The Acacia Plant

The Leaf | The Inflorescence | Pollination | Seeds | Growth

The seedling and leaf

Like all dicotyledons, the developing Acacia seedling commences its life with the formation of a pair of cotyledons. In acacias these seed leaves act as a food storage for the young plant for the first two weeks following germination¹⁹ (the seeds being nonendospermic). Following this initial rapid consumption of reserve proteins the cotyledons develop photosynthetic capability and begin producing food for the young seedling. After about three weeks a single, or a pair, of pinnate leaves develop. Continued growth usually results in the formation of bipinnate leaves. Sometimes the rachis of a bipinnate leaf is terminated at the apex by a terminal seta .

Fig. 1. Leaf and phyllode terms

One or more glands, which when young often secrete a nectar, occur on the topside of the petiole. Jugary glands may occur between the bases of a pair of pinnae, and interjugary glands may occur along the rachis between two or more pairs of pinnae. Occasionally glands may also occur on the rachilla. While ants, bees and other insects often feed on the nectar secreted by these glands, their primary function is not clear.

Phyllode development

Almost all non-Australasian and some Australasian species develop to maturity bearing only bipinnate leaves. A large number of the Australian species, however, soon develop phyllodes. The phyllode looks like and functions in the same way as an ordinary flat leaf. However, unlike a leaf, the phyllode is formed from the petiole and/or leaf rachis of the bipinnate leaf by becoming either vertically flattened or sometimes cylindrical, and with the loss of the pinnae. A few species lack both leaves and phyllodes. The apex of the phyllode may be acute , or acuminate , or obtuse , or occasionally emarginate. Frequently the apex is terminated by a mucro , which is a small, usually hard, sometimes pungent, projection.

A gland, or sometimes more than one gland, is usually situated on the upper margin of the phyllode. Glands may be of three types³², namely 'porate' in which a central cavity leads to an external pore; 'non-porate' which has no central cavity and no external pore; and 'flat', an uncommon type which differs from the non-porate type in its vascular structure. There is reasonable correlation between the gland type and the Sections of the Genus (refer to Chapter 3).

Phyllodes may be penninerved or parallel nerved. Further, the smaller or secondary nervature of parallel nerved phyllodes may form an intersecting network or reticulation, referred to as anastomosing, or if this is lacking, non-anastomosing.

Many phyllodes are flat, with outlines commonly being linear or oblong , elliptic, lanceolate, ovate, oblanceolate or obovate. They may, inaddition, be straight, curved or falcate. Some phyllodes are cylindrical or narrow linear and thick, and are referred to as being terete. Occasionally they may be square in cross section, and called tetragonus. Phyllodes which are thick and leathery are referred to as being coriaceous.

Nearly all species have articulate leaves or phyllodes which enables movement of the petiole. This is facilitated by a pulvinus - a small cushion-like swelling at the attachment of the petiole to the branchlet. A few species have decurrent leaves or phyllodes inwhich the petiole or phyllode, or part thereof, is continuous with the branchlet.

Stipules are small outgrowths at the base of the pulvinus, and they are often deciduous or become indurated and hardened. Sometimes they may be spine-like and pungent or very occasionally they may be like small phyllodes.

Branchlets and inflorescence axes are mostly straight, they may, however, be zig-zag between the nodes or phyllode bases (that is, flexuous). Occasionally the branches bear lenticels, which are slightly raised powdery spots consisting of bundles of cells in the bark and are permeable to atmospheric gases.

Indumentum and surface coverings

To reduce moisture loss and to dissuade insects from eating them, the parts of the plant, and particularly the young organs, may be variously covered. These covering include hairs, sticky resinous exudations, waxy films or powdery coatings.

A covering of hairs is often referred to as being pubescent or puberulous, depending upon the nature and size of the hairs. Terms such as hoary or hirsute are also used when the covering is of dense, very short white or grey hairs in the former, or long and coarse hairs in the latter, and hairs lying flat against the surface are appressed while spreading hairs diverge widely from the surface. Organs without hairs are glabrous.

A varnish-like or sticky resinous or viscid coating is quite common, its main function being to deter insect predation. This also appears to be the function of scabrous or asperulate parts which have the surface roughened with minute projections.

In drier environments species often have a whitish waxy (pruinose) coating which has a whitish-blue glaucous appearance. A covering of a very fine whitish grey or brownish scaly powder is refered to as being scurfy.

The Leaf | The Inflorescence | Pollination | Seeds | Growth