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| 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.
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| 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).
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| 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
