Page:EB1911 - Volume 16.djvu/402

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