An analysis o f the African Acacia species : their distribution , possible origins and relationships

The three subgenera recognized within the genus Acacia are outlined and the global distribution of each is indicated. The differences between the subgenera and the degree of relationship and levels of specialization are discussed briefly. It is suggested that the ancestral members of the genus were climbers or lianes. Past geological events considered likely to have influenced the distribution of the  Acacia species in Africa are outlined. The number of  species recorded from each African country is tabulated and the distribution and concentration of species within the genus Acacia as a whole and within each subgenus in Africa are illustrated. The highest concentrations of species within each subgenus occur in tropical east and south-east Africa. The distribution o f species within some o f the individual African countries and possible affinities are discussed and attention is drawn to the main centres of endemism. The distribution of the African species is correlated with the major phytogeographical regions recognized on the continent. The relationships between the African and the American, Madagascan, Indian and Australian  Acacia species are discussed briefly.


IN T R O D U C T IO N
The genus A cacia at present consists of about 1 100 species (perhaps as many as 1 200) which are widely dispersed in the Americas, the Carribbean and Pacific Islands, Africa, Madagascar and the Mascarenes, Asia, the Indo-Malesian region and A ustra lia.Europe is the only large geographical area devoid of indigenous A cacia species, and there are no indige nous species in New Zealand despite its relative proximity to Australia.The fossil record indicates that the genus was previously m ore widely distributed having been present formerly in the Ukraine (Shchekina, 1965) and in New Zealand (Mildenhall, 1972(Mildenhall, , 1975)).M ost species o f A cacia occur in regions where the rainfall is markedly seasonal or low, relatively few inhabiting rainforest areas, but even then the rainfall is usually unevenly distributed throughout the year and even in the wettest parts there is usually a short dry season.This does not imply that the genus originated in an arid or semi-arid region.On the con trary, it is considered probable that Acacia originated in the tropical lowlands and that most of the xerophytic features within the genus are secondary.
Much evidence has accum ulated in recent years to support the contention that there have been large scale movements of continents during geological time and Raven & Axelrod (1974) summarized the biogeo graphic support for the theory of plate tectonics.
Raven & Axelrod (I.e.) postulated that West G ondwanaland, com bined Africa and South America, was a primary area of evolution for many m ajor orders of angiosperms and perhaps the earliest angiosperms themselves, and that the initial radiation of the angiosperms occurred when direct migration was possible between South America, Africa, India, A n t arctica and Australia, and via Africa to Laurasia.West G ondw analand included vast arid to subhumid areas in tropical latitudes where the terrain and edaphic conditions were diverse and provided every opportunity for rapid evolution. T horne (1977), on the other hand, favoured south-eastern Asia and associated archipelagoes as the primary centre of origin of the most primitive angiosperms and m ain tained that prior to its fragmentation West G o n d wanaland was merely one o f a num ber o f importani centres for the development of the tropical angiosperm flora.
In support of their contention Raven & Axelrod (I.e.) argued that West G ondw analand was centrally located on routes of dispersal at the time when the prim ary evolutionary radiation o f the angiosperms was taking place.The climatic changes associated with the fragmentation of G ondw analand, which, ac cording to Raven & Axelrod, com m enced in the mid-Cretaceous approximately when the first angio sperms are encountered in the fossil record, probably had a profound influence on the evolution of the angiosperms and on the establishment o f m ajor lines within it.There is some disagreement over the timing o f the fragmentation o f G ondw analand, an event o f prim ary im portance from a phytogeographic point of view. Melville (1975), for example, was o f the opi nion that G ondw analand started to fragment earlier than indicated by Raven & Axelrod.In this paper I have followed Raven & Axelrod in assuming that G ondw analand started to fragment in the mid-Cretaceous and that the angiosperms were already fairly widely distributed.
The opening of the South Atlantic 125-130 million years ago heralded the spread o f m ore mesic climates over much of West G ondw analand and is thought to have started the sur£e o f angiosperms into the mesic lowland record about 110 million years ago (Raven & Axelrod, I.e.).The pattern o f appearance o f the angiosperms in the lowland record suggested to Raven & Axelrod that the primitive members of several extant orders and perhaps even a few families were already in existence by the close o f the early Cretaceous 110 million years ago. T horne (1978), however, maintained that few, if any, extant families, and certainly no extant genera, had evolved in West G ondw analand before the final sundering o f South America from Africa 100 million years ago.
Raven & Axelrod (I.e.) expressed the opinion that most m odern angiosperm families were in existence in the Paleocene about 65 million years ago before the connection between Africa and Eurasia was severed.The family Leguminosae is thought to have originated or at least have undergone its prim ary radiation and differentiation into three subfamilies in West G ondw analand and Raven & Axelrod expressed the view that Mimosoideae, Caesalpinioideae and perhaps Papilionoideae were in existence by the Paleocene.There are no reliable Cretaceous records o f Caesalpinioideae or of the other two subfamilies.From what can be inferred about its history and pre sent distribution patterns, Raven & Axelrod presume that M imosoideae migrated between Africa and South America during or prior to the Paleocene.After the Paleocene (54 million years ago) the evi dence suggests only limited migration between these two continents. Vassal (1972) recognized three subgenera within Acacia, namely, subgenera Acacia, A culeiferu m and H eterophyllum *, chiefly on the basis o f characters o f seeds and seedlings and on the occurrence of stipular spines and pollen characters.The three subgenera recognized by Vassal broadly correspond to grou p ings of B entham 's six series which is fortunate as most o f the characters on which Vassal's classifica tion are based are not obvious from the gross m o r phology of conventional herbarium specimens.The relationship between Bentham 's series and Vassal's subgenera is as follows: 1. Subgenus Aculeiferum Vassal ( = series Vulgares Benth. and series Filicinae Benth.) 2. Subgenus H eterophyllum Vassal ( = series P hyllodineae Benth., series B otryocephalae Benth. and series Pulchellae Benth.) 3. Subgenus Acacia ( = series G um m iferae Benth.)The following sections within each subgenus were proposed by Vassal (1972) and Guinet & Vassal (1978) and the names are used in this paper: 1. Subgenus Aculeiferum (Sections A culeiferum , M onacanthea and Filicinae) The distributions o f subgenera A cacia and A culei feru m are very similar but subgenus A cacia apparent ly enjoys a slightly wider distributional range than subgenus A culeiferum .Subgenus A culeiferum has a m ore restricted distribution in A frica than subgenus Acacia, is present in New G uinea while subgenus A cacia is absent, and only just reaches Australia (in the vicinity o f Coen in northern Q ueensland) where it is represented by a solitary species (A .albizioides Pedley) in contrast to subgenus A cacia which is widely distributed in northern A ustralia although represented by fewer than ten species.The vast m aj ority of species in the genus belong to subgenus H et erophyllum which is fundam entally an Australian group (including Tasm ania), while a further eighteen species (Pedley, 1975) occur in M adagascar and the Mascarenes, New Guinea, Form osa, the Philippines and the Pacific Islands to Hawaii.The genus reaches its southern limit o f distribution in Tasm ania.The position o f A .willardiana Rose, which occurs on the west coast o f N orth America (Mexico), is not clear (see later discussion) but if it is placed in subgenus H eterophyllum as advocated by Vassal & Guinet (1972) then the distribution of the subgenus shown in Fig. 3 should be extended eastwards from Hawaii to Mexico. Guinet & Vassal (1978) are o f the opinion that the three subgenera were differentiated by the Oligo-Miocene period ( ± 27 million years ago), and that no fundam ental difference seems to exist betw een their geographical distribution then and the present.They pointed out that the apparent absence o f the genus in the fossil record during the Paleocene is surprising, particularly if the genus is held to have had a monophyletic origin and if one considers that its distribu tion during the Neogene was what it is now.

O R IG IN A N D P O SSIBLE ID EN TITY O F T H E A N C E ST R A L M EM BER S O F TH E G E N U S A C A C IA
Like the origin of the angiosperms, the identity of the ancestral form o f A cacia has been the subject of much speculation and disagreement, but recently a broad consensus appears to have been reached which contradicts the earlier views of Andrews (1914) and Atchison (1948).Andrews and Atchison considered the G um m iferae (subgenus A cacia) to be the ancestral form as its members contained the m orpho logical characters of the genus that they considered to be primitive, namely, bipinnate leaves and persistent spinescent stipules.A tchison m aintained that chrom osom e num ber variation, m orphological uni formity and geographical distribution contributed toward establishing G um m iferae as the ancestral form o f the genus pointing out, in support of this contention, that G um m iferae is the only section of the genus with a cosm opolitan distribution (this is not strictly correct).The other sections o f the genus were held to have developed from the original forms in secondary centres where isolation through climatic or edaphic change was favourable to the survival of new types.Tindale & Roux (1975), on the basis of a limited sample, suggested that the chemical content of the South African species with non-spinescent stipules (subgenus A culeiferum ) is generally m ore a d vanced than that o f the species with spinescent stipules (subgenus A cacia), a suggestion that sup ports the above view.However, most o f the G um m i fe ra e are polyploids and polyploidy is now held to   correspond to a high degree of differentiation in the genus Acacia.Robbertse (1974) outlined the possible evolution of the inflorescence and flowering system in the South African acacias and considered subgenus Acacia (G um m iferae) to be m ore advanced than subgenus Aculeiferum ( Vulgares), a view which is supported by the detailed studies of Guinet & Vassal (1978).R ob bertse considered it probable that the paniculate flowering system, spicate inflorescence, pedicellate flowers, presence of a cup-shaped disc and a pedicel late ovary, all of which are found in subgenus A cu lei feru m , are primitive characters.Guinet & Vassal (1978) evaluated the degree o f relationship and specialization o f the m ajor sub divisions recognized within the genus on the basis of pollen, chrom osom e, seed, pod, inflorescence and vegetative characters.Each character was divided into three states, namely, unspecialized, specialized and highly specialized, and that section of the genus displaying the characters considered to be highly specialized was itself considered to be highly special ized.The division of some characters, for example seed size, into somewhat arbitrary size classes is ques tionable, especially as it was not disclosed why the range o f continuous variation in seed size was divided in such a m anner or what criteria were employed to established the degree of specialization of each size class.For example, seed were divided into the follow ing size classes, small (less than 5 m m long), m edium (5 -1 0 mm long) and large (more than 10 m m long).Small seed were regarded as unspecialized but it is quite conceivable that in some instances small seed may be specialized.In addition, the range of varia tion in seed size in many species obscures the limits of Guinet & Vassal's size classes.
Guinet & Vassal (I.e.) concluded that: 1. On the basis of pollen m orphology subgenus Acacia is the most specialized of the three subgenera and subgenus A culeiferum the least specialized.Although the pollen of subgenus H eterophyllum is generally m ore specialized than that of subgenus Aculeiferum , the two subgenera share im portant characters, for example, the absence of columellae and the presence of simple apertures.A culeiferum is the least specialized and subgenus A cacia the most specialized.

The characters of the seeds of subgenus A cacia
are often highly specialized, and the levels of special ization in the series Filicinae and M onacanthea of subgenus Aculeiferum are fairly close.
4. The cotyledonary and adult foliar characters selected did not appear to be specialized in subgenera A culeiferum and A cacia except in a few rare cases, while m any specializations occurred in subgenus H et erophyllum except in the development of spinescence.Section Filicinae of subgenus A culeiferum shows no specialized characters.

The characters of the inflorescence and pod selec ted indicated that certain characters in subgenus
A cacia are infrequently encountered in the other su b genera.
Guinet & Vassal attem pted to estimate the total levels of specialization within each subgenus (and within the sections within each subgenus) and co n cluded that: 1. On the basis of their average level o f specializa tion subgenus Acacia is the most specialized sub genus and A culeiferum the least specialized.

Section Filicinae of subgenus A culeiferum is
characterized by a preponderance o f unspecialized characters and shows the least diversity of all series in the genus.

Section M onacanthea of subgenus A culeiferum
contains m ore possible primitive states than section A culeiferum and appears to be less advanced than the latter.The rainforest areas o f the world were previously much more extensive and during the Paleogene humid forests stretched through m uch o f America, Africa, Arabia, India, Malaysia and Australia.
Acacia species are not well represented in rainforest areas at the present time and it is thought that this is probably due to their general intolerance o f low light intensities.In the absence o f any indications to the contrary, it seems reasonable to assume that m em bers of the genus have always been similarly intoler ant of low light intensities.The A cacia species which are currently the most successful in rainforest areas are the climbers and it appears as though the climb ing habit has enabled species to exploit situations in forests where light penetrates to the ground, for ex ample in clearings, on the banks of stream s or on forests margins, and reach and m aintain an emergent position in the canopy.If, as is considered likely, Acacia originated in lowland forests, it is suggested that the ancestral members were climbers or lianes and, this being the case, members o f \ixoio-A culei fe ru m which were similar in some respects to some members of subgenus A culeiferum .
Taking the African species as an example, a number of the members o f section M onacanthea sub genus Aculeiferum appear to be obligate climbers (A .lujae De Wild., A .kraussiana Meisn. ex Benth.) while others (A .brevispica H arm s, A .ataxacantha DC.) occur as climbers in forested areas or as scan-dent shrubs or even large spreading shrubs in neigh bouring w oodland or grassland areas.A .ataxacan tha occurs as a climber in forests and on forest margins, as a scandent shrub or non-scandent spreading shrub in w oodland or grassland, but on oc casions it grows as a substantial single-stemmed tree up to 10 m high in southern Africa.A . ataxacantha apparently exhibits the evolutionary potential that would have been necessary for a forest-dwelling climber to adapt and exploit the new habitats created in surrounding grassland and woodland areas as the forests retreated.
A .ataxacantha and the other climbers are in variably armed with scattered recurved non-stipular prickles but occasional plants are entirely or almost entirely unarm ed.Some species [A.caffra Thunb.)Willd., A .galpinii Burt Davy, A .polyacantha Willd.] in section A cu leiferu m o f subgenus A c u le i ferum which are typically armed with prickles in pairs at the nodes are likewise sometimes unarm ed.
In some species (A .caffra) in section A culeiferum occasional specimens are found where a few irregu larly scattered prickles occur in addition to the paired prickles at the nodes.This illustrates the apparent ease with which scattered or paired prickles can be lost and how scattered prickles could give rise to paired or solitary prickles at the nodes or vice versa.
Once again, the evolutionary potential for such change is still apparently present.Members of the American section Filicinae which Guinet & Vassal consider to be ancestral are typically unarmed but prickles could have been lost as indicated.
Although subgenera A cacia and A culeiferum share a com mon geographical area there are fundam ental differences between them, as indicated by Guinet & Vassal, and they do not appear to be closely related which suggests that subgenus A cacia did not arise di rectly from subgenus A culeiferum or vice versa.For example, the colporate pollen of subgenus A cacia with columellae is considered unlikely to have devel oped from the porate type without columellae.It seems m ore likely, therefore, that subgenus A cacia was derived from proto-A culeiferum rather than from subgenus A culeiferum itself.Subgenus H etero phyllum was possibly derived directly from subgenus Aculeiferum or, failing that, from proto-A cu lei ferum.
It is difficult to speculate on the identity o f the an cestral prolo-A cu leiferu m except very generally.It is suggested that the ancestral members were climbers or lianes, either unarm ed or armed with prickles, with many-jugate bipinnate leaves.Robbertse's (1974) findings suggest that they would have possessed a paniculate flowering system and that the flowers were pedicellate.The transition from capitate to spicate inflorescences and vice versa appears to have occurred several times during the development of the genus and there is no certainty as to which condition might be considered unspecialized.

O U T L IN E OF T H E C R E T A C E O U S -Q U A T E R N A R Y H ISTO RY O F A FR IC A
The present distribution patterns in A cacia in Africa have been determined by events that lie deep in the past but it is difficult to assess, except very generally, the effects of past geological events on the flora o f a continent  Fig. 6 (1978) along with that during the late Oligocene-early Miocene, m iddle-late Miocene and Recent.
By the close of the Cretaceous, Africa was isolated from South America and India-M adagascar and was surrounded by ocean.Although direct migration of plants to and from Africa was restricted after the mid-Cretaceous, direct interchange with South America was much easier than at present as the Atlantic was relatively shallow and num erous islands provided stepping stones between the two continents (Raven & Axelrod, 1974).
By the close of the Oligocene, the African plate had moved north to virtually its present position.During the late Oligocene-early Miocene (30-25 million years ago) the low relief in Africa was altered by uplift accompanied by warping (King, 1967), especially along the east coast, and the present land scape of the continent started to take shape.Volcanic activity started on a m ajor scale and the East African rift valleys were initiated.Uplift brought a cooler drier climate and the development and spread of dry climate over tropical Africa probably began near the close o f the Oligocene about 27 million years ago (Axelrod, 1972) and has continued to the present as the rift valleys continue to grow (Raven & Axelrod, 1974).The formation of a volcanic field from E thio pia southw ards down the rift valleys during the M io cene increased the development of rainshadows which in turn brought greater drought and tem pera ture extremes.
As a result of the Neogene trend to a drier climate brought on by the general uplift o f the continent, changes in circulation, and the resultant decrease in moisture, savanna started to spread at the expense o f rainforest and the African rainforest was progres sively impoverished.The development of rainshadow s in the rift valleys favoured the spread of savanna and then grassland, at first locally in small patches but then more extensively as the rainshadow effect increased.By the mid Miocene lowland rain forest is thought to have had only a patchy distribu tion along the northern parts of the east coast, and it seems probable that a tem porary dry season during which little or no rain fell was already evident in the Miocene.
By the close of the O ligocene-early Miocene the vegetation of Africa had assumed a near-m odern aspect although the com position and distribution o f vegetation differed in many im portant respects from that o f today (see Axelrod & Raven, Fig. 6, 1978).
A further major factor that affected the African flora was the development of the cold Benguella cur rent.By the early Miocene A ntarctica had moved to its present position and glaciation had been initiated.W hen glaciation commenced in Antarctica cold water started to bathe the west coast o f Africa bring ing to it a drier colder climate.A full ice sheet did not appear until the Pliocene about 5 million years ago and it waxed and waned for 2 -3 million years.As the m ajor ice sheet spread the Benguella current increas ed in strength and became progressively colder bring ing with it increased drought to the west coast of tropical Africa.The extensive Pliocene ice sheet w ould have brought a much drier climate not only to the coast of west tropical Africa but it may possibly also account for the dry global climate in the mid Pliocene (Raven & Axelrod, 1974).
As aridity spread and a seasonally dry climate became established, particularly during the Pliocene as the Antarctic ice cap developed, the African rain forest continued to be m ore and m ore restricted in distribution and impoverished and the taxa com pris ing the forests became progressively more discon tinuous.The strengthening high pressure systems brought a drier climate to the interior o f Africa and the spreading drought tended to disrupt and im poverish the African flora, the recurring aridity in the tropics during successive periods o f 'ice-age aridi ty' resulting in increased selection pressure for drought resistant taxa.As a consequence, rainforest areas were replaced by savanna and grassland, savan na and grassland by dry thorn scrub and dry thorn scrub by semidesert and desert vegetation.
The later phases of this trend in the Pliocene p ro bably resulted in the appearance o f local areas of semidesert, but widespread regional semideserts and deserts are apparently the consequence of later phases o f 'ice-age aridity '. According to Quezel (1979), a desert climate was probably initiated in the m ajor part o f the Sahara, at least in the lower altitudinal zones, during the Pliocene.
T hroughout the Tertiary there was a symmetrical distribution o f climate and vegetation in the central tropical belt.The present African vegetation shows much greater asymmetry than that o f the early to late Tertiary (see Axelrod & Raven, Fig. 6, 1978). W hite (1965) discussed the marked differences that exist at present between the Sudanian and Zam bezian floristic dom ains.The Sudanian Dom ain is m uch drier than the Zam bezian and its greater aridity has been largely responsible for the im poverishm ent of its flora.
The close of the Pliocene and the early Pleistocene were characterized by m ajor uplifting which raised the interior plateaux by over 1 700 metres.The alti tude of parts o f eastern A frica has increased by over 2 000 metres above that o f the M iocene and has brought to it a drier climate.The Pliocene-Pleistocene uplifting and associated climatic fluctuations favoured local speciation.
Fluctuations in the Q uaternary climate also had a significant effect on the tropical A frican rainforest flora, with the drier periods being times o f extinction of taxa requiring m ore or less continuously wet con ditions.Rainforest expanded during the hum id inter glacial periods and contracted again during the dry glacial periods.Wild (1968) reconstructed tentative vegetation maps o f Zim babw e showing how the vege tation would have differed from that o f today if rain fall decreased by 50% or increased by about 150% above present levels.Wild dem onstrated that if rain fall increased by 150% above present levels, Z im bab wean forests that are now isolated would have been sufficiently widespread to have been in contact with the main forest areas o f Zaire and West A frica which would explain why some species in isolated Z im bab wean forests have west African affinities.Wild sug gested that Q uaternary pluvials o f only 50% higher rainfall would probably have resulted in m ore or less continuous forest at lower altitudes through much of tropical Africa, but Axelrod & Raven (1978) con sidered this unlikely unless a considerable am ount of rain fell during the present dry winter season so that the rainfall was fairly evenly distributed throughout the year, a situation which was itself considered un likely because o f the strength o f the then prevailing anticyclonic circulatory systems.Axelrod & Raven suggested that the present links in forest taxa between the Zaire-West Africa and the relic forest patches in Zimbabwe may date from the early Miocene.
Even during the past 20 000 years there have been m ajor climatic changes over m uch of Africa (Van Zinderen Bakker, 1974) emphasizing that continued existence is not possible without continuous change.The tropical African rainforests continue to contract rapidly as a consequence o f hum an activities and p ro bably to a lesser extent because o f climatic changes.
It is against this background o f continuous change that the present distribution o f the A cacia species must be seen.Just as the present distributions differ from those of the past, so too will those o f the future differ from those of the present.Indeed, the present conservation status of a num ber o f species is uncer tain, especially o f some o f the endemic species with restricted distributions in the H orn o f Africa.

A N A L Y SIS O F T H E A F R IC A N A C A C IA SPE C IE S
The num ber o f species recorded from each country in Africa is indicated in Table 1, the countries corres ponding with the usual political boundaries except that, for the sake o f convenience, the territory o f the A fars and Issars has been included with Somalia.Table 1 was compiled from d ata contained in a co n spectus o f the African species (Ross, 1979) which was itself based on an exam ination of specimens in several African, British and European herbaria and on inform ation contained in the regional African floras.The African A cacia species remain incom pletely known and num erous taxonom ic problems await elucidation, especially in north-east tropical Africa.For the purpose of Table 1 and in the discus sion which follows the 115 species accepted by Ross (1979) have been taken as the num ber of species for the continent (this figure excludes A

. m acrothyrsa
Harms which is now considered (Hunde, 1979) to be a synonym of A .am ythethophylla A. Steud. ex A. Rich.).Taxa such as A .farnesiana (L.) Willd., which is not thought to be indigenous, and A .schlechteri Harms and A .andongensis Welw. ex Hiern, about whose precise taxonomic status there is some doubt, have been excluded as have the A .erioloba E. Mey x A. haem atoxylon Willd. hybrid and other hybrids, A. purpurea Bolle, A .m auroceana DC. and A .callicoma Meisn., which are names of uncertain applica tion, and A .sp. near Senegal, A .sp. near som alensis and others which are insufficiently known.The 115 species recognized by no means represent the final total num ber o f species for the continent but this figure does serve as a basis, imperfect as it is, for an analysis o f the African species.Because of the varia tion in the size of individual countries and because no country has species evenly distributed throughout it, the num ber of species per country is o f somewhat limited value alone.Furtherm ore, the distribution of species within countries in tropical east, south-east and southern Africa is far better docum ented than in a num ber of countries in other areas o f the continent.However, despite these limitations and, although perhaps the figures provided in Table 1 are incorrect in some details and likely to need alteration in the light of additional inform ation, it is believed that the overall patterns that emerge are sufficiently accurate to be of value.The general distribution of the genus A cacia in Africa and an indication o f the concentration o f species over the distributional range is shown in Fig. 4. As in the case of the inform ation in Table 1, Fig. 4 is m ore accurate in some areas than in others and this uneven treatment is a reflection of the uneven knowledge o f the genus over its range o f distribution.

Exam ination o f
The genus is widely distributed over the continent being absent only from the extreme northern portion o f north Africa, part of M auritania and western Sahara in West Africa, the vicinity of Cape Tow n in the extreme south-western tip of the continent and from parts of the west coast in South West Africa.*Tunisia is the only political entity on the continent in which no indigenous Acacia species are found.The greatest concentration of species occurs in tropical east and south-east Africa and, as one would expect, fewest species occur in desert regions to the north and south and in the rainforest areas, particularly in Zaire and in parts o f tropical west Africa.Although the genus is so widespread and form s such a con spicuous feature o f the landscape over much of the continent, the num ber o f species found in Africa is lower than one might expect and represents less then one-sixth o f the num ber o f species found in Australia.
Having noted the distribution o f the genus as a whole in Africa, the distributions o f subgenus Acacia and of subgenus A culeiferum are now examined (see  Acacia is apparently absent from Liberia, Sierra Leone, G abon, Equatorial Guinea and the densely forested areas in Zaire.
Apart from the exceptions noted above, the distri bution of species in subgenera A cacia and A culei feru m over the remainder of the continent is roughly similar although the number of species in individual areas within each subgenus varies.The highest con centration o f species in both subgenera occurs in tropical east and south-east Africa.
The pattern of distribution exhibited by A .sieber ana (see Fig. 7) is fairly representative o f that shown by a num ber o f widespread species in both sub genera, extending from Senegal in the west to the Sudan or Ethiopia in the north-east and down tropi cal east Africa through Kenya, U ganda and Tanzania skirting around the forested areas of central Zaire with one arm swinging westwards through Zam bia, south-east Zaire, Zimbabwe and Botswana to Angola and northern South West Africa and another arm continuing southwards through M ozam bique, the Transvaal and Swaziland into Natal.The two arms of distribution approxim ate roughly to the tem pera ture and rainfall belts.
The distribution of species within some o f the in dividual countries is now considered in m ore detail.Thirty-one species, none of which is endemic, occur in the Sudan.The highest concentration of species occurs in the south-east where the territory adjoins Ethiopia, Kenya and U ganda, and the numbers decrease quite sharply in the south-west, central and northern areas.The genus is very poorly represented in the north-western portion o f the coun try.
Ethiopia, with its diversified topography, has 43 species, the second highest num ber am ong the Afri can countries.In addition to being rich in species, Ethiopia is an im portant area o f speciation for Acacia, each subgenus having three endemic species.Among the endemics are A .walwalensis  U ganda, with 27 species, has far fewer species than either Kenya or Tanzania and the relative poverty evident in western Kenya is again evident over much o f Uganda.The distribution of A cacia in U ganda according to the provinces recognized in the Flora of Tropical East Africa is as follows: U l, N orthern : 24 species; U2, Western : 11 species; U3, Eastern : 16 species; U4, Buganda : 13 species.The N orthern Province which is well watered in the west is by far the richest province.
Tanzania, with 50 species, has significantly more species than any other country and both subgenera A cacia and A culeiferum are best represented in T a n zania.In addition, ten o f the species, three in sub genus A culeiferum and seven in subgenus A cacia, are endemic in Tanzania.The distribution of A cacia in T anzania according to the provinces in the Flora of Tropical East Africa is as follows: T l, Lake : 23 spe cies; T2, N orthern : 28 species; T3, Tanga : 28 spe cies; T4, Western : 29 species; T5, Central : 27 spe cies; T6, Eastern : 21 species; T7, Southern H igh lands : 14 species; T8, Southern : 16 species.These figures illustrate the relative poverty o f Acacia species in the Southern Highlands and Southern Province in contrast to the remainder of the country over which the species are fairly evenly spread.T an zania has been an im portant centre of speciation in the 'A .drepanolobium H arm s ex Sjostedt com plex', a complex of species with characteristically enlarged stipular spines ('ant-galls').In addition to the wide spread A .drepanolobium , six species with enlarged stipular spines, namely A .bullockii Brenan, A .burttii Bak.Burundi) in the north, north-east, east and south-east respectively No endemic species occur in Zam bia, Malawi or Botswana.The southern province of Zambia, which to some extent represents a northern extension of the western province of Zimbabwe, is by far the richest province, while the western province with only six species is the poorest.The north and south-east pro vinces o f Botswana each have 23 species but the genus is poorly represented in the arid south-west province where only 9 species occur.The southern limit of distribution o f the genus in Africa occurs in the Cape Province in South Africa and, as one would expect, the num ber of species in South Africa declines rapidly to the south particular ly along the east coast.Twenty-one species, all of which occur in Zululand and Tongaland in the north, are recorded from Natal.However, impoverishment to the south in Natal is fairly rapid as a num ber of species reach their southern limit o f distribution in Natal.O f the twenty-one species which occur in Tongaland and Zululand in the north, only thirteen occur south o f the Tugela River, and o f these thirteen only five species extend south of the U m tam vuna river into Transkei.The number o f species in Natal also falls away fairly rapidly with increasing altitude tow ards the interior and the widespread A .karroo Hayne is the only species in Lesotho.The majority of species in Swaziland occur in the lowveld in the east with fewer species in the higher areas.The Transvaal, with 35 species, has the highest num ber of species of all of the provinces in South Africa.Once again, the majority o f species occur in the lowveld in the east and in the north with fewer species occurring in the highveld region with its colder winters.A num ber of species favouring sandy soils occur in the western portion of the province.The complex o f species with glandular glutinous pods appears to be centred in the Transvaal where six of the seven species within the complex are found The countries in which endemic species occur and the num ber of endemics recorded within each are given in Table 2.
Because of the variation in the size of individual countries and because no country has an even spread of endemism, the number of endemic species per country is of limited value alone.For example, T a n zania with ten endemic species out of a total of fifty species has a greater num ber of endemics than Somalia with nine endemics out o f a total of 32 species, but the proportion of endemics in Somalia is greater.However, despite the limitations, the infor m ation in Table 2 is nevertheless fairly instructive.
Table 2 shows that the highest num bers o f endemic species occur in Tanzania, Somalia and Ethiopia in tropical east and north-east Africa.Interestingly, the highest num ber of endemic species in subgenus A cacia occur in Tanzania while the highest num ber of endemics in subgenus A culeiferum occur in Somalia.M orocco is the only country in north or west Africa which has an endemic species.Although the Sudan has one more species than Somalia, the Sudan has no endemic species while Somalia has nine, and Ethiopia with one more species than Kenya has six endemics and Kenya only one.The distribu tion of the narrow endemics each subgenus referred to in Table 2 are illustrated in Figs 11 and 12.
The endemism in the genus A cacia in Africa is shown in Table 3.
The high proportion of species endemic in one country in contrast to the much lower figures for those species endemic in two or three countries emphasizes the prevalence o f species with narrow ranges.
It is instructive to briefly com pare the distribution of the African A cacia species in Fig. 4 with the phytogeographical regions o f Africa (see Fig. 13).Fig. 13 is a slightly simplified version adapted from the scheme accepted by Brenan (1979)  The Sudano-Zam bezian Region, which is charac terized by a strong seasonal climate, corresponds to the tropical savanna and is by far the largest Region in Africa extending north and south o f the equator but physically continuous by a relatively narrow con nection in east Africa.To the north it is bounded by the deserts and semideserts o f the Sahara, in the cen tre it circumscribes the limits o f the forests of the Guineo-Congo Region, and in the south it extends to the deserts and semideserts o f the Karoo Namib Region and the Cape Region.As is to be expected, the majority of Acacia species occur within this region.

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The currently accepted sequence of events which m arked the begining of the fragm entation of West G ondw analand and the separation of the African and South American plates was outlined by Raven & Axelrod (1974) and is summ arized briefly here.Separation o f the African and South Am erican plates started about 127 million years ago with the final m arine connection between the N orth and South Atlantic occurring about 100 million years ago.The tw o continents remained in close proximity with only a narrow strait separating the present G abon from north-eastern Brazil for another 10 million years.By the end o f the Cretaceous, about 65 million years ago, the two continents are thought to have been separated at their closest points by about 800 km although they are said to have been linked by numerous volcanic islands.South A merica moved away from Africa and gradually converged with North America becoming equidistant between the two early in the Eocene about 50 million years ago.South America subsequently becam e m ore isolated from Africa and closer to N orth A merica until a direct land connection between N orth and South A merica occurred in the Pliocene 7 -5 million years ago.This sets the background against which the African and American species must be seen.
There is no evidence and it seems unlikely that A cacia or its prototype had differentiated when Africa separated from South A merica about 100 million years ago, and it is not clear whether the genus crossed the Atlantic in the late Cretaceous about 65 million years ago (by which time the three subfamilies of Legum inosae are know n to have been differentiated) when the two continents were separated but thought to have been linked by num erous volcanic islands, or w hether the genus was carried over the Atlantic in Paleocene or Eocene time.From what is known or can be inferred about its history and present distribution patterns Raven & Axelrod (1974) presum e that M im osoideae migrated between Africa and South A m erica during or prior to the Paleocene when the Atlantic was m uch narrower than at present, while T horne ( 1978) is o f the opinion that migration could have occurred in the late Cretaceous, Paleocene or Eocene.T he evidence sug gests that only limited migration occurred between these continents after the Paleocene.
Both subgenera A cacia and A culeiferum are present in A frica and in America.However, the representation o f subgenus A culeiferum in each con tinent is different: sections M onacanthea and A culeiferum occurring in Africa and sections M onacanthea and Filicinae in A merica, i.e.Filicinae is absent from Africa and A culeiferum is absent from America.T he position o f A .willardiana Rose, which has a horizontally flattened petiole and is endemic to the west coast o f N orth A merica (Mexico), is uncer tain and opinions differ as to whether it should be referred to subgenus A culeiferum or subgenus H eterophyllum .Vassal & Guinet (1972) (1975) argued that the species could be accom m o dated without difficulty in subgenus A culeiferum as it showed relationships to A m erican species of the subgenus.In support of their earlier contention that A .willardiana belonged to subgenus Ffeterophyllum Guinet & Vassal (1978) pointed out that the seed of A .willardiana contains the am ino-acid 'willardine' (Gmelin, 1959) which has also been recorded from the Australian species A .p o d a lyriifo lia A. Cunn. ex G. Don and A . dealbata Link both o f which are members of subgenus H eterophyllum .However, 'willardine' is also present in the Asian A .m odesta Wall., a m ember of subgenus A culeiferum (Evans et al, 1977).The available evidence suggests that A. w illardiana is in fact referable to subgenus A culeiferum .
No species in section M onacanthea is com m on to A feature shared by some species of subgenus Acacia in central America and in Africa is the pro duction o f swollen stipular spines and a mutualistic association of these spines with several species of ants, but the degree of mutualism in each continent differs.Some of the neotropical acacias with swollen spines, com m only referred to as swollen thorn acacias or 'bulls-horn acacias', have spicate inflores cences and others capitate inflorescences and the species do not appear to constitute a close phyletic unit, yet they share m any adaptive ecological and morphological traits and, according to Janzen (1974), provide outstanding examples of evolution ary convergence.Janzen (I.e.) observed that the species o f obligate acacia-ants in the New World are not specific to a swollen-thorn A cacia species, but rather to its life form.As an example he cited A .collinsii Safford, w hich has at least eight species of obli gate acacia-ants living in it over its range from Mexico to Colum bia, all of which also live in other swollen-thorn acacias.
The acacias with swollen spines in central America do not appear to occur south of Venezuela or C olum bia (Janzen, 1966).The mutualistic interaction be tween ants and acacias in central America has been detailed in a series o f papers by Janzen (1966, 1967a, 1967b) and subsequently summarized (Janzen, 1969a).Janzen (1969a) noted that the central A m eri can acacias with swollen spines differ from the other A cacia species in the area in having: 1. partially hollow spines which are occupied by ants.2. modified leaflet tips called Beltian bodies which constitute the primary source of protein and oil for the ant colony.3. greatly enlarged foliar nectaries which supply the sugar requirements for the ant colony.4. all-year-round leaf production on most individ uals which provides a relatively constant source of food for the ants. 5. an absence of chemical and structural traits that protect other acacias from most herbivores in the environment.The ants are functionally analagous to the chemicals released by some plants in their com petitive interactions with other plants; like these chemicals the ants are 'produced'at a metabolic cost to the plant (Janzen 1969b).Janzen (1966) expressed the view that the swollen-thorn acacias of central America have lost, apparently through evolutionary change, their ability to withstand the phytophagous insect dam age and competitive pressure of neigh bouring plants without the protection of the obligate acacia-ants.
Janzen concluded that those A cacia species with ants do not normally duplicate their defence systems and thus do not m ake toxic com pounds such as cyanogenic glycosides in quantity.One exception he noted was A .chiapensis Safford which possesses both types of defence systems.Janzen (1974) con cluded that A .chiapensis is a marginal host for obli gate acacia-ants and in m any features of growth and habit resembles non-ant acacias (Rehr et al., 1973).However, Siegler et al. (1978) found specimens of A .hindsii Benth., a species inhabited by an obligate acacia-ant, in Oaxaca and Jalisco to be strongly cyanogenic which is another exception to Janzen's earlier observation.
The swollen-thorn acacias occur in the wettest areas o f tropical central America.As the drier areas are approached the acacia cannot retain its leaves long enough to keep the ant colony alive and the unoccupied shoot does not survive to maturity because of insect damage.In cooler areas the growth o f the acacia is slower and the ants are apparently insufficiently active in cool weather to deter the phytophagous insects and vertebrate browsers adap ted to cool weather and thus the acacia receives more dam age than it can tolerate and the ant colony starves to death owing to a lack o f leaf products.
All o f the central American swollen-thorn acacias have a sweet pulp around the seeds and the seeds are dispersed by birds. Janzen (1969a) noted that there appeared to be 'a selective pressure acting on all the swollen-thorn acacias that favours bird-dispersal of seeds'.The species of swollen-thorn acacia with the widest distributions are those whose seeds are most readily removed by birds while those with seeds that are less easily removed have more restricted distribu tions.The birds begin dispersal of seeds as soon as the pods are ripe which is im portant to plants that lose 6 0 -100% o f a particular seed crop through the predations o f the larvae o f Bruchidae.As the initial infestation usually destroys 4 0 -8 0 % o f the seeds and all seeds remaining on a tree have usually been killed within two months of seed m aturation, the rapid removal o f the seeds by birds is possibly critical to the survival of the Acacia species.
Like the central American species, the African spe cies with swollen stipular spines, com m only called 'ant-galls', do not consist of a group o f closely related species.Most o f the African 'ant-gall' acacias have white or pale yellowish white flowers in capitate heads but some have deep yellow flowers and two species have spicate inflorescences.The African A cacia species with swollen spines vary from those which are apparently partially ant-dependent to those which have no regular mutualistic association with ants.
Hocking (1970) investigated the East African swollen-thorn acacias and, although he worked on several different species, concentrated on A .drepanolobium H arm s ex Sjostedt which is probably the most ant-dependent African species.Hocking found that while A .drepanolobium can be grown to at least flowering stage in the absence of ants and probably the ants can be raised without the Acacia, in nature the association is essentially an obligate one as the ants and A .drepanolobium seldom persist in dependently.No more than 1% of the A .drepan olo bium plants in the study area were found to be without ant associates.It follows that advantage must accrue to both parties so that these associations are also mutualistic as in the case of the New World A .cornigera L. (Janzen 1966L. (Janzen , 1967b)).However, although converging on the central American system, the association between A .drepanolobium and the ants has not reached the same degree o f development as that in the neotropics.
Several species in the 'A .drepanolobium com plex' are associated with ants to a lesser extent.At the other extreme are a num ber of species with swollen spines, for example A .luederitzii Engl. var.retinens (Sim) J. H. Ross & Brenan, which have no mutualistic association with ants, the ants and other insects merely taking advantage of the hollow spines as suit able dom atia.The hollow spines in A .luederitzii var.retinens and in several other species are frequently unoccupied and often entire plants lack any enlarged spines.
The African acacias with swollen spines differ from the American swollen-thorn species in the fol lowing respects: 1. they lack Beltian bodies at the tips of the leaflets.
H ocking (I.e.) suggested that the occurrence of Beltian bodies at the tips of the leaflets in the New W orld A cacia species may ensure a more uniform distribution of ants on the foliage and be an ad ap ta tion to an environment in which phytophagous insects are a relatively greater threat than browsing herbivores.2. they lack all-year-round leaf production.3. they are not confined to the wettest areas.O n the contrary, in Africa they occupy areas which experi ence a pronounced dry season.H ocking (I.e.) sugges ted that the establishment of ants on the African acacias may improve their adaptation to a dry environm ent through the pruning out, by the ants, of the axillary buds of the swollen stipules.4. the extra-floral nectaries do not appear to be developed to the same extent.5. the seeds are not surrounded by sweet pulp.6. the seeds are not distributed by birds.In Africa some of the large herbivorous m am m als rapidly dis perse the seed of certain A cacia species, for example A .tortilis (Forssk.)Hayne, away from the parent plant and in so doing play a similar role to that played by birds in central America.
It is not known whether or not the African species lack the chemical and structural traits that protect the A m erican acacias from most herbivores in the en vironm ent and the matter needs investigation.
The genera o f ants involved in the mutualistic asso ciation with species of A cacia in Africa and in Am erica differs as one might expect.P seu dom yrm ex is the im portant ant genus in America while Crem atogaster is the most im portant genus in Africa.The development of swollen spines and the mutualistic association with ants in Africa and in America appears to have taken place independently in each continent and represent an example o f convergent evolution.
the Mascarenes.Subgenus H eterophyllum is essen tially Australian so the occurrence o f A .xiphoclada Bak. in M adagascar and A .heterophylla (Lam.)Willd. in the Mascarenes is o f considerable phytogeographic interest.As species with phyllodes do not occur on the m ainland o f any other continent it is probably reasonable to assume that any species of Acacia with phyllodes now occurring outside of Australia must either have com e from the Australian region or have been derived from species which have (Pedley, 1975). Bell & Evans (1978) found that the seed of A .heterophylla and all o f the Australian species analysed showed a single characteristic amino acid pattern which led them to suggest that Australia and the M ascarene Islands once form ed part o f the same land mass and that the seed chemistry of subgenus H eterophyllum is the seed chemistry that characterized the ancestral species o f G ondw ana land.A .heterophylla is superficially very similar to A .koa A. Gray which is endemic in the Hawaiian Islands some 15 000 kilometres away but differs in characters o f the corolla, pod, seed and seedlings (Vassal, 1969). Carlquist (1965) postulated that A. koa and A .heterophylla are probably descendants of seeds which floated from A ustralia into the Pacific and Indian Oceans respectively.While this may be true, both species are, however, tetraploid and pro bably not primitive.
Despite the close proximity o f M adagascar to Africa, as far as is know n only one indigenous spe cies in subgenus A culeiferum , namely A .rovum ae Oliv., is com m on to both A frica and M adagascar.In tropical east A frica A .rovum ae occurs on or near the coast and the appearance of the pods suggests that they are indehiscent and water-borne.If the pods are indeed dispersed by water this m ay possibly account for the occurrence o f the species in Africa and in M adagascar and it seems reasonable to assume that migration o f the species between the two areas took place in geologically recent time.There is a very doubtful and unlikely record o f A .pervillei Benth., a M adagascan species, from Delagoa Bay, M ozam bi que, but the most likely explanation is that the label does not belong with the specimen (Ross, 1973).Bentham (1875) was o f the opinion that A .pervillei was more closely allied to the South American A. lacerans Benth.than it is to any other Old World species.There is no recent taxonom ic revision of the Madagascan species and several o f the species are in sufficiently known.

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M adagascar was connected with Africa into the mid-Cretaceous when it was situated against Tanzania-Kenya about 15° N of its presenty position (Axelrod & Raven, 1978).M adagascar then form ed part of the now largely submerged M ascarene Plateau which joined India in the east into the late Cretaceous.Precisely when M adagascar-India separ ated from Africa is still not certain but it could have occurred at any time between the mid-and late C reta ceous.India separated from the M adagascar-M ascarene subcontinent early in the Paleocene about 65 million years ago and moved north to meet the Asian land mass by the middle Eocene about 45 million years ago (Axelrod & Raven, 1978).
Both subgenera Acacia and A culeiferum are pre sent in Africa and in M adagascar but, in addition, subgenus H eterophyllum occurs in M adagascar and

R E L A T IO N SH IP BETW EEN T H E A F R IC A N A N D IN D IA N SPE C IE S
There are far fewer A cacia species in India than in Africa but both subgenera A cacia and Aculeiferum are present in A frica and in India and, m oreover, two species in each subgenus are com m on to each land mass which provides clear evidence o f a close rela tionship between the Indian and African acacias.Al though migration between Africa and India is now extremely difficult or perhaps impossible because of the intervening arid areas, in form er times the two areas were connected by a belt o f tropical forest and savanna and direct migration was possible.The fact that a num ber o f the acacias in India and in Africa, in areas that are now widely separated, cannot be distinguished at specific level suggests that their separation is geologically relatively recent and was attained when direct m igration was possible between India and Africa and not when India 'rafted ' north-  (Raven & Axelrod, 1974) or perhaps earlier (Schuster, 1976) otherwise one would have expected greater morphological diver sification to have occurred.The morphological dif ferences between the species com m on to Africa and India are mostly slight but the differences, taken together with the geographical discontinuity between the African and Indian populations, have been con sidered sufficiently significant to warrant the popula tions being regarded as subspecifically or varietally distinct (see Table 4).
In addition to these species which are com m on to both Africa and India, the Indian A .pennata (L.) Willd., one of the climbers with scattered recurved prickles in subgenus A culeiferum , is extremely close ly related to a num ber of African species such as A .brevispica, A .schw einfurthii, A .pentagona and allies.Once again, the degree of similarity between A. pennata and the African species suggests that their separation is geologically relatively recent.It is clear that there is a much closer affinity between the African and Indian species than there is between the African and South American species.
Although not present in India, A .tortilis, a member o f subgenus A cacia which is widespread in Africa where it is represented by a number of sub species, extends into A rabia as do several other A fri can species.A .gerrardii Benth. is represented in the Negev Desert by subsp.negevensis Zohary, this sub species being separated from all of the other variants in Africa by a wide geographical discontinuity.
The Indian species A .ferruginea (Roxb.)DC., a member of subgenus A culeiferum , has some pollen characteristics which are specific to the Australian subgenus H eterophyllum , while at least twenty-six species in subgenus H eterophyllum have a porate type o f pollen with simple appertures which is char acteristic of subgenus A culeiferum (Guinet & Vassal, 1978) illustrating the apparent close relationship be tween subgenera A culeiferum and H eterophyllum .

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The vast m ajority of the      A nother possibility, however, is that the genus may have been established in A ustralia prior to the Lower Miocene but confined to the northern part of the continent.The suggestion that A cacia first becam e established on the northern part o f the A ustralian plate and later spread to other parts o f the continent when suitable conditions prevailed for it to do so was advanced by Andrews (1914) and sup ported by Pedley (1975). Vassal (1972), on the other hand, considered that the primitive section of subgenus H eterophyllum probably occupied the whole o f the A ustralian continent at the beginning of the Tertiary.1977).During the late Miocene and Pliocene a m oderate rainfall and drier type o f vegetation existed and it is thought that remnants o f lower Tertiary flora existed in small refuge areas in the eastern highlands and migrated westwards when the climate becam e wetter for a relatively brief period in the late Pliocene (M artin, 1977).There is every indication that aridity increased subsequent to Pliocene time and the climate became more seasonal with a wellm arked dry period and it is not until the Pleistocene that grasslands an d /o r savanna w oodlands became prom inent in south-eastern Australia. According to Gill (1975) it was only at this stage about two million years ago that the full opportunity for speciation in A cacia and E ucalyptus occurred.As aridity inten sified closed forest would have been eliminated from all but locally favourable sites in north-eastern Australia.
In rainforest areas A cacia species are confined to m arginal areas and clearings and it is only when the canopy is disturbed that the light requiring elements o f the Australian flora become established (Burbidge, 1960) A part from the obvious differences such as the d e velopment of phyllodes in many members of su b genus H eterophyllum in Australia and the differen ces between the three subgenera in pollen, chrom o somes, seeds, seedlings, inflorescences and pods alluded to by Guinet & Vassal (1978), there are other differential tendencies between subgenus H etero phyllum and the African representatives of sub genera A cacia and A culeiferum which are briefly dis cussed here.Brenan are absent am ongst the A ustralian species.One explanation advanced for the flattened crowns in Africa is that it is an adaptive response to browsing (Brown, 1960).A nother suggestion that has been offered is that it is the result o f insolation damaging the apical growing buds, but if this was the case it is strange that none of the A ustralian species has developed the same adaptive response where the effects o f insolation are as great as they are in Africa.The African species in general 'lo o k ' different to most o f the Australian m embers o f subgenus H etero phyllum .

The stipules in all African mem bers of subgenus
A cacia are spinescent and invariably very prominent and in the Australian m embers of the subgenus the stipules are typically spinescent at least when young although they are usually small or occasionally absent.The stipules in subgenus H eterophyllum are, with few exceptions, small, inconspicuous and often deciduous.M any o f the species are entirely unarmed but in others spinescence has arisen in various ways through the m odification o f phyllodes, branchlets and peduncles.A .paradoxa DC. and A . victoriae Benth.are exceptional in having stipular spines although those of the latter are sometimes reduced to blunt outgrow ths, and stipular spines also occur in some o f the Western A ustralian species (Pedley, 1978).Stipular spines are far better developed in the African species o f subgenus A cacia than they are in the Australian members o f subgenus A cacia or in subgenus H eterophyllum .Brown (1960)  However, the argum ent loses some validity as many species in other genera have survived browsing pres sure without the aid o f spines although admittedly they may have developed other deterrents such as of fensive chemical attributes to discourage large browsers.No similar radiation o f large browsers appears to have occurred in A ustralia, the endemic Australian m am m al fauna consisting o f marsupials, monotrem es, rodents and bats.O f these, the kanga roo is the largest survivor but kangaroos are chiefly grazers and it seems reasonable to assume that the extinct giant kangaroos were also grazers.Brown (I.e.) considered the development of spininess in many of the smaller shrubby acacias, grasses and other shrubs of the closed forest understandable see ing that kangaroos, smaller marsupials and rodents feed on them.

None of the Australian members of subgenus
Acacia has swollen stipular spines which are so characteristic of some of the African and American species and none o f the Australian species has form ed any mutualistic association with ants although the seeds o f m any Australian plants, including some Acacia species, are dispersed short distances by ants (Berg, 1975).The Australian continent is richly endowed with ants so the lack of a mutualistic association between acacias and ants cannot be due to a scarcity of ants. Hocking (1970) suggested that if the Australian members of subgenus Acacia ever had the tendency to produce swollen spines then presumably the expression of the character has been lost under reduced selection pressure from her bivorous animals.The extra-floral nectaries in the Australian members of subgenus Acacia are small.

A num ber of the A ustralian members o f subgenus
H eterophyllum flourish in a cold and wet environ ment in southern Victoria and Tasm ania far further south than on any other continent and have occupied a habitat that is generally avoided by the indigenous African species.The African species tolerate hot and dry, hot and wet, and cold and dry habitats but where cold and wet conditions persist for any great period o f the year acacias are usually infrequent in their occurrence or absent.A cacia species by Evans et al. (1977) revealed that the genus can be divided into four biochemically dif ferent groups on the basis of their seed chemistry and Southgate (I.e.) suggested that one o f the factors in fluencing the ability o f bruchid larvae to survive within a seed may be the level o f certain am ino acids, notably pipecolic acid and some heteropolysac charides.The am ino acid composition of the seed of the Australian members of subgenus H eterophyllum differs from that o f the seed o f members of other subgenera, and a possible explanation for this may be that members of subgenus H eterophyllum have apparently evolved without the selection pressure of bruchid predation.

DISC U SSIO N
As a result of the multidisciplinary approach to Acacia in recent years much evidence has accum u lated which indicates that fundam ental differences exist between subgenera Acacia and A culeiferum , and that subgenus A culeiferum is m ore closely related to subgenus H eterophyllum despite the fact that they occupy basically different geographical areas which show relatively little overlap, than are subgenera Acacia and Aculeiferum which share a com m on geographical area.The differences between subgenera A cacia and Aculeiferum are such that it is considered unlikely that the one gave rise to the other directly but rather that they arose from a com m on or similar prototype.Many questions concerning the origin, evolution and dispersal of the genus and o f the relationships within it remain to be answered to enable a better understanding to emerge.
Although the African Acacia species have received a considerable am ount of attention during the last few decades they remain inadequately known and n u merous taxonom ic problems await elucidation.Des pite the incompleteness of the inform ation on the dis tribution of the African species the overall patterns that emerge are probably sufficiently accurate to be o f value.Further collecting, especially in tropical north-east Africa and in west tropical Africa, will resolve some o f the taxonomic problems and provide m ore accurate inform ation on the distributions of m any species.Some species in subgenus A culeiferum are almost as widespread in Africa as the most widespread m em bers o f subgenus Acacia, but the distribution o f sub genus A cacia as a whole in Africa exceeds that of subgenus A culeiferum .That subgenus A cacia enjoys a wider range of distribution than subgenus A culei fe ru m suggests that the former has been able to occupy habitats from which the latter has been ex cluded and the possibility exists that it has been assisted in this by the greater genetic plasticity con ferred on it as a consequence of its members being polyploid.On the other hand, climbing members of subgenus Aculeiferum have been successful in forested areas o f the continent in which subgenus A cacia is not represented, the climbing habit, which is not known in subgenus Acacia in Africa, enabling species to take advantage of suitable sites in forested areas.The highest concentration o f species in each subgenus occurs in tropical north-east, east and south-east Africa but different parts of the continent have been im portant areas of local speciation for each subgenus, the highest concentration of endemic species in subgenus Acacia occurring in T anzania and the highest concentration of endemics in subgenus Aculeiferum in Somalia.
Despite the advances in our knowledge o f the A fri can species in recent decades, detailed population studies are required and inform ation is needed on their biology and autecology.Only when such a reservoir of inform ation is available will a better understanding o f the African species emerge.A nd, what is true for the African species applies equally to those in other continents.It would be highly benefic ial as a first step to have a conspectus of the A cacia species occurring on each continent reflecting the current state of taxonomic knowledge along the lines o f that produced for the African species (Ross, 1979) or the m ore detailed revision of Queensland species (Pedley, 1978(Pedley, , 1979)), and ultimately a conspectus of the genus as a whole.This is, of course, a fairly for m idable task especially when one considers the A us tralian species but much valuable work has already been done on the Australian species.Hopefully, a conspectus of the Australian species will be prepared before too long.Acacia is a fascinating genus, which com m ends itself to further study.

Die drie sub genera wat in die genus Acacia erken word, w o rd in h o oftrekke beskryw e en die globale verspreiding van elk w ord aangedui. D ie verskille tussen die subgenera en die graad en verw antskap en vlakke van spesialisasie w ord kortlik s bespreek. Die voorstel w ord getnaak dat die voorouers van die genus klim -o f slingerplante was. G eologiese gebeure in die verlede wat 'n m oon llik in vloed op die ver spreiding van Acacia-spesies in A frik a kon gehad het, w ord geskets. Die aantal spesies wat vir elke land in A frik a aangeteken is w ord getabuleer en die ver spreiding en konsentrasie van spesies binne die genus Acacia as geheel en binne elke subgenus in A frik a w ord geillustreer. Die hoogste konsentrasie van spesies binne elke subgenus kom in oo s en su id-oos tropiese A frik a voor. Die verspreiding van spesies in som m ig e van die afsonderlike A frika-lan de en m o o n tlik e verw antskappe w o rd bespreek en die aandag w ord o p die hoofsentra van endem ism e gevestig. Die verspreiding van die spesies van A frik a w o rd m et die h o o f fito-geo g rafiese streke wat o p die vasteland erken word, in verband gebring. D ie verw antskappe tussen die
2. Subgenus Heterophyllum (Sections H etero ph yllu m , Uninervea and Pulchelloidea) 3. Subgenus Acacia (Section A cacia) ♦The subgeneric name Phyllodineae (D C .)S er in g e has priority and will have to be adopted in place o f H eterophyllum Vassal, but the nam e H eterophyllum is retained for the purpose o f this paper.G LO B A L D IST R IB U T IO N O F T H E G E N U S A C A C IA An indication o f the global distribution o f each subgenus is given in Figs 1-3.

Fig
Fig. 2 .-A n in d ic a tio n o f th e g lo b a l d is tr ib u tio n o f s u b g en u s A culeiferum .

Fig
Fig. 3 .-An in d ic a tio n o f th e g lo b a l d is tr ib u tio n o f s u b g e n u s H e te r o p h y llu m ( e x clu d in g A .willardiana w hich p r o b a b ly b e lo n g s in su b g e n u s Aculeiferum ).
2. Subgenus Acacia is clearly distinguished from the other two subgenera by the high level o f specializa tion of the chromosom e characters (chrom osom e num bers and the degree of homogeneity of the karyotype).Subgenera A culeiferum and H eteroph yl lum are m ore homogeneous and have similar levels o f differentiation of chrom osom e characters.On the basis o f c h ro m o so m e c h a ra c te rs, su b g en u s 4. The P hyllodineae are more specialized than the B otryocephalae and the Pulchellae in subgenus H eterophyllum but the persistence of unspecialized states occurs to a similar degree in the three series.Guinet & Vassal (I.e.) stressed that if the correla tion of the characters selected reflects a true relation ship between the m ajor subdivisions o f the genus, then subgenera Aculeiferum and H eterophyllum are more closely related to one another despite the fact that they occupy basically different geographical areas which show relatively little overlap, than are subgenera Aculeiferum and A cacia which share a com m on geographical area.Guinet & Vassal favoured the concept that Acacia originated in West G ondw analand in an area that approxim ates to the area presently occupied by C en tral America (from Mexico to Bolivia).In support of this contention Guinet & Vassal pointed out that characters which are absent in the genus A cacia itself in America are nevertheless found in indirectly related genera.For example, the fundam entally A ustralian ex trap o rate pollen type (subgenus H eterophyllum ) exists in some South American genera closely related to Piptadenia, and phyllodes are present in some South A m erican species of M im osa.These occurrences were regarded by Guinet & Vassal as evidence that the A m erican continent contains most of the evolutionary potential for the characters now found in the genus A cacia and accords with G uinet's (1969) earlier suggestion that phyllodes and the pollen type com m only found in subgenus H eterophyllum m ay have originated in South America and that A ustralia was a secondary centre of development and differentiation.However, because o f the occurrence in A ustralia o f phyllodes and the pollen type alluded to, it can equally well be argued that A ustralia also contains most o f the evolutionary potential for the characters now found in the genus A cacia and that these characters originated in Australia.Guinet & Vassal are o f the o p inion th at section F ilicin ae o f subgenus A culeiferum preserves the m orphological characters closest to those postulated as being ancestral in the genus.Section Filicinae is poorly know n and much m ore inform ation is required.

Figs 5
Figs 5 & 6).O f the 115 species accepted for Africa, 52 belong to subgenus Aculeiferum and 63 to subgenus Acacia.It is at once apparent that there are significant dif ferences in the distributional ranges o f the two sub genera.Subgenus Acacia extends far further north than subgenus Aculeiferum being found for the most part as far north as 30° N latitude except in Algeria and M orocco where the subgenus occurs even further north and in the Nile valley in Egypt where a species occurs in the Nile delta.Subgenus A culeiferum , on the other hand, does not occur m uch north of 20° N latitude except in Egypt and m ore particularly in the Nile Valley.In the extreme south of the continent subgenus A cacia occurs to within 100 km o f Cape Tow n while subgenus A culeiferum has not succeeded in penetrating the south-west tip o f the Cape P ro vince and is also absent from the high country in Lesotho and the eastern O range Free State.Both subgenera are absent from parts o f the west coast of

Fig
Fig. 5 .-Th e gen era l d istrib u tio n o f su b g e n u s Acacia in A fr ic a and an in d ic a tio n o f th e c o n cen tration o f sp ecies over the d is tr ib u tio n a l ra n g e o f th e su b gen u s.
Two species, both members of subgenus Acacia, have been recorded from southern M orocco in the extreme north-west of the continent, namely, A .AN ANALYSIS OF THE AFRICAN AC A C IA SPECIES: THEIR DISTRIBUTION, species and, according toQuezel (1979), the evidence indicates that they penetrated into the Sahara only at the end o f the last pluvial, probably after several previous phases of extension, but did not reach the M editerranean regions because of their therm al dem ands.In Egypt most species occur in a zone along the Nile River with fewer species occurring in the ad ja cent desert areas.A .nilotica follows the course o f the Nile into the delta itself.
Gilliland and A .pseudonigrescens Brenan & J. H. Ross, two very distinctive species confined to the Ogaden.The highest num ber o f species occurs in the north-east adjacent to the Red Sea, but the H arar Province and Ogaden are also rich in species.A .reficiens W awra has a very disjunct distribution (see Fig. 8) : subsp.misera (Vatke) Brenan occurring in north-eastern U ganda, south-eastern Sudan, Kenya, eastern Ethiopia and Somalia and subsp.reficiens occurring in south-western Angola and northern South West Africa, the species providing a good example of the well known distributional discontinuity between the more arid areas of South West A frica and the north east H orn o f Africa.Verdcourt (1969) and De Winter (1971) provided examples o f disjunctions in other genera and families which furnish evidence of a former arid corridor across the continent from the north-east to the south-west.Conditions for direct migration were probably best when arid phases o f the Pleistocene were at a maxim um and distances bet ween the arid zones were least or possibly when a continuous arid belt extended across Africa from the north-east to the south-east.The distribution of A .stuhlmannii Taub.provides a less extreme example of the disjunct distribution illustrated by A .reficiens, while some species have a m ore or less continuous distribution from the north-east to the south-west.Somalia has a remarkable flora and the A cacia species are no exception.Although only 32 A cacia species have been recorded from Somalia, nine are endemic (seven in subgenus A culeiferum and two in subgenus Acacia).Somalia has been an im portant centre o f speciation in the 'A .Senegal (L.) Willd.complex' where six very distinctive endemic taxa have arisen, namely, A .ankokib Chiov., A .caraniana Chiov., A .cheilanthifolia Chiov., A .ogadensis Chiov., A .som alensis Vatke and A .sp. near A .somalensis.A .zizyph ispin a Chiov., another member of the 'A .Senegal com plex', although not endemic in Somalia itself, has a very restricted distribution in southern Somalia and in the Ogaden in Ethiopia.A .bricchettiana Chiov.(subgenus Acacia) is similarly confined to the Ogaden region in Somalia and Ethio pia.Several other species, for example A .condyloclada Chiov., yet another member of the 'A .Senegal complex', and A .edgew orth ii T .Anders, (subgenus Acacia) are confined to Ethiopia, Somalia and the Northern Frontier Province o f Kenya (A .edgeworthii also occurs on Socotra), while A .turnbulliana Brenan is confined to Somalia and the N orthern Frontier Province o f Kenya.A .leucospira Brenan, another endemic, is a very distinctive species with minute laterally compressed leaflets which are remi niscent of those o f A .haem atoxylon in southern Africa (see Fig. 9).Small laterally compressed leaf lets are unknow n elsewhere am ongst the African species but A .leucospira and A .haem atoxylon are not closely related.The H orn of Africa has been an im portant centre of speciation in Acacia.Kenya, with 42 species, has only one endemic spe cies, namely, A .thom asii Harm s, a m em ber of the 'A .Senegal com plex', while A .p a o lii Chiov.subsp.paucijuga Brenan is endemic in the north-west o f the country.The poverty o f endemism in Kenya is in marked contrast to Tanzania.The distribution of A cacia in Kenya according to the provinces recog nized in the Flora o f Tropical East Africa is as fol lows: K l, N orthern Frontier : 26 species; K2, F ig .9 .-T h e k n o w n d istr ib u tio n s o f Acacia leucospira and A .haem atoxylon.
f., A .erythrophloea Brenan, A .m alacocephala H arm s, A .m buluensis Brenan and A .p se u d o fistu la Harm s, are endemic in Tanzania and two of them, A .bullockii and A .erythrophloea, are endemic in the Western Province.The W estern P ro vince is outstanding on account of the num ber of endemics found within it: seven of the ten endemic A cacia species in Tanzania are found within the W estern Province although only the above two species are confined to it.A .taylorii Brenan & Exell, a m em ber of the 'A .pennata com plex' with scattered recurved prickles, is endemic along the coast in the Southern Province towards the M ozam bique border, A .tephroderm is Brenan in the same complex is endemic in the Eastern Province and A .latistipulata H arm s, yet another member of the complex, occurs from central Tanzania to central M ozam bique.A .ancistroclada Brenan, although not endemic in Tanzanzia itself, has a restricted distribution in n o rth east Tanzania and south-east Kenya.A .stuhlm annii has a discontinuous distribution being recorded from Ethiopia, Somalia, Kenya and northern and central T anzania in the north and from Zimbabwe, Botswana and the Transvaal in the south.Zaire illustrates very well the point m ade earlier that A cacia species are not spread uniform ly throughout a country.Tw enty-four species are recorded from Zaire but the great m ajority are co n fined to the mainly w ooded grasslands of the Ubangi-Uele, Lac Albert, Lacs Edouard et Kivu, Bas-K atanga and H aut-K atanga regions (phytogeographical regions used in Flore du C ongo, du Rw anda et du

F
ig . 1 0 .-Th e k n o w n d istr ib u tio n s o f Acacia lujae a ndA .kraussiana.
The highest num ber of species in South West Africa occurs in the north-west and the highest number in Angola in the south-west in the Huila, Mossamedes and Benguela Districts.Two of the 23 species in South West Africa, namely A .m ontis-ustiMerxm.& Schreiber and A .robynsiana  Merxm.& Schreiber, are endemic in the north-west and two of the 25 species in Angola, A .antunesii Harms and A .quintanilhae Torre, are endemic in the south-west.While both endemics in South West Africa are members o f subgenus A culeiferum , the endemics in Angola both belong to subgenus Acacia.In addition to these endemics with narrow distributional ranges, A. hebeclada D C. subsp.tristis Schreiber is confined to much the same area in north-western South West Africa and south-western Angola.Reference has already been m ade to the discontinuous distribution shown by A .reficiens but A .m ellifera (Vahl) Benth.subsp.m ellifera also has a disjunct distribution occurring in northern South West Africa and south western Angola (Mossamedes and Benguela Dis tricts) in the south and in Tanzania and territories to the north.The num ber of Acacia species in Angola falls away from the south-west to the north and east particularly towards the forested areas in the north although A .welwitschii Oliv.subsp.w elwitschii is endemic in northern Angola.South-western Angola and north-western South West Africa (with the exception of a narrow strip along the coast from which the genus is absent) appear to be one of the m ore im portant centres of speciation in Acacia in southern tropical Africa.
which was itself based on the earlier works o f Wickens (1976), White (1965) and C hapm an & W hite (1970).Wickens (I.e.) recognized eight Regions in Africa, three of which were themselves divided into domains.
Africa and the Americas although some tropical species on each continent show close similarities, for example, A .brevispica H arm s, A .schweinfurthii Brenan & Exell, A .pen tagon a (Schum ach.)H ook. f. and allies in Africa and A .riparia H. B. K. and A .paniculata Willd. in tropical America and the West Indies.Although subgenus Acacia is well represented in both Africa and America no species is com mon to Africa and America; A .farnesiana is not indigenous in Africa having been introduced and subsequently become naturalized in some areas.A .sieberana in Africa shows some relationship to the American A .macrantha H um b. & Bonpl.ex Willd., an observa tion noted by Bentham (1875).
Brenanwards after breaking from Africa with M adagascar about 100 million years ago

Fig. 16 )
Fig. 16).If A .farnesiana is not indigenous in A ustra lia then presumably it must have been introduced prior to European settlement as the species was en countered in the inland areas o f Australia by early explorers.The Australian members of subgenera Acacia and Aculeiferum are not particularly well known; some members o f subgenus Acacia are not yet described although the subgenus is currently being revised.Ex cept for the pantropical A .farnesiana which has a diploid chrom osom e num ber of 52 and the natural ized A .nilotica subsp.indica (Benth.)Brenan in which 2n = 44, 52, 104 (Vassal, 1974), there are no chrom osom e data available for other Australian

F
Acacia farnesiana in Australia.memberso f subgenus A cacia and none for the soli tary representative o f subgenus A culeiferum .
m igration between A ustralasia and Africa via India and M adagascar was probably relatively direct, but with fairly long steps over w ater, after the start o f the Tertiary 65 million years ago.The first recorded appearance o f A cacia pollen in the A ustralian fossil record is in mid-M iocene deposits ( + 16 million years ago) in southern Vic toria (C ookson, 1954).T o explain the apparent absence o f A cacia pollen from Paleogene beds Cookson concluded that either the genus A cacia was not represented in the N o th o fa g u s -conifer forests which are know n to have covered large areas of Australia during the early Tertiary period or else the genus did not become an integral part o f the A ustralian flora as a whole until after the Lower M iocene period (25 million years ago).A lthough the evidence was not conclusive, C ookson favoured the latter possibility.

A
lthough the centre of present-day development of species in a subgenus need not necessarily reflect the centre o f past development, the great diversity exhi bited by subgenus H eterophyllum in A ustralia and the lack o f close relatives on other continents sug gests that subgenus H eterophyllum developed in Australia from which source a few species such as A .confusa, A .koa and A .heterophylla were subse quently dispersed to other areas and that the sub genus did not enter Australia after developing else where.It is tempting to speculate that subgenus A cacia entered northern Australia where it has since largely rem ained in the mid-Tertiary (or perhaps even later) when A ustralia came into contact with the south-east Asian plate but it is difficult to account for the poor representation of subgenus Aculeiferum in Australia.As A ustralia was separated from A frica and India by a considerable marine gap and was distant from any other tropical land mass for millions o f years u n til late in the Oligocene when contact was m ade with the south-east Asian plate,Melville (1975)  concluded that 'the characteristic Australian flora -excluding the Indo-M alaysian element of relatively recent origin -must have evolved in situ from ancestors of Perm ian age'.Melville (I.e.) continued that 'Accep tance o f this conclusion implies that evolutionary trends in m any families such as Proteaceae, Restionaceae, Legum inosae and C om positae, must have been initiated already in the Perm ian for the o b served parallel evolution to have taken place subse quently on separated G ondw anic fragm ents'.Irrespective o f when the genus first becam e estab lished in A ustralia and from where it came its devel opm ent and subsequent spread over the continent have been influenced by past geological and climatic changes.During the period from the Triassic, until northw ard drift began in the Eocene, A ustralia was situated about 15° south of its present latitude (Jones, 1971).The Australian Tertiary pollen record is largely that o f rainforest which must have been widespread, though not necessarily continuous, over the southern part of the continent (M artin, 1978).The high co n tent of gymnosperm s in the A ustralian Paleocene assemblages is thought to indicate a cool temperate climate similar to present-day T asm ania (M artin, I.e.).T he subsequent increase in M yrtaceae and other angiosperms is taken to indicate a relative increase in tem perature to a warm temperate or subtropical climate.Tem peratures reached the m axim um for the Tertiary in the early Eocene and the dram atic in crease in N othofagus in the mid-Eocene marks the onset o f a cooling trend although further fluctuations were experienced.The Oligocene to early Miocene was a period o f an equable climate with very high rainfall and stable temperatures while the m id -la te Miocene was a time of profound change when N oth ofagu s and m any other taxa disappeared from the fossil record in south-eastern Australia (M artin, . The retreat of closed forest with in creasing aridity would have favoured the dispersal o f A cacia species and the fossil finds discussed by C ookson (1954) possibly indicate an expansion in the distributional range of A cacia which coincided with the retreat of N othofagus.H opper & Maslin (1978) suggested that the recent speciation in A cacia in W estern Australia has been prom oted by recurrent m igration, extinction and isolation of populations as a result o f Pleistocene climatic fluctuations and their erosional consequences in climatically transitional areas.The possible dispersal of A cacia in A ustralia as a whole is dealt with by Pedley (1980).
1.The African species (except A .albicia) are invari ably deciduous during the dry season, either regularly or irregularly so, while the Australian members of subgenus H eterophyllum are invariably evergreen.
suggested that 'A cacia in Australia passed through an earlier period in which spininess had little adaptive value, followed by a time in which selective pressures again arose favouring the development o f spines de n ovo in shrubs of the forested country and in the lower shrubs o f the open co untry'.Brown (I.e.) attributed the lack o f spinescence in many of the Australian Acacia species and in other dom inant genera like E ucalyptus to the 'longcontinued absence or scarcity o f effective large brow sers' until the recent introduction o f domesticated animals.Large browsing herbivores are now or have recently been abundant in A frica and in tropical A merica in areas where spinescent acacias occur and certainly in Africa the acacias constitute an im port ant source o f food for m any browsing mammals.
5. The African species invariably flower in spring orearly sum m er and the inflorescences are usually p ro duced with or before the young leaves.In contrast, the Australian members of subgenus H eterophyllum are evergreen and a num ber of species flower in winter.6.The anthers of nearly all of the African species (except A .albida and A .redacta, neither of which it has been suggested is referable to Acacia,Robbertse   & Von Teichm an, 1979)  are adorned with a small apical deciduous gland but none of the Australian members of subgenus H eterophyllum appear to have glands on the anthers.The function of the glands in the African species is not clear but Hocking(1970)   suggested that the tissue filling the glands may con tain useful am ounts o f nitrogenous material which may be utilised by phytophagous insects.A .bidw illii Benth.and A .sutherlandii (F.Muell.)F. Muell., both Australian members of subgenus Acacia, have anthers with small deciduous apical glands but I have not seen suitable material of other Australian members o f the subgenus, except of A .farnesiana which lacks the glands, to establish whether they also have the deciduous glands.This needs further in vestigation.7. Some o f the Australian members of subgenus H eterophyllum have seed with conspicuous brightly coloured arils, whereas none of the African species does.8. Bruchids are responsible for the destruction of vast quantities o f seeds of some American and A fri can species in subgenera Acacia and Aculeiferum whereas seeds o f the Australian species are relatively unaffected by bruchids, probably because few indige nous species o f bruchid occur in Australasia and the Pacific Islands (Southgate, 1978).A survey of the free non-protein am ino acids in the seeds o f 106 to Mr L. Pedley, Assistant Di rector, Queensland H erbarium , for answering several queries, for inform ation concerning the distribution o f subgenus Acacia in Australia and for providing a draft account of his manuscript on the derivation and dispersal of Acacia, to Mr J. R. M aconochie, N orth ern Territory H erbarium , and Mr B. R. Maslin, Western Australian H erbarium , for inform ation on the occurrence of subgenus Acacia in the N orthern Territory and Western Australia respectively, to Mr N. Hall, Castle Hill, New' South Wales, for inform a tion on the distribution of Acacia farnesiana in Australia, to Mr C. H. Stirton, South A frican Liai son Officer, Royal Botanic G ardens, Kew, England, for providing inform ation on some M adagascan A cacia species, and to Mrs G. Bray, N ational H erb arium o f Victoria, for typing the m anuscript.
Acacia spesies van A frik a , A m erika, M algassie, Indie en A ustralie w ord k o rtlik s bespreek.REFER E N C ES A n d r e w s .E. C ., 1914.D evelopm ent and distribution o f the

LE 1. -The representation o f the Acacia species within each African country Country N o. o f species in subgenus Acacia N o . o f species in subgenus
Table 1 reveals, not surprisingly, that the highest concentrations o f species occur in countries in tropical north-east, east and south-east Africa.The figures suggest that there is a tendency in most countries in tropical north-east, east, south east, southern and south-west Africa for subgenus Acacia to be proportionately better represented than subgenus Aculeiferum , although M ozam bique is an obvious exception to this generalization, while in Zaire and in countries to the north-west subgenus A culeiferum is often numerically as im portant as or T A Beven proportionately more im portant than subgenus A cacia although once again there are a num ber o f ex ceptions.This apparent proportional preponderance o f subgenus A culeiferum in Zaire and parts of tropical west Africa may possibly be due to the presence o f extensive forested areas which climbing species in subgenus A culeiferum have been able to colonize but from which subgenus Acacia has been largely excluded.
Coast : 23 species.These figures illustrate the richness o f the N orthern F ro n tier Province, mainly because of the presence there of the Somali element, the central and Coastal P ro vinces, and the relative poverty of western Kenya.
Zambezia : 16 species; Tete : 21 species; Manica e Sofala : 20 species; Sul do Save : 17 species; Lourenco Marques : 23 species.The highest numbers o f species occur in the extreme north and south of the country and in the Tete province.O f the 38 species occurring in Zimbabwe only A .chariessa M ilne-Redhead, which is almost always Acacia species in Zimbabwe according to the prov inces recognized in Flora Zambesiaca is as follows: North : 19 species; East : 17 species; Central : 15 spe cies; West : 27 species; South : 27 species.The great est numbers o f species occur in the south and west, the numbers there being increased by the presence of several members of the complex with glandular glutinous pods, namely A .borleae Burtt Davy, A .exuvialis V erdoorn, A .nebrow nii Burtt Davy and A .permixta Burtt Davy, and species such as A .erioloba E. Mey. and A .luederitzii Engl, which favour dry woodland and often occur on the Kalahari sands.
. Only four species, A .kamerunensis Gandoger, A .ciliolata Brenan & Exell, A .pentagona (Schumach.)Hook.f. and A .lujae, all climbing species in subgenus A culeiferum , occur in the Forestier Central region.A .lujae is the only endemic A cacia species in Zaire.A .lujae seems more closely related to A .kraussiana, which is endemic along the coast in southern M ozam bique and Natal and from which it is separated by a wide interval, than it is to any members o f the 'A .pennata com plex' (see Fig. from the territory.The type locality o f A .rovumae was given as 'Rovum a Bay' without any indication of whether it was collected from the north or south side, crucial inform ation as the Rovum a river forms the boundary between M ozam bique and Tanzania.The absence o f any definite records o f the species from M ozam bique suggests that it does not occur in the territory.The distribution o f A cacia in M ozambique according to the provinces recognized in Flora Zambesiaca is as follows: Niassa : 22 species;

LE 2 .-The countries within which endem ic Acacia species occur and the number o f endem ic species within each country
T A B

h e distribution o f the narrow en d em ics within F ig . 1 2 .-T he distribution o f the narrow en d em ics within subgenus Acacia referred to in Table 2.
subgenus A culeiferum referred to in T able 2.

. farnesiana (apart from A . nilotica, A . karroo and A . albida Del
A cacia is represented in Africa by subgenera A cacia and A culeiferum and in Australia by sub genera A cacia, Aculeiferum and H eterophyllum . Acacia species (830 spe cies fide H opper & Maslin, 1978) occur in Australia where considerable morphological diversity has oc curred.
The only A ustralian species thought to be deciduous is A .ditricha Pedley, a m ember o f subgenus Acacia, but it is possible that some other m embers o f this subgenus are also deciduous.2. The flattened spreading crowns which are so characteristic o f some of the African species such as A .tortilis subsp.heteracantha (Burch.)Brenan and subsp.spirocarpa (Hochst.ex A. Rich.)Brenan, A. sieberana var.w oodii (Burtt Davy) Keay & Brenan, A .lahai Steud.& H ochst. ex Benth.and A. abyssinica H ochst. ex Benth.subsp.calophylla