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

[摘要] English: The genus Acacia is an extensive one with representatives in Africa,America, Asia and Australia. When Bentham revised the subfamily Mimosoideaein 1875 he divided the taxon into six subgenera. All the South African representativesof the genus were placed in the two subgenera Vulgares and Gummiferae.Bentham distinguished between the two subgenera by the differences inmorphology of the stipules. Bentham's classification of the subgenera differsfrom that of Oliver (1871), whose grouping of the South African Acacia speciesis based on the morphology of the inflorescence. Species which are affectedby 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 isnecessary that the whole genus be revised. Due to the extent of such a projectit was decided to limit this investigation to the South African species with themain emphasis on characters which could be of taxonomic importance. Where itwas deemed necessary a few species from other geographical areas were includedin the investigation.Various characters were found which were valuable for considerations ofphylozenetic history of the genus and for delimitation of taxa.On the basis of the presence of starch grains in the cotyledons and themorphology of the seedlings, A.albida, A.kraussiana and A.schweinfurthii wereplaced in a new subgenus, Farinosae. On the basis of these characters the subgenusFarinosae shows affinity with the subgenera Phyllodinae, Bothryocephalaeand Pulchellae of Australia and certain Vulgares species of America. The subgenusFarinosae can possibly be phylogenetically regarded as the oldest extanttaxon of the genus Acacia.With the species A.albida, A.krau8siana and A.schweinfurthii in a separatesubgenus, the remaining South African members of the subgenera Vulgares and Gummiferaecan be separated on the basis of classical features such as the morphologyof the stipules and inflorescences. Other clear differences which werefound between these two subgenera are as follows:(a)The flower of the Vulgares (and Farinosae) species contains a cup-like discto which the filaments are adnate, and the ovary has a prominent pedicel(gynophore). In the flower of the Gummiferae species the disc is lackingand the ovary is almost sessile. A disc and pedicelled ovary are alsofound in the flower of A.albida and the name Feidherbia albida whichChevalier proposed for this taxon in 1934 is therefore not justified. The originof the cup-like disc on which the stamens are found can probably be regardedas a relict of an ancient branched system on which stamens were borne andwhich later became reduced.(b) In the pods of the Vulgares (and Farinosae) species the fibre zone consistsof both cross and longtitudinally arranged fibres, while the pods of theGummiferae species have only longtitudinally arranged fibres, or none at all.(c) In the pinnules of the Vulgares (and Farinosae) species a few layers ofspongy parenchymatous cells are found between the abaxial and adaxial palisadeparenchyma cells. In the pinnules of the Gummiferae species theabaxial and adaxial palisade cells are continuous.(d) The seedlings of the Vulgares species have membranous or leaf-like stipuleswhile the stipules of the Gummiferae species are spinescent. Hooked spinesare absent on the Gummiferae seedlings but appear on the Vulgares seedlingseither dispersed on the internodes or arranged in groups of two or threedirectly below the nodes.On the basis of the morphology of the flower, the anatomy of the pod and themorphology of the seedlings, the Vulgares and Gummiferae species can be dividedinto smaller groups and, in certain cases, into definite species. Keys havebeen compiled to facilitate the classification.Interesting differences between the Vulgares and Gummiferae species werenoted with regard to the nocturnal movements of the leaves. These differenceswere, however, not very clear in all cases. In the case of certain species suchas A.polyacantha and A.robusta the nocturnal movements are very conspicuous andspecific (see Fig. 125).A hypothesis was postulated concerning the evolution of the inflorescencesystems and inflorescences of the genus Acacia. The hypothesis includes atheory concerning the development of the involucel (cupula). Flowering datesof the different Acacia species are determined to a large extent by the morphologyof the inflorescence system. Date of flowering can therefore be consideredas a genetic character which nevertheless can be influenced to a certainextent 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 ofseed ontogeny produced information concerning the presence of endosperm inmature seeds of A.kirkii, A.tortilis and A.xanthophloea. This characteristiccannot be used as a criterium to remove these three species, as well as otherexotic 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 shootsare discussed. Judging from the arrangement of the primary vascular bundles inthe seedling there is probably a relationship between a monolacunar node with twoleaf traces, a trilacunar node and a node which is called the fifth type byTakhtajan (1969).It may be concluded that the extant South African taxa of the genus Acaciacan be distinguished from each other on a specific level. By using variablecharacters such as degree of pubescence and numerical taxonomic characters suchas 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 dividedinto smaller taxa until more information is available concerning the abovementionedcharacteristics.As a final summary, a key has been composed in which as many characters aspossible have been used to distinguish between the South African Acacia species.

[发布机构] University of the Free State