Phytogeography of African Commelinaceae

Africa (including Madagascar) has nearly twice as many species of Commelinaceae as any other continent (approximately 270 species, or about 40% of the total in the family). Of the 17 genera which are native, seven (Anthericopsis, Coleotrype, Palisota, Polyspatha, Pseudoparis, Stanfieldiella and Triceratella) are endemic, the highest percentage generic endemism of any continent. Within Africa gcneric diversity is slightly higher in western than in eastern tropical floras. Species richness, however, is greatest in eastern Africa, mainly due to a high diversity of species of Commelina and Aneilema. Africa shares more genera with Asia (nine) than with any other continent. Only one African genus, Buforrestia, is neither endemic nor shared with Asia. Its western African/northeastern South American distribution is unique in the family. Besides Buforrestia, only five other genera of Commelinaceae (out of a total of 50 in the family), occur in both the Old and New Worlds. These genera. Aneilema, Commelina, Floscopa, Murdannia and Pollia are all very widespread in the Old World, occurring in Australia and Asia in addition to Africa (both continental and Madagascar). Madagascar is relatively poor in species (31). but these include the endemic Madagascan genus Pseudoparis, the sole African species of Rhopalephora, and the largest number of species of the Afro-Malagasy endemic genus Coleotrype. The high rate of generic endemism of Commelinaceae in Africa probably indicates that Africa was one of the ancient centres of diversity for the family. The high species diversity is more likely due to relatively recent radiations by genera pre-adapted to survival in non-forest habitats. The occurrence of only a small number of genera on both sides of the Atlantic suggests that the Commelinaceae have been evolving independently in the eastern and western hemispheres for a long period.

The high rate of generic endemism of Commelinaceae in Africa probably indicates that Africa was one of the ancient centres of diversity for the family.The high species diversity is more likely due to relatively recent radiations by genera pre-adapted to survival in non-forest habitats.The occurrence of only a small number of genera on both sides of the Atlantic suggests that the Commelinaceae have been evolving independently in the eastern and western hemispheres for a long period.endemism.In the following account, the distribution of each African genus is summarized briefly.In the discussion distribution patterns in Africa for the family as a whole and the phytogeographic relationships of the African Commelinaceae are considered.

GENERIC CONCEPTS
Generic concepts within the Commelinaceae are presently in a state of flux, particularly for the New World genera.The generic concepts for the Old World genera used here differ from those of Brenan (1966) in that Dictyospermum, Tricarpelema and Rhopalephora are treated as distinct from Aneilema (see Faden, 1975;1977); Ballya is included in   Aneilema (Faden, 1975); Aclisia is included in Pollia (Faden, 1975;Faden & Suda, 1980); and Porandra (Hong, 1974) is recognized as doubtfully distinct from Amischotolype.For the New World genera the differences from Brenan (1966) are as follows: Setcreasea and Separotheca are included in Tradescantia, following Hunt (1975); Cuthbertia is considered distinct from Phyodina; and the new genera Elasis, Gibasoides and Matudanthus (Hunt, 1978) are accepted.A soon to be described genus related to Geogenanthus, ranging from Panama to Peru, is included in the total of 51 genera recognized for this treatment.
DISTRIBUTION OF GENERA Amischotolype Amischotolype (syn.Forrestia A. Rich.) is represented in Africa by a single western African species, A. tenuis (C.B. Clarke) Rolla Rao (Nigeria to Zaire).The 10-15 other species in the genus range from north-eastern India to New Guinea.

Aneilema
Fifty-six of the 62 species of Aneilema occur in Africa (Faden, 1975, and unpublished).Two of the African species, A. umbrosum (Vahl) Kunth and A. forskalii Kunth extend to tropical America and the Arabian Peninsula, respectively.Of the remaining six species, one (underscribed) is endemic to the Yemen Arab Republic and the other five to Australia and the surrounding islands including New Guinea.
Within Africa the principal centre of taxonomic and morphological diversity of Aneilema is in eastern and north-eastern Africa; from southern Ethiopia and central Somali Republic south to central Tanzania.Secondary concentrations of taxa occur in the miombo region of south-western Tanzania and adjacent Zambia and Zaire; from southern Mozambique and south-western Zimbab we to northern and eastern Transvaal and the north eastern Cape Province of South Africa; and in the savannah region of West Africa; Sierra Leone and Senegal to Nigeria (Faden, 1975).The single Madagascan species, the endemic A. aparine Perrier, is the sole African species of section Aneilema which otherwise includes all of the Australian region taxa.

Polyspatha
The two species of this endemic African genus, Polyspathapaniculata Benth.and P. hirsuta Mildbr., are widespread in western Africa (Sierra Leone to Angola), extending east to Uganda (Brenan, 1968).

Pseudoparis
Pseudoparis, with two described species, P. monandra Perrier and P. cauliflora Perrier, is the only genus of Commelinaceae which is confined to Madagascar.

Rhopalephora
The single African species of this genus is the endemic Madagascan R. rugosa (Perrier) Faden.The three other species range from India and Sri Lanka to the Fiji Islands (Faden, 1977).

Stanfieldiella
The six taxa (four species plus two varieties) of the African endemic genus Stanfieldiella are nearly restricted to West Africa (Brenan, 1960(Brenan, . 1968).Only S. imperforata (C.B. Clarke) Brenan extends further east (to eastern Tanzania) or south (to Angola) of Cameroun.

Tricarpelema
This genus was collected in Africa for the first time in 1964.The species, which will be described elsewhere, was gathered at N' Kolbisson,Cameroun (De Wilde et al. 3728,BR,K,WAG) and, to this author's knowledge, it has not been recollected.The remaining six species of the genus (as used in the sense of Dictyospermum subgen.Tricarpelema of Faden, 1975) are Asiatic, ranging from northeastern India to the Philippines and Borneo.

Triceratella
This morphologically eccentric, monotypic, Afri can endemic genus is known from a single collection made in Zimbabwe in 1958(Brenan, 1961).

DISCUSSION
The distributions of the African genera of Commelinaceae have been outlined above and are summarized in Table 1.Several generalizations may be made.The largest genera, except Palisota, are the most widely distributed.The five genera which occur in all eleven African floral regions, Aneilema, Commelina, Cyanotis, Floscopa and Murdannia are also widespread outside of Africa, occurring in Asia, Australia and, except Cyanotis, South and North America.These five genera include 228 African species or about 84% of the total species in the continent and Madagascar.
Significantly, all five of these genera comprise species principally of non-forest habitats.Among the remaining 12 genera only the monotypic African endemics Anthericopsis and Triceratella could be characterized as non-forest genera.Five of the 10 forest genera are endemic.Three of these endemic genera, Palisota, Polyspatha and Stanfieldiella, have their centres of diversity in western Africa; a fourth, Pseudoparis, is confined to Madagascar; and the fifth, Coleotrype, is most diverse in Madagascar, but is also scattered on the continent.Of the five non-endemic forest genera, the African species of Amischotolype, Buforrestia and Tricarpelema are restricted to western Africa; Rhopalephora occurs only in Madagascar; and only Pollia could be described as widespread: it is the sole forest genus in the family to occur in Asia, Australia and the New World, in addition to Africa.
The preponderance of forest genera in western Africa and Madagascar is a distribution pattern that is prevalent in many groups of flowering plants as well as in pteridophytes.This pattern is undoubtedly relictual, being the result of the increasing aridity in Africa during the Neogene and again since the late Pleistocene (Axelrod & Raven, 1978;Richardson & Richardson, 1972).This desiccation was in part associated with the onset of the formation of the Rift Valley system, and hence it was most severe in eastern Africa.Lowland forest taxa tended to become isolated in western Africa and Madagascar where climatic deterioration was less intense.Some taxa also survived in isolated coastal and subcoastal forests of eastern Africa, sometimes with subsequent morphological differentiation (Faden, 1973).
The occurrence of western African forest taxa or clear relatives thereof in these East African forests provides strong evidence that these genera and, by inference, others were once more widespread in the continent.In the Commelinaceae three species span the gap of 650-850 km of unsuitable habitat:  . disperum (Brenan, 1968) as conspecific with the East African plants.
The increasing aridity in Africa during the Neogene led to a great increase in drier habitats, ranging from woodland to semidesert.This would have provided non-forest genera ample opportuni ties to diversify and speciate.The major radiations of Commelina and Aneilema, which are apparently still ongoing, probably began during this period.Therefore whereas the reduction of lowland rain forests in the Miocene may have decreased diversity among the forest genera of Commelinaceae, possibly leading to the extinction of some, its overall effect would probably have been to have greatly increased the total number of species in the family.
Phytogeographically the African Commelinaceae are most closely related to those of Asia (including Malesia).Nine of the ten nonendemic African genera -all but Buforrestia -also occur in Asia (vs.six in Australia and five each in North and South America).Three of those nine genera, Amischotoly pe, Rhopalephora and Tricarpelema, are restricted to these continents.Of the 23 African species (8% of the total), which are not endemic to the continent or Madagascar, 22 also occur in Asia (all but Aneilema umbrosum).No doubt the floristic similarity between Africa and Asia can be accounted for in part by the present direct land connection between the two continents.In a future paper the effects of the altering positions of the continents during the Tertiary will be considered.
* Department of Botany, National Museum of Natural History, Smithsonian Institution.Washington D.C. 20560.U.S.A.

TABLE 1 .
-Distribution of Commclinaceae in Africa by floral region sepalosa (C.B. Clarke) Engl., ranges from Ethiopia to Malawi and Zaire.Buforrestia The two African species of Buforrestia are endemic and allopatric: B. mannii C. B. Clarke from Nigeria to Cameroun and B. obovata Brenan from Guinea to Ghana (Brenan, 1968).The third species, B. candolleana C. B. Clarke, is South American and stretches from Surinam to north-eastern Brazil.the only endemic African genus of Commelinaceae which occurs both on Madagascar (six species) and on the mainland (three species).Perrier (1936; 1938) informally separated the Madagascan taxa from the continental species.He compared only C. natalensis C. B. Clarke on the basis of corolla and androecial morphology, a distinction which appears to be sound.The three mainland species are allopatric: Coleotrype natalensis (South Africa to Mozambique and Zimbabwe); C. bruecknerana Mildbr.(eastern Kenya and Tanzania); and C. laurentii K. Schum.(West Africa to Uganda).Commelina The nearly cosmopolitan genus Commelina is the largest in the family, with about 170 species.It is best developed in Africa where approximately 125 species, many still undescribed, occur.All but about 11 of the African species are endemic to the continent and/or Madagascar.Within Africa, Com melina has its greatest number of species in the Flora o f Tropical East Africa and Flora Zambesiaca areas.It is especially diverse in the miombo areas of southern Tanzania, northern Zambia and south eastern Zaire.Asiatic genus of about 50 species, has 11 taxa in Africa of which seven (or perhaps six) are endemic.The only questionable endemic is M. axillaris Brenan of the East African coast which is uncomfortably close to certain collections of the Asiatic M. blumei (Hassk.)Brenan.Except for M. simplex (Vahl) Brenan and M. tenuissima (A.Chev.)Brenan, most African species are rather local.There is no concentration of species in any part of the continent.In Africa, M. gigantea (Vahl) Briickn. is restricted to Madagascar.
ColeotrypeColeotrype, a genus of 9 species, plus many infraspecific taxa described from Madagascar, is Palisota schweinfurthii C. B. Clarke, Pollia condensata C. B. Clarke, and Stanfieldiella imperforata (C.B. Clarke) Brenan.Three genera are represented by vicariant forest species in the two areas: Coleotrype by C. laurentii K. Schum. in western Africa (east to Uganda) and C. bruecknerana Mildbr. in eastern Kenya and Tanzania; Palisota by P. ambigua (P.Beauv.) C. B. Clarke in western Africa and P. orientalis K. Schum. in eastern Tanzania; and Aneilema by A. beniniense (P.Beauv.)Kunth in western Africa and A. dispermum Brenan in eastern Tanzania and Malawi. 1 do not regard the West African plants treated as A