'n Morfologiese studie van die genus Acacia Miller in Suid-Afrika

English: The genus Acacia is an extensive one with representatives in Africa, America, Asia and Australia. When Bentham revised the subfamily Mimosoideae in 1875 he divided the taxon into six subgenera. All the South African representatives of the genus were placed in the two subgenera Vulgares and Gummiferae. Bentham distinguished between the two subgenera by the differences in morphology of the stipules. Bentham's classification of the subgenera differs from that of Oliver (1871), whose grouping of the South African Acacia species is based on the morphology of the inflorescence. Species which are affected by this difference are A.albida, A.schweinfurthii and A.kraussiana. Because of this problem and the fact that, judging from the literature, there is some uncertainty regarding the application of taxonomic criteria, it is necessary that the whole genus be revised. Due to the extent of such a project it was decided to limit this investigation to the South African species with the main emphasis on characters which could be of taxonomic importance. Where it was deemed necessary a few species from other geographical areas were included in the investigation. Various characters were found which were valuable for considerations of phylozenetic history of the genus and for delimitation of taxa. On the basis of the presence of starch grains in the cotyledons and the morphology of the seedlings, A.albida, A.kraussiana and A.schweinfurthii were placed in a new subgenus, Farinosae. On the basis of these characters the subgenus Farinosae shows affinity with the subgenera Phyllodinae, Bothryocephalae and Pulchellae of Australia and certain Vulgares species of America. The subgenus Farinosae can possibly be phylogenetically regarded as the oldest extant taxon of the genus Acacia. With the species A.albida, A.krau8siana and A.schweinfurthii in a separate subgenus, the remaining South African members of the subgenera Vulgares and Gummiferae can be separated on the basis of classical features such as the morphology of the stipules and inflorescences. Other clear differences which were found between these two subgenera are as follows: (a)The flower of the Vulgares (and Farinosae) species contains a cup-like disc to which the filaments are adnate, and the ovary has a prominent pedicel (gynophore). In the flower of the Gummiferae species the disc is lacking and the ovary is almost sessile. A disc and pedicelled ovary are also found in the flower of A.albida and the name Feidherbia albida which Chevalier proposed for this taxon in 1934 is therefore not justified. The origin of the cup-like disc on which the stamens are found can probably be regarded as a relict of an ancient branched system on which stamens were borne and which later became reduced. (b) In the pods of the Vulgares (and Farinosae) species the fibre zone consists of both cross and longtitudinally arranged fibres, while the pods of the Gummiferae species have only longtitudinally arranged fibres, or none at all. (c) In the pinnules of the Vulgares (and Farinosae) species a few layers of spongy parenchymatous cells are found between the abaxial and adaxial palisade parenchyma cells. In the pinnules of the Gummiferae species the abaxial and adaxial palisade cells are continuous. (d) The seedlings of the Vulgares species have membranous or leaf-like stipules while the stipules of the Gummiferae species are spinescent. Hooked spines are absent on the Gummiferae seedlings but appear on the Vulgares seedlings either dispersed on the internodes or arranged in groups of two or three directly below the nodes. On the basis of the morphology of the flower, the anatomy of the pod and the morphology of the seedlings, the Vulgares and Gummiferae species can be divided into smaller groups and, in certain cases, into definite species. Keys have been compiled to facilitate the classification. Interesting differences between the Vulgares and Gummiferae species were noted with regard to the nocturnal movements of the leaves. These differences were, however, not very clear in all cases. In the case of certain species such as A.polyacantha and A.robusta the nocturnal movements are very conspicuous and specific (see Fig. 125). A hypothesis was postulated concerning the evolution of the inflorescence systems and inflorescences of the genus Acacia. The hypothesis includes a theory concerning the development of the involucel (cupula). Flowering dates of the different Acacia species are determined to a large extent by the morphology of the inflorescence system. Date of flowering can therefore be considered as a genetic character which nevertheless can be influenced to a certain extent by ecological conditions. The development of the ovule and the ontogeny of the seed was investigated. Ideas were put forward as to the possible homology of the ovule. A study of seed ontogeny produced information concerning the presence of endosperm in mature seeds of A.kirkii, A.tortilis and A.xanthophloea. This characteristic cannot be used as a criterium to remove these three species, as well as other exotic species containing endosperm in the seed, out of the genus Acacia (cf. Vassal, 1968). The arrangement of vascular tissue in the seedling, leaves and young shoots are discussed. Judging from the arrangement of the primary vascular bundles in the seedling there is probably a relationship between a monolacunar node with two leaf traces, a trilacunar node and a node which is called the "fifth type" by Takhtajan (1969). It may be concluded that the extant South African taxa of the genus Acacia can be distinguished from each other on a specific level. By using variable characters such as degree of pubescence and numerical taxonomic characters such as seed and leaf size it is possible to carry the subdivision of species too far. It is therefore recommended that the taxa with an extensive distribution, manifesting considerable variation, such as A.karroo Hayne, A.caffra Willd., A.reficiena Wawra (= A.luederitzii Engl.) and A.robusta Burch., should not be divided into smaller taxa until more information is available concerning the abovementioned characteristics. As a final summary, a key has been composed in which as many characters as possible have been used to distinguish between the South African Acacia species.