The common red clover (_Trifolium pratense_) has its flowers in such dense heads that it is difficult at first sight to discover that they are Papilionaceous. On examination, however, it will be found that each separate flower has its standard, wings, and keel, though the wings are so large as to hide the keel, and nearly to obscure the standard. The calyx is tubular at the base, but divided above into five long, awl-shaped teeth, that stand widely apart from each other. The legume has only one or two seeds, and it is so small as generally to be hidden by the calyx. TRIBE II.—MIMOSÆ. [Illustration: FIG. 15.—FLOWERS AND SPRIG OF ACACIA ARMATA.] The second division of Leguminosæ comprises those plants which have heads of flowers either in spikes or balls, like those shown in _fig._ 15. This figure represents two heads of flowers of _Acacia armata_, a well-known greenhouse shrub, of their natural size; and _fig._ 16 shows a head of similar flowers magnified. In the latter, _a_ shows the calyx, which is five-toothed, and _b_ the petals, which are five in number and quite regular in shape; _c_ are the stamens, which vary from ten to two hundred in each flower, and which are raised so high above the petals as to give a light and tuft-like appearance to the whole flower. The legumes are very large in proportion to the flower; and consequently, by a wise provision of nature, only a very few of the flowers produce seed. The valves of the legumes are not fleshy like those of the pea, but dry and hard, and when they open they do not curl back. [Illustration: FIG. 16.—FLOWER OF ACACIA MAGNIFIED.] The flowers in the different kinds of Acacia, differ in the corolla, which has sometimes only four petals, which are occasionally united at the base, and in the calyx, which is sometimes only four-cleft. The flowers also in many species are in spikes instead of balls. [Illustration: FIG. 17.—THE BI-PINNATE LEAF OF AN ACACIA.]