Aeschynomene elaphroxylon

The holotype, which is kept in Paris, was collected by George Samuel Perrottet in northwestern Senegal on a small island in the lake of "N'Gher, or Panié-Foul" at the mouth of the "Marigot de Taoué River".[8][9][10] It was first described as Herminiera elaphroxylon by Perrottet and Jean Baptiste Antoine Guillemin in 1830–1833, who mention that the natives knew the plant by the name bilor or billeur, and that Michel Adanson had already written about it under that name growing on a floating island in his book about his time in Senegal, Voyage au Sénégal.[10] It was moved to the subgenus Ochopodium in the genus Aeschynomene by Paul Hermann Wilhelm Taubert in 1894 in the book series Die Natürlichen Pflanzenfamilien.[11]

Smithia grandidieri, collected by Alfred Grandidier on the southwest coast of Madagascar and thence described in 1883, was synonymised with Aeschynomene elaphroxylon in 1971.[5][9]

It is placed in the Tribe Dalbergieae.[12][13]

Aeschynomene elaphroxylon is indigenous to most of tropical Africa, and is also found in Madagascar, where it was possibly naturalized.[1][4][5] In Madagascar it is found in Mahajanga and Toamasina Provinces,[3] and grows in dense stands along the shoreline of Lake Alaotra.[14] It occurs southwards to northern Mozambique,[15] southern Malawi,[8] northern Zimbabwe[16] to Angola.[8]

In Ethiopia the species can be found in the Nechisar National Park.[17]

It appears to have naturalised at Lake Okeechobee, Florida.[18]

Aeschynomene elaphroxylon is an evergreen,[19] semi-aquatic shrub or small tree often found growing in water.[5][4][20] It grows two to nine, exceptionally up to twelve, metres high.[8][9] The straight trunk is swollen (caudiciform) and somewhat conical in shape,[5][8] and can reach a diameter of up to 50cm at breast height, at a height of ten metres.[21] There can be spines on the swollen trunk.[7][14] The wood is spongy and extremely lightweight, it is often mistakenly called pith, this acts to help the plant stay afloat.[5][8][22] 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.[5][21] The stems are spiny, with pairs of substantial thorns under the base of leaves.[23] The short sharp spines are 2–15mm long, and nearly always present in this species.[8] Both the trunk and the branches are covered in sticky, glandular, hispid (bristly) hairs,[7][15] both long and short.[5][8]

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 (uncommon, resembling phloem, but very slender and relatively thick-walled). The vascular bundle is very small.[21]

It has a chromosome count of 2n=40.[24]

The leaves are compound. Their petiole and rhachis together measure 4–16cm long, and both are densely covered with both stiff bristly hairs and shorter pubescence, and also often with short, minute spinelets.[8][9][15] The petiolules measure 0.5–1mm in length.[8][9] It has peltate stipules which have an erose (irregularly notched) appendage below the point of attachment, 2—4mm in diameter.[7] 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 (7-)10–13mm in length, (3-)5-6(-9)mm in width, and eventually fall off as the leaf matures.[7][8][9] The leaves have 20-40 leaflets which have a single rib or raised vein below. The leaflets are (5-)8–26.5(-30)mm long and 4–10mm 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 (not a mirror image).[7][8][9][15] 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 (feeling somewhat rough to the touch).[7][9][15] The secondary veins on the underside of the leaflets are usually blackish or dark-coloured.[7][8][9]

Aeschynomene elaphroxylon flower. Note indumentum and spines on the stem.

The 30—45mm long,[7] relatively large,[22] beautiful flowers[11] are single or usually bunched together in groups of two to four in an axillary, racemous inflorescences.[7][15] The inflorescence has ovate bracts which are 6–7mm long and 3–5mm wide, as well as bracteoles which are 10–16mm long and 5–10mm wide.[8][9] Both the bracts and bracteoles end with a sharp point, and are pubescent adaxially.[7] These bracteoles are ovate or elliptic, and asymmetrical, in shape, and covered in velvety hairs. The peduncle is 1.3cm long, the inflorescence rhachis is 1–4.5cm long and the pedicels are 1.2–1.6cm long.[8][9] Both are hispid like the stems.[7] The sticky, green calyx[14] 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, 1.9–2.5cm long and 0.6–1.2cm wide, the other lip is ovate-oblong, 1.5–2cm long and 0.6–1.1cm wide.[8][9] All of the petals are somewhat pubescent.[7] The 3–4.7cm long and wide (sometimes longer than wide), rounded or spatulate standard is yellow to orange;[7][9][15][22] the wings and keel are yellow,[8][9] or greenish-yellow.[14] The standard is positioned upright,[14] 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—30mm long and 10—15mm wide.[7][9] The keel petals are slightly shorter than the standard, their claws are about 5mm long.[7] The keel opens at the base for the emergence of the stamens.[14] The glabrous, oblique wings are slightly longer than the standard, 35—40mm long and 15—20mm wide; their claws are about 5mm long.[7] The ten stamens are 28—40mm long.[7][11]

The seeds are black or dark purplish-brown, 5-6mm long, 3-3.8mm wide,[7][8][9] 2mm thick, and have a slightly eccentric hilum. The seeds are reniform and slightly beaked in shape, the beak resembling the front of a canoe.[8][9] The seeds are contained in flattened, erect pods that grow in contorted into twisted full or half spirals on the tree.[9][15][20] The pods are 10-14cm 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.[8][9][15] The (5-)6–8(-10)mm long and 7–9mm wide chambers are more or less oblong or trapezoid.[7][8][9] The fruit has short stipules.[7]

Aeschynomene elaphroxylon grows by or in water and in waterlogged soils in swamps, lakes and along rivers,[4][5][21][20] typically growing in one to two metre deep of standing water.[1][5] It is only found in freshwater; it can be seen as a freshwater mangrove.[3] The dense tangle of roots are adapted to supporting the tree when the water subsides, in areas where there are fluctuating water levels over the course of the year.[21] It will occur in dense monospecific stands.[14] It sometimes grows in large floating mats of trees or on floating islands.[21][22] It occurs in dry and subhumid climates.[3] It is extremely fast-growing,[4][11][22] and often grows in stands which can rapidly colonise rivers and swamps. Throughout its entire African range, it has been registered at altitudes of 70–1,850 metres.[5] It is found at 520–1,350m in altitude in East Africa,[9] 470–1,060m in the Zambezi region,[8] or very roughly 500–1,499m in Madagascar.[3]

A large dense monospecific stand of small 4-5m trees of Aeschynomene elaphroxylon, some knocked over, with swollen trunks, along the shoreline of Lake Alaotra, near Andrebagara at 756m in Ambatondrazaka District, Madagascar.

In Mangochi District in southern Malawi, it is found growing along the lake shores and in seasonally flooded areas associated with the shrubs Aeschynomene afraspera, Ae. cristata, Sesbania sesban, S. sericea and S. rostrata, and the grasses Eriochloa borumensis, Panicum repens, Phragmites mauritianus and Vossia cuspidata.[25]

In the lagoons and extensive swamps of the Volta Delta in southern Ghana it is the dominant plant growing in floating mats and islands together with two other frequent woody plants, Ficus congensis and Alchornea cordifolia. The ambatch tree is associated with the fern Cyclosorus striatus here, as well as the grasses Leersia hexandra and African wild rice Oryza barthii, the curcubit vine Zehneria capilacea and the herbs Ipomoea aquatica, Pentodon pentandrus and Polygonum tomentosum. It also often forms thickets which cover large stretches of banks and shallow waters.[21]

Flooded areas are not the only places where it can grow, a study of the phytosociology on the islands of Lake Ziway, an Ethiopian Rift Valley lake, identified a common plant community dominated by Ae. elaphroxylon and Sesbania sesban, with Euphorbia tirucalli and Acacia tortilis as co-dominant species, and characterised by the presence of Withania somnifera. This community occurred at an altitude of 1,673m and slope of 5%.[26] The vegetation along the shores of Lake Ziway is composed of bulrush (Paspalidium geminatum), papyrus (Cyperus papyrus), blue water lily (Nymphaea nouchali) and Ae. elaphroxylon.[27] In Arsi Negele (woreda) in Oromia region in southeastern Ethiopia, where it is called boboffee, it occurs in Kimphe Lafa forest, a remnant of dry afromontane forest. It also occurs in the Gughe-Amaro Mountains in the southwest Ethiopian highlands.[19]

In a historical study of how different plant species have reacted to differing water levels in Lake Okeechobee, Florida, Ae. elaphroxylon was found to grow after the native aquatic plants which emerge from the water were killed off as a result of an extended decline in water level (a future scenario considered relatively unlikely). Conversely, very low water on a regular basis or consistent high water levels would both likely favour expansion of other plant species.[18]

Seed dispersal is achieved by wind and water.[22] The seeds of Ae. elaphroxylon have been known to stay viable after being buried for a few years in the waterlogged banks where it grows.[28] The seeds are specifically preyed upon by the larvae of the seed weevil Bruchidius kiliwaensis, which has not been found in any other species as of 2008.[29] The leaf-defoliating sesbania beetle, Mesoplatys ochroptera, earlier said to feed on Aeschynomene, was found not to do so in the wild. The sap-sucking shield bug Brachyplatys testudonigro, which is common in Malawi, likely does feed upon it, as it has regularly found on other Aeschynomene and many other similar legumes like Crotalaria, Indigofera and Tephrosia.[25]

The bark is attacked by Phomopsis herminierae, a type of Coelomycetes fungus.[30]

Although it is sometimes seen as a weed, the conical poles are also harvested for various purposes.[4][36] The spongy wood, often mistakenly termed pith, is lighter than cork, yet very resistant to rot.[22] Unlike pith, it contains thick tree rings. It is extremely lightweight and soft.[10] The trunk wood is traditionally used to make floats for fishing nets, and used in raft, canoe[1][9][21] and boat construction, for example around Lake Chad.[22] The trunks were cut in foot-long sections for use as net floats in coastal Senegal already in the 1820s.[10] At least in the 1950s, sandals were manufactured from the wood in Ghana, as well as screens for blocking the sun.[21] Some French entomologists also used thin slices of the wood as a base in which to pin their specimens in the early 1800s.[10]

On the remote Lake Keilack in South Kordofan, much visited by nomads with their cattle, three bundles of trunks are bound together to make a small rafts propelled using a long pole like a punt.[37] It is sometimes seen as a nuisance in Lake Chad, as the rapidly growing stands are so very densely matted with interlocking roots, they can make it very difficult to fish. Wildlife proliferates protected in the stands.[22] Ae. elaphroxylon is locally called bofoffe by the Zay people who live on islands as well as shore areas of Lake Ziway in the Ethiopian Rift Valley. Bofoffe is used here for making raft boats and stools.[27][38]

Ae. elaphroxylon of Lake Ziway was found to bioaccumulate a high concentration of the trace element manganese (1.6 × 103mg per kg of dry weight). Nonetheless, among the ten plants studied, it was not considered the best bioindicator of pollution.[39]

Pollen or phytoliths of this species found in drill cores of lake sediments can be used to track the presence of past lakes and fluctuations in lake-levels and climates in Africa.[40][41]

Of all the aquatic Aeschynomene species, especially Ae. elaphroxylon is larger-flowered, and has therefore been planted as an ornamental in some areas.[7]

It has been cultivated in Malaysia,[7] Sri Lanka,[6] and South America. It has also been grown as an ornamental in Bogor on Java, at least in the 1950s or earlier.[7][42] It is said to be cultivated in Egypt,[9] as it was said to be studied in the Nile in 1891 by Klehbahn during the British military occupation of the country.[21] In actually, Klehbahn was sent specimens of the plant by the explorer Georg August Schweinfurth, who was in Cairo at the time and related that a number of fine specimens were grown there in a small basin in the botanical garden of the medical school, in such a luxuriant state as he had not seen during his travels in the upper Nile and beyond. The plant was also grown in the botanical garden in Berlin in the 1880s, but this accession died despite the best efforts of Ignatz Urban -cuttings from Schweinfurth were later grown here.[31]

Based on the description as related by Schweinfurth, it would appear that the plants in cultivation, in Egypt at least, grew differently than those in the wild, as the plants lacked the aerial roots described by earlier writers and seen by him elsewhere in Africa, although we can assume that Schweinfurth observed the brachyrhizae, from his description of "filzartiges Geflecht" of roots.[31]

In Brazil, it has been cultivated in the Jardim Botânico do Rio de Janeiro since 1915, collected in Africa. It was grown in the "dry arboretum" at "Lago do Frei Leandro" (the pond with the Victoria lilies) in the 1980s. It has been grown in the Jardim Botânico de São Paulo since 1941.[43]

• Dressler, S.; Schmidt, M. & Zizka, G. (2014). "Aeschynomene elaphroxylon". African plants – a Photo Guide. Frankfurt/Main: Forschungsinstitut Senckenberg.