1911 Encyclopædia Britannica/Leguminosae

Fig. 1.—Leaf of an Acacia (A. heterophylla) showing flattened leaf-like petiole (phyllode), p, and bipinnate blade.

LEGUMINOSAE, the second largest family of seed-plants, containing about 430 genera with 7000 species. It belongs to the series Rosales of the Dicotyledons, and contains three well-marked suborders, Papilionatae, Mimosoideae and Caesalpinioideae. The plants are trees, shrubs or herbs of very various habit. The British representatives, all of which belong to the suborder Papilionatae, include a few shrubs, such as Ulex (gorse, furze), Cytisus (broom) and Genista, but the majority, and this applies to the suborder as a whole, are herbs, such as the clovers, Medicago, Melilotus, &c., sometimes climbing by aid of tendrils which are modified leaf-structures, as in Lathyrus and the vetches (Vicia). Scarlet runner (Phaseolus multiflorus) has a herbaceous twining stem. Woody climbers (lianes) are represented by species of Bauhinia (Caesalpinioideae), which with their curiously flattened twisted stems are characteristic features of tropical forests, and Entada scandens (Mimosoideae) also common in the tropics; these two suborders, which are confined to the warmer parts of the earth, consist chiefly of trees and shrubs such as Acacia and Mimosa belonging to the Mimosoideae, and the Judas tree of southern Europe (Cercis) and tamarind belonging to the Caesalpinioideae. The so-called acacia of European gardens (Robinia Pseudacacia) and laburnum are examples of the tree habit in the Papilionatae. Water plants are rare, but are represented by Aeschynomene and Neptunia, tropical genera. The roots of many species bear nodular swellings (tubercles), the cells of which contain bacterium-like bodies which have the power of fixing the nitrogen of the atmosphere in such a form as to make it available for plant food. Hence the value of these plants as a crop on poor soil or as a member of a series of rotation of crops, since they enrich the soil by the nitrogen liberated by the decay of their roots or of the whole plant if ploughed in as green manure.

The leaves are alternate in arrangement and generally compound and stipulate. A common form is illustrated by the trefoil or clovers, which have three leaflets springing from a common point (digitately trifoliate); pinnate leaves are also frequent as in laburnum and Robinia. In Mimosoideae the leaves are generally bipinnate (figs. 1, 2, 3). Rarely are the leaves simple as in Bauhinia. Various departures from the usual leaf-type occur in association with adaptations to different functions or environments. In leaf-climbers, such as pea or vetch, the end of the rachis and one or more pairs of leaflets are changed into tendrils. In gorse the leaf is reduced to a slender spine-like structure, though the leaves of the seedling have one to three leaflets. In many Australian acacias the leaf surface in the adult plant is much reduced, the petiole being at the same time flattened and enlarged (fig. 1), frequently the leaf is reduced to a petiole flattened in the vertical plane; by this means a minimum surface is exposed to the intense sunlight. In the garden pea the stipules are large and foliaceous, replacing the leaflets, which are tendrils; in Robinia the stipules are spiny and persist after leaf-fall. In some acacias (q.v.) the thorns are hollow, and inhabited by ants as in A. sphaerocephala, a central American plant (fig. 2) and others. In some species of Astragalus, Onobrychis and others, the leaf-stalk persists after the fall of the leaf and becomes hard and spiny.

From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.
Fig. 2.—Acacia sphaerocephala.
I, Leaf and part of stem; D, hollow thorns in which the ants live; F, food bodies at the apices of the lower pinnules; N, nectary on the petiole. (Reduced.)	II, Single pinnule with food-body, F. (Somewhat enlarged.)

Leaf-movements occur in many of the genera. Such are the sleep-movement in the clovers, runner bean (Phaseolus), Robinia and acacia, where the leaflets assume a vertical position at nightfall. Spontaneous movements are exemplified in the telegraph-plant (Desmodium gyrans), native of tropical Asia, where the small lateral leaflets move up and down every few minutes. The sensitive plant (Mimosa pudica) is an example of movement in response to contact, the leaves assuming a sleep-position if touched. The seat of the movement is the swollen base of the leaf-stalk, the so-called pulvinus (fig. 3).

Fig. 3.—Branch with two leaves of the Sensitive Plant (Mimosa pudica), showing the petiole in its erect state, a, and in its depressed state, b; also the leaflets closed, c, and the leaflets expanded, d; p, pulvinus, the seat of the movement of the petiole.

The stem of the lianes shows some remarkable deviations from the normal in form and structure. In Papilionatae anomalous secondary thickening arises from the production of new cambium zones outside the original ring (Mucuna, Wistaria) forming concentric rings or transverse or broader strands; where, as in Rhyncosia the successive cambiums are active only at two opposite points, a flat ribbon-like stem is produced. The climbing Bauhinias (Caesalpinioideae) have a flattened stem with basin-like undulations; in some growth in thickness is normal, in others new cambium-zones are found concentrically, while in others new and distinct growth-centres, each with its cambium-zone, arise outside the primary zone. The climbing Mimosoideae show no anomalous growth in thickness, but in some cases the stem becomes strongly winged. Gum passages in the pith and medullary rays occur, especially in species of acacia and Astragalus; gum-arabic is an exudation from the branches of Acacia Senegal, gum-tragacanth from Astragalus gummifer and other species. Logwood is the coloured heartwood of Haematoxylon campechianum; red sandalwood of Pterocarpus santalinus.

The flowers are arranged in racemose inflorescences, such as the simple raceme (Laburnum, Robinia), which is condensed to a head in Trifolium; in Acacia and Mimosa the flowers are densely crowded (fig. 4). The flower is characterized by a hypogynous or slightly perigynous arrangement of parts, the anterior position of the odd sepal, the free petals, and the single median carpel with a terminal style, simple stigma and two alternating rows of ovules on the ventral suture of the ovary which faces the back of the flower.

Fig. 4.—Acacia obscura, flowering branch about 1/3 natural size.
1, Part of stem with leaf and its subtended inflorescence, about natural size.	2, Flower, much enlarged. 3, Floral diagram of Acacia latifolia.   (After Eichler.)

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 (fig. 4 [3]), and the sepals and petals have a valvate aestivation, and are generally pentamerous, but 3-6-merous flowers also occur. The sepals are more or less united into a cup (fig. 4 [2]), and the petals sometimes cohere at the base. The stamens vary widely in number and cohesion; in Acacia (fig. 4) they are indefinite and free, in the tribe Ingeae, indefinite and monadelphous, 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 (fig. 5) 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 (fig. 5 [2]), valvate. The corolla shows great variety in form; it is imbricate in aestivation, the posterior petal being innermost. In Cercis (fig. 5) it clearly resembles the papilionaceous type; the odd petal stands erect, the median pair are reflexed and wing-like, and the lower pair enclose the essential organs. In Cassia all five petals are subequal and spreading; in Amherstia the anterior pair are small or absent while the three upper ones are large; in Krameria, the anterior pair are represented by glandular scales, and in Tamarindus are suppressed. Apetalous flowers occur in Copaifera and Ceratonia. The stamens, generally ten in number, are free, as in Cercis (fig. 5) 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 zygomorphic in the median plane (figs. 6, 7). The five sepals are generally united (figs. 7, 9), and have an ascending imbricate arrangement (fig. 6); the calyx is often two-lipped (fig. 9 [1]). The corolla has five unequal petals with a descending imbricate arrangement; the upper and largest, the standard (vexillum), stands erect, the lateral pair, the wings or alae, are long-clawed, while the anterior pair cohere to form the keel or carina, in which are enclosed the stamens and pistil. The ten stamens are monadelphous as in gorse or broom (fig. 9), or diadelphous as in sweet pea (fig. 8) (the posterior one being free), or almost or quite free; these differences are associated with differences in the methods of pollination. 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 (fig. 10) 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 (fig. 11); in Astragalus it is divided by a longitudinal septum.

Fig. 5.—Flowering branch of Judas-tree (Cercis siliquastrum) reduced.
1, Flower, natural size. 2, Floral diagram.

Fig. 6.—Diagram of Flower of Sweet Pea (Lathyrus), showing five sepals, s, two are superior, one inferior, and two lateral; five petals, p, one superior, two inferior, and two lateral; ten stamens in two rows, a, and one carpel, c.	Fig. 7.—Flower of Pea (Pisum sativum), showing a papilionaceous corolla, with one petal superior, st, the standard (vexillum), two inferior, car, the keel (carina), and two lateral, a, wings (alae). The calyx is marked c.

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 structures 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. Indehiscent one-seeded pods occur in species of clover and in Medicago, also in Dalbergia and allied genera, where they are winged. In Colutea, the bladder-senna of gardens, the pod forms an inflated bladder which bursts under pressure; it often becomes detached and is blown some distance before bursting. An arillar outgrowth is often developed on the funicle, and is sometimes brightly coloured, rendering the seed conspicuous and favouring dissemination by birds; in such cases the seed-coat is hard. In other cases the hard seed-coat itself is bright-coloured as in the scarlet seeds of Abrus precatorius, the so-called weather-plant. Animals also act as the agents of distribution in the case of fleshy edible pods containing seeds with a hard smooth testa, which will pass uninjured through the body, as in tamarind and the fruit of the carob-tree (Ceratonia). In the ground-nut (Arachis hypogaea), Trifolium subterraneum and others, the flower-stalks grow downwards after fertilization of the ovules and bury the fruit in the earth. In the suborders Mimosoideae and Papilionatae the embryo fills the seed or a small quantity of endosperm occurs, chiefly round the radicle. In Caesalpinioideae endosperm is absent, or present forming a thin layer round the embryo as in the tribe Bauhinieae, or copious and cartilaginous 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
Fig. 8.—Stamens and Pistil of
Sweet Pea (Lathyrus). Thestamens
are diadelphous, nine of them
being united by their filaments
f, while the uppermost one (e)
is free; st, stigma, c, calyx. 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 encloses 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 Müller distinguishes four types of papilionaceous flowers according to the way in which the pollen is applied to the bee:

(1) Those in which the stamens and stigma return within the carina and thus admit of repeated visits, such are the clovers, Melilotus and laburnum. (2) Explosive flowers where stamens and style are confined within the keel under tension and the pressure of the insect causes their sudden release and the scattering of the pollen, as in broom and Genista; these contain no honey but are visited for the sake of the pollen. (3) The piston-mechanism as in bird’s-foot trefoil (Lotus corniculatus), Anthyllis, Ononis and Lupinus, where the pressure of the bee upon the carina while probing for honey squeezes a narrow ribbon of pollen through the opening at the tip. The pollen has been shed into the cone-like tip of the carina, and the heads of the five outer stamens form a piston beneath it, pushing it out at the tip when pressure is exerted on the keel; a further pressure causes the protrusion of the stigma, which is thus brought in contact with the insect’s belly. (4) The style bears a brush of hairs which sweeps small quantities of pollen out of the tip of the carina, as in Lathyrus, Pisum, Vicia and Phaseolus.

Fig. 9.—Broom (Cytisus scoparius). (2-7 slightly reduced.)
1, Calyx. 2, Standard. 3, Wing. 4, Keel.	5, Monadelphous stamens and style. 6, Pistil. 7, Pod.

From Vines’s Students’ Text-Book of Botany, by permission of Swan, Sonnenschein & Co.	Fig. 11.—Lomentum or lomentaceous legume of a species of Desmodium. Each seed is contained in a separate cavity by the folding inwards of the walls of the legume at equal intervals; the legume, when ripe, separates transversely into single-seeded portions or mericarps.
Fig. 10.—Dry dehiscent Fruit. The pod (legume) of the Pea. r, The dorsal suture; b, the ventral; c, calyx; s, seeds.

Leguminosae is a cosmopolitan order, and often affords a characteristic feature of the vegetation. Mimosoideae and Caesalpinioideae are richly developed in the tropical rain forests, where Papilionatae are less conspicuous and mostly herbaceous; in subtropical forests arborescent forms of all three suborders occur. In the temperate regions, tree-forms are rare—thus Mimosoideae are unrepresented in Europe; Caesalpinioideae are represented by species of Cercis, Gymnocladus and Gleditschia; Papilionatae by Robinia; but herbaceous Papilionatae abound and penetrate to the limit of growth of seed-plants in arctic and high alpine regions. Shrubs and undershrubs, such as Ulex, Genista, Cytisus are a characteristic feature in Europe and the Mediterranean area. Acacias are an important component of the evergreen bush-vegetation of Australia, together with genera of the tribe Podalyrieae of Papilionatae (Chorizema, Oxylobium, &c.). Astragalus, Oxytropis, Hedysarum, Onobrychis, and others are characteristic of the steppe-formations of eastern Europe and western Asia.

The order is a most important one economically. The seeds, which are rich in starch and proteids, form valuable foods, as in pea, the various beans, vetch, lentil, ground-nut (Arachis) and others; seeds of Arachis and others yield oils; those of Physostigma venenosum, the Calabar ordeal bean, contain a strong poison. Many are useful fodder-plants, as the clovers (Trifolium) (q.v.), Medicago (e.g. M. sativa, lucerne (q.v.), or alfalfa); Melilotus, Vicia, Onobrychis (O. sativa is sainfoin, q.v.); species of Trifolium, lupine and others are used as green manure. Many of the tropical trees afford useful timber; Crotalaria, Sesbania, Aeschynomene and others yield fibre; species of Acacia and Astragalus yield gum; Copaifera, Hymenaea and others balsams and resins; dyes are obtained from Genista (yellow), Indigofera (blue) and others; Haematoxylon campechianum is logwood; of medicinal value are species of Cassia (senna leaves) and Astragalus; Tamarindus indica is tamarind, Glycyrrhiza glabra yields liquorice root. Well-known ornamental trees and shrubs are Cercis (C. siliquastrum is the Judas-tree), Gleditschia, Genista, Cytisus (broom), Colutea (C. arborescens is bladder-senna), Robinia and Acacia; Wistaria sinensis, a native of China, is a well-known climbing shrub; Phaseolus multiflorus is the scarlet runner; Lathyrus (sweet and everlasting peas), Lupinus, Galega (goat’s-rue) and others are herbaceous garden plants. Ceratonia Siliqua is the carob-tree of the Mediterranean, the pods of which (algaroba or St John’s bread) contain a sweet juicy pulp and are largely used for feeding stock.

The order is well represented in Britain. Thus Genista tinctoria is dyers’ greenweed, yielding a yellow dye; G. anglica is needle furze; other shrubs are Ulex (U. europaeus, gorse, furze or whin, U. nanus, a dwarf species) and Cytisus scoparius, broom. Herbaceous plants are Ononis spinosa (rest-harrow), Medicago (medick), Melilotus (melilot), Trifolium (the clovers), Anthyllis Vulneraria (kidney-vetch), Lotus corniculatus (bird’s-foot trefoil), Astragalus (milk-vetch), Vicia (vetch, tare) and Lathyrus.