Africa - Flora

AFRICA - FLORA The character of the vegetation of Africa is largely determined by the distribution of heat and moisture coupled with the great range in altitude. Phytogeographically the continent may be pri marily divided into three sections : (I) a northern extratropical region, bordering the southern shores of the Mediterranean and gradually fading into the northern Sahara desert with a very scanty flora; (2) a tropical African region, bounded approximately on the north by the Tropic of Cancer ; and (3) a South African re gion, reaching as far north as the Limpopo, Lake Ngami and southern Angola. According to Thonner (Fl. Pl. of Africa, 1913), there are in Africa about 3,712 genera and over 40,000 species of flowering plants, a number constantly being added to.

The Northern Area.

The northern area contains a consid erable proportion of genera and species common to the south of Europe, besides a large number of endemics in and south of the Atlas mountains and in Morocco. In the Atlas mountains the At lantic cedar (Cedrus atlantica) is a characteristic tree, and is very closely related to species in Asia Minor and the western Hima layas, some authorities having considered them all to be forms of one variable species. The flora of Morocco is related to that of the Canary islands, whilst the somewhat meagre indigenous vegeta tion of Egypt is largely the same as that of Palestine and Persia.

The tropical African flora may be roughly divided into four regions : (I) a northern desert or semi-desert region; (2) a savan nah region; (3) a high mountain region ; and (4) the tropical rain forest. The first type is characterized by a very scanty rain fall, and its flora consists largely of plants which are short-lived, such as annuals, or those specially adapted to resist long periods of drought. These include the date-palm (Phoenix dactylif era) which grows where other vegetation is practically non-existent. The transition from the semi-desert to that of the savannah is usually gradual with an increase in annual rainfall and pronounced wet and dry seasons. In the savannah the low, thorny bushes of the desert are replaced by scattered trees and shrubs, giving the country a park-like appearance, with an undergrowth of grasses and herbaceous flowering plants, including numerous annuals and species with underground root-stocks. This vegetation is typical of the elevated plateau-region which occupies a large portion of the continent. Characteristic trees are the baobab (Adansonia digi tata), the Shea butter tree (Butyrospermum Parkii), Anogeissus Schimperi, Afzelia africana, Entada sudanica, Parkia filicoidea, the fan-palm (Borassus flabellifer), etc.

In general it may be stated that the desert type is steadily en croaching on the savannah, which in turn is gradually penetrating the northern limits of the forest region, a condition resulting from a combination of desiccating winds during the prolonged dry period and of human agency (shifting cultivation and burning of the grass). In the drier parts of the savannah, for example in the eastern Sudan, large areas are covered by species of thorny acacia, including A. arabica, producing gum-arabic. In somewhat less arid but similar country farther south, in Rhodesia, south-east Belgian Congo and parts of Angola, another leguminous genus, Bracny stegia, is very plentiful, and forms almost pure low forest, whilst in the drier semi-desert areas of south Angola and Damaraland a unique example of the vegetable kingdom, W elwitschia mirabilis, is found. In the savannah and neighbouring forest regions there are numerous plants with edible fruits and not a few poisonous plants such as species of Dichapetalum (Chailletiaceae), the ar row-poison (Akokanthera), Lasiosiphon Kraussianus, Strophan thus, etc.

The high mountains capping the savannah region, such as the Cameroon mountains, Mt. Elgon, Mt. Kenya, Mt. Ruwenzori, Mt. Kilimanjaro, etc., carry a flora of great interest. On one or other of these mountains it is possible to traverse almost as varied a §uccession of types of vegetation as might be encountered in a journey from the equator to the vicinity of either of the. poles. Many of the genera found on their slopes are represented else where only in the temperate regions of the northern or southern hemispheres. Some of the better known examples are berberis, cornus and anemone, whilst outliers from the Cape region occur, such as Protea. The weird forests of tree-groundsels (Senecio) and the giant species of lobelia are characteristic features of the East African mountains.

The evergreen forest occupies mainly the low-lying country with a high rainfall distributed throughout the year, and extends in an ever-widening belt from the colony of Sierra Leone to the Cameroons, whence it spreads out over a great part of the Belgian Congo and the Gabun as far as the Victoria Nyanza. This forest sometimes penetrates for a considerable distance into the savan nah regions along the banks of rivers. It is particularly rich in endemic species of trees, shrubs and woody climbers, some of which are of great commercial value. The most important timber trees are the African mahoganies (Khaya, Pseudocedrela, Entan drophragma, etc.) and numerous trees of the families Caesalpini aceae, Combretaceae, Simarubaceae and Euphorbiaceae. Amongst ornamental shrubs the numerous species of Mussaenda (Rubi aceae), with enlarged, brightly coloured calyx-lobes, and the bril liant-flowered combretums, are conspicuous. The silk-cotton or kapok tree (Ceiba pentandra), with its enormous buttressed trunk, often attains gigantic proportions, whilst in West Africa especially the valuable oil-palm (Elaeis guineensis) is abundant. As many of the peculiar genera found in this primaeval forest, especially in the Cameroons and Gabun regions, are also found in the Guianas and Brazil on the opposite side of the Atlantic, it seems probable that it is but the remains of a once much more extensive and con tinuous tract of forest. Apart from Madagascar, the connection between the East African forests and those of tropical Asia is not nearly so marked.

South African Flora.

The South African flora, broadly speaking, consists of two main types : the typical Cape or south western region occupies a narrow strip within the southern coast line. In some respects it resembles certain features of the Aus tralian flora, the peculiar families Proteaceae and Restionaeceae and certain groups of Leguminosae being common to the two areas and rare in other parts of the world. The vegetation of this coastal area is of a marked xerophilous character and bears a super ficial resemblance to that of certain parts of the Mediterranean. It is of the so-called bushwood type, and here and there are tracts of grassland. Intermingled with the shrubs are numerous annuals, herbaceous perennials, succulents, etc. Farther inland, between the ranges of mountains, much of the surface is occupied by the "rhenosterbosch," Elytropappus rhinocerotis (Compositae). True forests occur only towards the eastern part of the region, in the districts of Knysna and Humansdorp, and the trees compos ing them nearly all belong to tropical genera. In the north-west a few trees of a conifer, Callitris juniperoides, are found, being the sole remains of a once extensive forest. The silver-tree, Leucaden dron argenteum, is now confined to the Cape peninsula, which har bours as many as a hundred species of Erica. Other striking plants in the same district are the brilliant scarlet Disa uniflora and the blue D. graminifolia, two beautiful ground orchids. There are also numerous species of the geranium family and many beautiful oxalis, large numbers of which are in cultivation.

Of the remainder of the South African regions perhaps the most interesting and remarkable is the Karroo, which occupies a broad belt in the hinterland of the south-western coast region. It is mainly a vast shallow basin which in former times was the bed of a large lake. Its altitude, accord ing to Bolus, ranges from 1,800 2,5ooft., the mountains on the northern margin reaching 8,000ft. altitude. The vegetation is of an intensely xerophytic type, being composed mainly of succulents of weird and diverse form, thorn bushes and numerous plants with bulbous and tuberous root stocks. Trees are almost entirely absent. The predominant f am ilies in this region are Corn positae, Leguminosae and Ficoi daceae, the last-mentioned be ing almost entirely represented by about 70 species of the genus Mesembryanthemum and its segregates. In this area the exotic Opuntia from Central America has become a pest.

The upper and Kalahari regions distinguished by Bolus may for convenience be treated as one. Whilst the families mentioned above are still highly represented, the grass family, Gramineae, takes a very high place, much of the country being grassland (high veld). Here also there is a considerable influx of families and genera characteristic of the tropical regions to the north. Amongst the Compositae the everlastings (Helichrysum) are conspicuous.

The south-eastern coast region is bounded on the south-west by the Van Stadens mountains and extends northwards east of the Drakenberg as far as the tropic into which it gradually merges. It is essentially a southerly extension of the tropical flora of East Africa, and contains a large number of such tropical families as Asclepiadaceae, Acanthaceae, Rubiaceae, etc. True rain-forest occurs in Natal. The plants so characteristic of the south-western coast region diminish rapidly northward. Some of the most striking plants in the area are a large tree-euphorbia (E. grandidens) and several species of Eucephalartos (Cycadaceae) and numerous species of aloe. Although nearly opposite the island of Madagascar, there is scarcely anything in common between the two floras.

As in the case of the tropical flora, it seems clear, judging by the history of the vegetation, that the climate of South Africa is gradually becoming drier. (J. Hu.)