Aeschynomene elaphroxylon
Aeschynomene elaphroxylon, also known as an ambatch, pith-tree, balsa wood tree, or umburu, is a common large shrub to small tree in the genus Aeschynomene in the family Fabaceae, growing in swamps, lakes and rivers in Tropical Africa. This is an unusual leguminous tree in that it grows in water as a freshwater mangrove, with an extremely lightweight wood acting as a float and a specialised root system sprouting from the trunk which forms a tangled web hanging through the water and sprawling through the mud. It has adventitious roots and roots which are differentiated into special structures adapted to the swamp environment. It can even grow as floating islands of drifting forests.
The name 'ambatch' derives from the name of the plant in the Arabic dialect spoken in Nigeria known as Shuwa by the British. It is called Nile pith tree in Sri Lanka, ambach there and in Malaysia, or ambatsch in Malaysia.
The extremely light, spongy, but rot-resistant wood is traditionally used to make floats for fishing nets and the trunks are lashed together to make rafts, as well as more sophisticated items.
Description
Aeschynomene elaphroxylon is an evergreen, semi-aquatic shrub or small tree often found growing in water. It grows two to nine, exceptionally up to twelve, metres high. The straight trunk is swollen and somewhat conical in shape, and can reach a diameter of up to 50 cm at breast height, at a height of ten metres. There can be spines on the swollen trunk. The wood is spongy and extremely lightweight, it is often mistakenly called pith, this acts to help the plant stay afloat. As the trees grow larger, their trunks are frequently knocked over, and later curve back towards the sun. Their branches touching the soil or water surface, or submerged underwater, are densely covered in adventitious roots. The trees or saplings can even dislodge and are sometimes found floating as components of a mat of vegetation which covers the waters. The trunk bole just above the waterline also forms a tangle of roots. A group of plants form thick mats due to these roots. The stems are spiny, with pairs of substantial thorns under the base of leaves. The short sharp spines are 2–15 mm long, and nearly always present in this species. Both the trunk and the branches are covered in sticky, glandular, hispid hairs, both long and short.The wood is largely built up of regular layers of palisade-like tracheids with un-lignified cell walls. Isolated vessels are scattered in this mass of tracheids, accompanied by tangentially arranged groups of libriform cells. The vascular bundle is very small.
It has a chromosome count of 2n=40.
The leaves are compound. Their petiole and rhachis together measure 4–16 cm long, and both are densely covered with both stiff bristly hairs and shorter pubescence, and also often with short, minute spinelets. The petiolules measure 0.5–1 mm in length. It has peltate stipules which have an erose appendage below the point of attachment, 2–4 mm in diameter. These stipules have a lobe on one side only, are velvety in texture, broadly ovate in shape with the upper portion deltoid-ovate and ending in an acute tip, are 10–13 mm in length, 5–6 mm in width, and eventually fall off as the leaf matures. The leaves have 20–40 leaflets which have a single rib or raised vein below. The leaflets are 8–26.5 mm long and 4–10 mm wide, entire, mucronulate, their shape is elliptic and oblong or obovate, with their apex truncate, retuse or slightly emarginate, and their bases where they connect to the rhachis are obliquely rounded. The leaflets are mostly hairless and coloured blue-green on their upper sides, and somewhat glaucous on the undersides, which are covered in slightly tubercular-based, short, coarsely pubescent hairs, especially along the edges and on the mid-vein on the underside, where the minute hairs are often asperulous. The secondary veins on the underside of the leaflets are usually blackish or dark-coloured.
The 30–45 mm long, relatively large, beautiful flowers are single or usually bunched together in groups of two to four in an axillary, racemous inflorescences. The inflorescence has ovate bracts which are 6–7 mm long and 3–5 mm wide, as well as bracteoles which are 10–16 mm long and 5–10 mm wide. Both the bracts and bracteoles end with a sharp point, and are pubescent adaxially. These bracteoles are ovate or elliptic, and asymmetrical, in shape, and covered in velvety hairs. The peduncle is 1.3 cm long, the inflorescence rhachis is 1–4.5 cm long and the pedicels are 1.2–1.6 cm long. Both are hispid like the stems. The sticky, green calyx is densely covered in both pubescent and longer, glandular, hispid hairs. The calyx has two lips. These lips are entire or sometimes have two or three teeth at their apex, one lip is ovate-lanceolate in shape, 1.9–2.5 cm long and 0.6–1.2 cm wide, the other lip is ovate-oblong, 1.5–2 cm long and 0.6–1.1 cm wide. All of the petals are somewhat pubescent. The 3–4.7 cm long and wide, rounded or spatulate standard is yellow to orange; the wings and keel are yellow, or greenish-yellow. The standard is positioned upright, and is emarginate and adaxially puberulous near its apex. The petals of the keel are laciniate along their lower edges where the two petals unite, and pubescent on the adaxial surface. Their blades are falcate, oblique, and about 25–30 mm long and 10–15 mm wide. The keel petals are slightly shorter than the standard, their claws are about 5 mm long. The keel opens at the base for the emergence of the stamens. The glabrous, oblique wings are slightly longer than the standard, 35–40 mm long and 15–20 mm wide; their claws are about 5 mm long. The ten stamens are 28–40 mm long.
The seeds are black or dark purplish-brown, 5–6 mm long, 3–3.8 mm wide, 2 mm thick, and have a slightly eccentric hilum. The seeds are reniform and slightly beaked in shape, the beak resembling the front of a canoe. The seeds are contained in flattened, erect pods that grow in contorted into twisted full or half spirals on the tree. The pods are 10–14 cm long when unrolled, and have 6–17 joints or chambers. They are densely covered in glandular bristly hairs standing on little tubercles, also with short pubescent hairs. There are veins beneath the dense indumentum. The 6–8 mm long and 7–9 mm wide chambers are more or less oblong or trapezoid. The fruit has short stipules.
Roots
The root system of Ae. elaphroxylon is quite peculiar. As mentioned before, it produces numerous adventitious roots from the trunk and branches, and these roots are strictly separated into two different types, a handful differentiated as strongly growing macrorhizae and the majority being short brachyrhizae. The macrorhizae grow as a mass of long branching roots through the mud and water, the brachyrhizae grow laterally along them in dense rows. These brachyrhizae all look very similar to each other, they do not branch or elongate further, but grow a special bark containing corky inner tangential cell walls. The loose skeleton of macrorhizae roots, relatively restricted in volume, covered by a thick coat of brachyrhizae hanging in the shallow waters and in the waterlogged bottom of lakes are a unique adaptation to the free movement of nutrients in water.The macrorhizae grow indeterminately in length, split into different branches as they grow, thicken as they age, are 3–5 mm thick at their tips, and light brown in colour. They have no genuine root hairs above the zone of their elongation, and very rarely branch, unless the growing tip hits the soil or is damaged. They may arise from the bole of the tree, but can also occur as big adventitious roots on submersed stems and branches. Unlike mangroves and other swamp trees such as Taxodium distichum, this species does not make specialised pneumatophores, required for gas exchange in the anoxic swamp mud; instead, this species relies upon the adventitious roots which are exposed with drops in water level, and sprout from the lower part of the bole above the water level.
The brachyrhizae are adventitious roots which resemble rust-brown hairs in this species, and conspicuously coat the older lower stems, covering the base of the bole and adjoining root spurs, and also on the underwater roots and in the waterlogged soil. They are all uniform in form: unbranched, of equal thickness and of similar length. There are no root hairs, and they can form on roots and stems in any stage of growth. There is no secondary growth: no cambial activity, no periderm formation. Under the thin rhizodermis there is a clear one-celled layer of exodermis, with a thickened inner tangential cell wall containing layers of suberin, clothing a thin cortex containing only three parenchymatous layers of cells. Most of the brachyrhizae are connected in the area of lenticels through a single of the bigger medullar rays with a single one of the isolated longitudinal vessel elements scattered in the secondary xylem of the macrorhizae. This small connection does not allow for any subsequent anastomosis with other vessels in the wood and this prevents further growth of the brachyrhizae. Nonetheless, the brachyrhizae are remarkably long-lasting compared to those of most trees. Older brachyrhizae are connected to an inner transverse vessels linking the lateral root with the longitudinal vessel in the mother root, younger ones are linked with one of the outer vessels. Thus some brachyrhizae can be seen to persist for many years. The durability of such simple primary tissues in these roots is thought to be due to the protection offered by a particularly strong exodermis.
Neither forms of roots seem to have endophytic fungi in their cortex. Brachyrhizae also lack signs of epiphytic fungi.