III. A. SENEGAL

3.1 A. Senegal: Recognition and Relationship

A. senegal has flowers in spikes but belongs to a sizeable and complex group of spicate-flowered acacias with prickles either in threes or singly (not in pairs) on the branchlets.

Several species are closely related to and may be confused with A. senegal. The following features of A. senegal are important in separating it from its relatives:-

Bark on branchlets not conspicuously peeling off. Gland on petiole small, 0.5–0.75 mm in diameter. Pinnae mostly 3–8 pairs, rarely as many as 12, or as few as 2. Leaflets in 7–25 pairs per pinna, small, mostly 0.5–2 mm wide, not glaucous. Corolla 2-75-4 mm long. Stamen filaments to 7 mm long. Pods more or less puberulous or pubescent, 1.2–3.4 cm wide.

A full account of the complex of species involving A. senegal is given by Ross (1979), with a key on pp. 24–5. The following species are those most similar to and likely to be confused with A. senegal; under each is given the countries of occurrence and the principal distinction from A. senegal:-

A. asak (Forsk.) Willd. Sudan, Ethiopia, Arabia.
Leaflets glaucous, 1–25–3.75 mm wide. Pods glabrous, 0.9–1.5 (-1.8) cm wide.

A. cheilanthifolia Chiov. Somalia.
Leaflets 3–4(-5) pairs per pinna. Pods 1–1.1 cm wide.

A. dudgeoni Craib ex Holland. West Africa (Mali to Nigeria)
Pinnae in 7–16 pairs per leaf. Pods glabrous when mature.

A. hamulosa Benth. Ethiopia, Somalia, Kenya, Arabia.
Axes of pinnae each with a single hooked prickle on lower side at or near apex (absent in A. senegal).

A. hunteri Oliv. Arabia.
Internodes very short. Inflorescences very short, about 1 cm. Pods short, 2–5 cm long.

A. sp. near senegal (Ross 1979, p.21). Somalia.
Pinnae in 1 (rarely 2) pairs per leaf and prickles on branchlets solitary.

A. thomasii Harms. Kenya.
Shrub or tree with long wand-like branches and corollas 6.5–7 mm long.

3.2 A. senegal: Summary of previous taxonomic work

Although A. senegal has been known since the eighteenth century and is an economically important species as the source of gum arabic, its delimitation has presented constant difficulty (as can be seen from its synonymy) and is not altogether clear even today. Brenan (1953) recognised var. leiorhachis and var. rostrata as geographical variants and clarified the distinctness of A. asak and A. hamulosa from A. senegal. Ross (1968) gave an excellent general account of A. senegal and further clarified the variation. The best account of the relationship between A. senegal and allied species is given by by Ross (1979).

Fig.4. Acacia senegal var. senegal. (A) Flowering branch (approx. x.5); (B) detail of leaf (approx. x 2); (C) flower (approx. x 5); (D) pod (approx. x .5) with detail of hair on surface (approx. x 10); (E) seed (approx. x .5);
Acacia senegal var. rostrata. (F) pod (approx. x.5).

3.3 A. senegal: Brief description

A shrub or tree usually 2–15 mm high, rarely smaller. Bark on trunk yellowish to grey or greyish-brown, rough or smooth and papery and peeling off (see description of varieties, pp. 15–18 Crown variable, loose and rounded to dense and flattened, sometimes open with long whippy branches. Young branchlets glabrous to densely pubescent. Prickles 2–8 mm long, in threes, with the central one curved downwards and the two laterals more or less curved upwards, or singly with the laterals absent. Petiole usually glandular. Pinnae 2–6(-12) pairs, with glands on the rhachis between the top 1–5 pairs of pinnae. Leaflets 7–25 pairs per pinna, glabrous to somewhat pubescent, 1–7 mm long, 0.5–2 mm wide. Flowers whitish or yellowish-white, more or less scented, in spikes 2–10 cm long, or peduncles 0.5–2 cm long. Pods dehiscent, straight, grey to brown or yellowish-brown, pubescent or puberulous, oblong or elliptic, 3–14(-19 in var. leiorhachis) cm long, 1.3–3.4 cm wide. Seeds (2-)4–8 per pod, more or less transverse.

Fig.5. Acacia senegal. Map showing approximate distribution of the species and varieties.

3.4 A. senegal: Distribution

Widespread in tropical and subtropical Africa, in eastern Africa from South Africa northwards to the Sudan, on the west side in southern Angola and Namibia/South West Africa and from Senegal eastwards. Also in Arabia, Pakistan and India.

3.5 A. senegal: Variation

(i) Introduction

Four varieties are at present recognised within the widespread and variable A. senegal. As emphasized by Ross (1968, 1979), the delimitation of these varieties is far from satisfactory and more information about them is badly needed. Specimens are often difficult to assign and even the actual number of varieties involved is not certain. However, there is no doubt that much of the variation is recognisable in the field and taxonomically significant.

(ii) Key to varieties

Fig.6. Acacia senegal var. senegal. Habit. of trees Kenya, Nakura District (J. Knight 34C)

(iii) Descriptions, synonymy and distributions of varieties

1. A. senegal var. senegal

   Synonyms:-

   Mimosa senegal L. (1753)

   Mimosa senegalensis Houtt. (1774), non Forsk.

   A. verek Guill. & Perr. (1832)

   A. rupestris Stocks ex Boiss. (1872)

   A. virchowiana Vatke & Hildebr. (1880)

   A. senegal var. typica Fiori (1911)

   A. senegal var. platyosprion chev. (1932)

   A. senegal subsp. modesta (Wall.) Roberty var. rupestris (Stocks ex Boiss.) Roberty (1948)

   A. senegal subsp. senegalensis (Houtt.) Roberty var. verek (Guill. & Perr.) Roberty (1948)

   Tree up to 15 m high with lax and rounded or flat and spreading crown. Bark on trunk not papery and peeling but greyish to yellowish-brown, rough, fissured, sometimes flaking. Young branchlets sparsely to densely pubescent. Inflorescence-axes sparsely to densely pubescent, rarely glabrous. Pods rounded to acute, rarely acuminate at apex.

   Typical var. senegal occurs in Africa and Asia: in Africa it is recorded from Senegal, Gambia, Mali, Ghana, Niger, Nigeria, Sudan, Ethiopia, Somalia, Central African Republic, Cameroun, Zaire, Rwanda, Uganda, Kenya and Tanzania; in Asia it is recorded from Pakistan (Sind, Baluchistan) and India (Rajasthan, Punjab, Delhi) (see Ali, 1973).

2. A. senegal var. kerensis Schweinf. (1896)

   Shrub or bush 1–5 m high, branching from base with trunk short or almost absent. Young branchlets usually pubescent. Leaves with up to 4 pairs of pinnae. Inflorescence-axes pubescent. Pods rounded to acute at apex, rarely acuminate.

   The var. kerensis is recorded from Ethiopia, Somalia, Uganda, Kenya and Tanzania.

   Based as it is primarily on growth-habit, the var. kerensis is often hard to assess and identify from dried specimens. Field evidence from north-eastern Africa indicates that it is distinctive and easily recognisable, but further information is much needed, both from there and from east Africa. It is far from certain if all shrubby forms of A. senegal are taxonomically identical.

   

   Fig.7. Acacia senegal var. rostrata. Habit. South Africa Transvaal, Dongola Reserve (R. A. Dyer 166/7).

3. A. senegal var. rostrata Brenan (1953)

   Synonyms

   A. spinosa Marl. & Engl. (1888), non E. Mey.

   A. trispinosa Marl. & Engl. ex Schinz (1900), non Stokes

   A. rostrata Sim (1909), non Humb. & Bonpl. ex Willd.

   A. oxyosprion Chiov. var. oxyosprion (1932)

   A. senegal subsp. trispinosa (Stokes) Roberty (1948)

   A. volkii Suesseng. (1954)

   Usually a tree 2–8 m high with a flattened and spreading or slightly rounded crown; sometimes a shrub (Natal). Young branchlets more or less pubescent. Leaves with 4–12 pairs of pinnae (most pairs in Natal). Inflorescence-axes pubescent. Pods markedly rostrate or acuminate at apex.

   The main range of var. rostrata is in southern Africa, in Mozambique, Zimbabwe, Botswana, Namibia/South West Africa and South Africa. Here it occurs to the exclusion of var. senegal.

   However, a few specimens from Kenya and Somalia have pods similar to those of var. rostrata (Brenan, 1953; Ross 1968) and there is a similar specimen at Kew from Oman (Miller 2390). They may, as Ross (1979) maintains, be var. rostrata, but the identity of these outlying specimens is not certain, and more evidence is required.

   Ross (1979) has pointed out that Acacia oxyosprion Chiov. var. pubescens Chiov. (1932) may be the same as var. rostrata and an earlier name. However, the type has not been traced.

4. A. senegal var. leiorhachis Brenan (1953)

   Synonym:-

   A. circummarginata Chiov. (1915)

   A tree up to about 12 m high, with (in most of its range) long straggling branches forming an open irregular crown, but (in Kenya) crown sometimes well-formed and rounded. Bark on trunk usually yellow, papery and peeling. Young branchlets subglabrous to glabrous, often rather smooth and purplish. Inflorescence-axes glabrous to subglabrous, often purplish. Pods rounded to acute at apex, occasionally acuminate, sometimes larger than in other varieties (up to 19 cm long).

   Var. leiorhachis is confined to eastern Africa, where it occurs in Ethiopia, Somalia, Kenya, Tanzania, Mozambique, Zambia, Zimbabwe, Botswana and South Africa.

   

   Fig.8. Acacia senegal var. leiorhachis showing long whippy branches.
   Tanzania, Masai District, 16 km S. E. of Longido. (Milne-Redhead & Taylor 7157).

   The var. leiorhachis is often very distinct from the other variants of A. senegal in its habit and bark. But it is quite possible that future evidence will show that there is more than one entity included in var. leiorhachis.

(iv) Doubtful synonyms

Ross (1974, 1979) has pointed out that A. cufodontii Chiov. (1939) from Ethiopia, while conspecific with A. senegal, cannot be clearly referred to any of its varieties; also that A. senegal var. pseudoglaucophylla Chiov. (1916) may be a distinct variety or a synonym of var. senegal.

3.6 A. senegal: Further work needed

The taxonomy of A. senegal presents far more uncertainties and unsolved problems than that of any of the other three species considered in this report. The main need is for further field study and carefully collected material in north eastern Africa, especially Somalia, which is probably the centre of diversity for this group. In particular, var. kerensis needs careful observation in the field as its taxonomy is far from certain. The growth forms of var. leiorhachis need clarification: is there more than one variant? The occurrence of rostrate pods in Kenya and Somalia needs further study: are they chance abnormalities, or are they indeed outliers of var. rostrata? The same comments apply to Oman, from which available material is very scanty. Good material, with habit observations, from Pakistan and India is also needed. Photographs of the habit of A. senegal would be welcome from most parts of its range.

In Somalia also there are a number of species of Acacia very closely related to A. senegal. Their taxonomic limits are often not clear, and some indeed may still await scientific recognition. Thus it is not only A. senegal but its relatives also that need further study before the picture clarifies.

3.7 A. senegal: Literature consultad and cited