tree, valuate. The corolla shows great variety in form ; it is imbricate in aestiva tion, the posterior petal being innermost.
The stamens, generally ten in number, are free, as in Cercis, or more or less united, as in Amherstia, where the posterior one is free and the rest are united. In tamarind only three stamens are fertile. The largest suborder, Papilionatae, has a flower zygomor phic in the median plane. The five sepals are generally united, and have an ascending imbricate arrangement ; the calyx is often two lipped. The corolla has five unequal petals with a descending im bricate arrangement; the upper and largest, the standard (vexil lum), stands erect, the lateral pair, the wings or alae, are long clawed, while the anterior pair cohere to form the keel or caring, in which are enclosed the stamens and pistil. The ten stamens are monadelphous, as in gorse or broom, or diadelphous, as in sweet pea (the posterior one being free), or almost or quite free ; these differences are associated with differences in the methods of pol lination. The ten stamens here, as in the last suborder, though arranged in a single whorl, arise in two series, the five opposite the sepals arising first. The carpel is sometimes stalked and often surrounded at the base by a honey-secreting disk; the style is terminal and in the zygomorphic flowers is often curved and somewhat flattened with a definite back and front. Sometimes as in species of Trifolium and Medicago the ovules are reduced to one. The pod or legume splits along both sutures into a pair of membranous, leathery or sometimes fleshy valves, bearing the seeds on the ventral suture. Dehiscence is often explosive, the valves separating elastically and twisting spirally, thus shooting out the seeds, as in gorse, broom and others. In Desmodium,
Entada and others the pod is constricted between each seed, and breaks up into indehiscent one-seeded parts; it is then called a lomentum ; in Astragalus it is divided by a longitudinal septum.
The pods show a very great variety in form and size. Thus in the clovers they are a small fraction of an inch, while in the common tropical climber Entada scandens they are woody struc tures more than a yard long and several inches wide. They are generally more or less flattened, but sometimes round and rod like, as in species of Cassia, or are spirally coiled as in Medicago. In Colutea, the bladder-senna of gardens, the pod forms an in flated bladder which bursts under pressure; it often becomes detached and is blown some distance before bursting. In some cases the hard. seed-coat itself is bright-coloured as in the scarlet seeds of Abrus precatorius, the so-called weather-plant. In the ground-nut (Arachis hypognea), Trifoliurn subterraneum and others, the flower-stalks grow downwards after fertilization of the ovules and bury the fruit in the earth. In the suborders Mimo soideae and Papilionatae the embryo fills the seed or a small quantity of endosperm occurs, chiefly round the radicle. In Caesal pinioideae endosperm is absent, or present forming a thin layer round the embryo as in the tribe Bauhinieae, or copious and carti laginous as in the Cassieae. The embryo has generally flat leaf like or fleshy cotyledons with a short radicle.
Insects play an important part in the pollination of the flowers. In the two smaller suborders the stamens and stigma are freely exposed and the conspicuous coloured stamens serve as well as the petals to attract insects; in Mimosa and Acacia the flowers are crowded in conspicuous heads or spikes. The relation of insects ' to the flower has been carefully studied in the Papilionatae, chiefly in European species. Where honey is present it is secreted on the inside of the base of the stamens and accumulated in the base of the tube formed by the united filaments round the ovary. It is accessible only to insects with long probosces, such as bees. In these cases the posterior stamen is free, allowing access to the honey. The flowers stand more or less horizontally; the large, erect, white or coloured standard renders them conspicuous, the wings form a platform on which the insect rests and the keel en closes the stamens and pistil, protecting them from rain and the attacks of unbidden pollen-eating insects. In his book on the fertilization of flowers, Hermann Muller distinguishes four types of papilionaceous flowers according to the way in which the pollen is applied to the bee (see POLLINATION).