The Acacia Species with Glandular Glutinous Pods in Southern Africa

T h e com plex of Acacia species with glandular g lutinous pods, consisting of seven spec ies, is endem ic to southern Africa. A m ap showing the distribution o f these species is given. A new key to the identification o f these species is provided and characters that enable m embers o f the com plex to be distinguished from A. karroo H ayne are discussed. The South African species of Acacia with glandular glutinous pods were dealt with by Verdoorn in Bothalia 6 : 153-160 (1951). Recently Brenan in Kew Bull. 21 : 480 (1968) has described yet another species with glandular pods from Mozambique. As there is now an additional species within the complex and, as more material and information has become available since Verdoorn’s treatment, it seems worthwhile to re-examine the entire complex in southern Africa. The complex now consists of seven species, namely, with species enumerated chronologically: A. nehrownii Burtt Davy. A. horleae Burtt Davy. A. pernii.xta Burtt Davy. A. swazica Burtt Davy. A. e.xuvialis Verdoorn. A. tenuispina Ver­ doorn and A. torrei Brenan. The complex is endemic to southern Africa. The distribution of each species may be seen in Fig. 1. Fig. 1 reveals that no species within this complex occurs in the Orange Free State, in Lesotho or in the Cape. The Transvaal has the greatest number of species; of the seven within the complex only A. torrei is absent. A. e.xuvialis and A. permixta are endemic to the Transvaal. A. exitvialis occurring in the eastern portion and A. permixta in the western. A. torrei is endemic to the Manica e Sofala region of Mozambique. A. horleae occurs in southern M ozam ­ bique, the eastern Transvaal, eastern Swaziland and northern Zululand. A. swazica occurs in the south-eastern Transvaal, in Swaziland and just south of Abercorn Drift in northern Zululand. A. tenuispina is found in eastern Bots­ wana and in the western and central Transvaal, whilst A. nehrownii ranges from South West Africa to Botswana and the northern Transvaal. This complex of species is taxonomically difficult. The species are all very closely related to one another and to A. karroo Hayne and appear to have been derived from A. karroo ‘stock’ during earlier times. It is not clear whether each species was independently derived from the ancestral stock or whether some of the species have given rise to others. Each species seems to have different habitat preferences, the different species being possibly the outcome of differing environmental conditions acting upon a common or similar gene pool. Some specimens cannot be referred either to A. karroo or to A. tenuispina with

This complex of species is taxonomically difficult.The species are all very closely related to one another and to A. karroo Hayne and appear to have been derived from A. karroo 'stock' during earlier times.It is not clear whether each species was independently derived from the ancestral stock or whether some of the species have given rise to others.Each species seems to have different habitat preferences, the different species being possibly the outcome of differing environmental conditions acting upon a common or similar gene pool.Some specimens cannot be referred either to A. karroo or to A. tenuispina with certainty.For example, Codd 7047 (PRE) from north of Pienaars River (Grid Reference: 2528 AB Pretoria) was described by the collector as "possibly a hybrid between A. karroo and A. tenuispinu/' Some plants from the Spring bok Flats agree vegetatively with A. tenuispina, but lack glandular pods and are therefore hesitantly referred to A. karroo.The two species may well hybridize but this requires careful field studies.
Apart from certain specimens referred to above as being possible inter mediates between A. karroo and A. tenuispina, each of the species can usually be distinguished from A. karroo fairly easily.The species differ from A. karroo in that they are usually small, slender plants and are smaller in all of their parts.With the exception of A. borleae and A. torrei the remaining species have typically fewer pinna and fewer leaflet pairs than A. karroo.The young branchlets, leaf petioles, rachides and rachillae are slender and not robust as in /!. karroo.Unlike A. karroo, the flowers are not aggregrated into such dense terminal panicles, but tend often to occur in axillary fascicles along the young stems.In addition, the pods are smaller, of a different shape and texture.The species perhaps most likely to be confused with A. karroo are A. swazica and A. exuvialis.However, the bark in both species is different being pale greyish to chestnut or reddish-brown and often peeling off in strips in contrast to the dark brownish-black or reddish-brown rough bark of A. karroo.
A new key to the identification of the species within this complex has been drawn up which makes provision for A. torrei.In contrast to the key given by Verdoorn (I.e.), where the first dichotomy was based on pod shape and whetiier the glands on the pods are conspicuous or not, the emphasis in this key is initially on vegetative characters.This is bccause pod shape in some of the species is more variable than initially recognized and also because it enables some of the species to be identified when in the flowering or even in the vege tative state.However, even in the presence of pods, it is felt that the vegetative characters used in the key enable certain species to be identified far more readily than by relying on pod characters.All species usually have flowers and pods in various stages of maturity contemporaneously.It may be argued that in the absence of pods it is not possible to establish whether the plant in question is a member of the complex with glandular glutinous pods.Whilst this may occasionally be true, it is usually possible and, certainly with a little practice, relatively simple to establish whether or not a plant belongs to this complex.
The position of the involucel on the peduncle is the important character in differentiating A. nehrownii from the remaining species.The stoloniferous habit, slender spines and small leaflets distinguish A. temdspina from A. swazica and A. exuviulis.In A. exiivialis the thin bark peels off, but this feature has also been noted on some specimens of A. swazica.The spines of A. swazica are slender, whereas in A. exuviulis they arc often somewhat enlarged and the members of each pair united basally.The conspicuous midrib and veins on the lower surface of the leaflets provide a useful means of identification in dried specimens of A. swazica, but it must be mentioned that the midrib and veins are not so conspicuous in living plants.They apparently only become conspicuous when the tissues have dried out.Pod shape also enables the two species to be differentiated.

Fig. 1
Fig. 1 reveals that no species within this complex occurs in the Orange Free State, in Lesotho or in the Cape.The Transvaal has the greatest number of species; of the seven within the complex only A. torrei is absent.A. e.xuvialis and A. permixta are endemic to the Transvaal.A. exitvialis occurring in the eastern portion and A. permixta in the western.A. torrei is endemic to the Manica e Sofala region of Mozambique. A. horleae occurs in southern Mozam bique, the eastern Transvaal, eastern Swaziland and northern Zululand.A. swazica occurs in the south-eastern Transvaal, in Swaziland and just south of Abercorn Drift in northern Zululand.A. tenuispina is found in eastern Bots wana and in the western and central Transvaal, whilst A. nehrownii ranges from South West Africa to Botswana and the northern Transvaal.

F
ig .1.-The known distribution of the Acacia species with glandular glutinous pods in Southern Africa.