Fig. 17.—The Bi-pinnate leaf of an Acacia.
The rest of the plant of Acacia armata is very curious; what appear to be the leaves (see d in fig. 15) are, in fact, only the petioles of the leaves dilated into what are called phyllodia; the true leaves, which were of the kind called bi-pinnate, having fallen off, or never unfolded. The true leaves, however, often appear on seedling plants; and thus, when seeds are sown of several kinds of Acacia, it is sometimes difficult to recognise them till they have attained a considerable age. The stipules of the leaves, (which are to ordinary leaves what bracts are to flowers,) are in Acacia armata, converted into spines, as shown at e. In some kinds of Acacia the true leaves, with the petioles in their natural state, (see fig. 17,) are retained in the adult plants, as in Acacia dealbàta; and in others, the bi-pinnate leaves are occasionally found attached to the phyllodia, as in A. melanoxylon. The bi-pinnate leaves are composed of from six to twenty pairs of pinnæ, or compound leaflets (see f in fig. 17), each of which consists of from eight to forty pairs of small leaflets (g). The Gum Arabic tree, Acacia vera, has leaves with only two pairs of pinnæ, but each has eight or ten pairs of small leaflets. The branches and spines are red, and the heads of flowers are yellow. There are above three hundred known species of Acacia.
The genus Mimosa differs from Acacia in the corolla being funnel-shaped, and four or five cleft. There are seldom above fifteen stamens, which are generally on longer filaments than those of the Acacia; and the legume is compressed and jointed or articulated between the seeds, so that the part which contains one seed may be broken off, without tearing the rest. The Sensitive-plant (Mimosa pudica) is a familiar example of this genus.
The cotyledons of the plants belonging to this tribe are generally leafy; and the seeds are not eatable. The plants themselves are easily recognised by their ball or tassel-shaped heads or spikes of flowers; by the small cup-shape and inconspicuous corolla of each; by the great number and length of the stamens; and by their bi-pinnate leaves, or phyllodia supplying the place of leaves—though the phyllodia are sometimes found in Australian plants with papilionaceous flowers, as, for example, in Bossiæa ensata.
TRIBE III.—CÆSALPINEÆ.
Fig. 18.—Flower of the Barbadoes Flower-fence.