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A. nilotica: Recognition and relationship 
In practice A. nilotica is unlikely to present serious difficulty in its recognition. As a tree with bright or golden yellow flowers in round heads, straight stipular spines in pairs but never inflated into “ant-galls” and indehiscent but compressed pods, there are few species with which it is likely to be confused. A. seyal, widespread in tropical Africa, also has yellow flowers but powdery white to red or yellow bark on the trunk, not dark and rough, and the pods dehisce. A. karroo in southern Africa has similar flowers but normally glabrous branchlets (in A. nilotica usually more or less pubescent to tomentose except in the northern part of its range) and, again, dehiscent pods. The only other species perhaps closely related to A. nilotica is A. gummifera, a rare endemic to Morocco (where A. nilotica does not occur), differing in having only 1–3 pairs of pinnae and eglandular petioles to the leaves. An often useful field character for A. nilotica is that some of the spines, though straight, are directed downwards (deflexed).
A. nilotica: Summary of previous taxonomic work 
Although A. nilotica was described by Linnaeus as long ago as 1753 (as Mimosa nilotica), its subsequent history has been confused nomenclaturally and taxonomically. It has often been incorrectly called A. arabica or A. scorpioides. It was recognised as variable and several varieties were described under one name or another up to the 1920's. At the same time some of the subspecies of A. nilotica were until fairly recently thought to be distinct species: for example, eastern African plants were usually called A. subalata and southern African ones A. benthamii. A major step forward in understanding A. nilotica as a widespread and complex species was taken by Hill, A.F. (1940), where he recognised five varieties. However, he failed to deal satisfactorily with the complex in East Africa. Brenan (1957) completed and refined the picture for Africa, recognising the races more appropriately as subspecies and Ross (1979) gives the latest account for the continent. Until recently observations and material from Asia have been inadequate to allow a clear picture of the remarkable complexity of A. nilotica there. Although Cooke, T.(1903) drew attention to a cypress-like growth variation, it was not until Ali & Faruqi (1969) that the pattern in Asia was clarified. Fig. 1 File:Acacia nilotica.gif Fig. 1. Acacia nilotica. Drawing of subsp. kraussiana but general appearance representative; however, compare pod variation in Fig. 2. (1) flowering branch (approx. x .3); (2) part of leaf-rhachis showing glands (approx. x 3); (3) flower (approx. x 5) with enlargement of anthers to show glands; (4) flower opened out to show ovary (approx. x 5); (5) pods (approx. x .3); (6) seed (approx. x .5). Reproduced from Flora Zambesiaca.
A.F.Hill, A.F.1940 Some nomenclatural problems in Acacia. Bot. Mus. Leafl.Harvard Univ. 8: 94–100