Acacia

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Acacia

Description

Armed or unarmed trees, shrubs or lianas. Leaves bipinnate or modified to phyllodes by dilatation of the petiole and the proximal part of the rachis; The leaves of the mature specimens of subg. Aculeiferum and subg. Acacia are bipinnate. In the major part of subg. Phyllodineae (excl. sections Botrycephalae and Pulchellae) the leaves are phyllodinous and look simple, being formed by dilation of the rachis and the petiole. The extrafloral nectaries, widespread in the Mimosoideae, can be observed in all three subgenera (except sect. Filicinae). In the phyllodinous species a nectary can often be observed as a slit or narrowly elliptic gland on the front margin of the phyl- lode close to the pulvinus. So-called 'reversion-shoots', i.e. shoots which develop bipinnate leaves, often with a compressed rachis and petiole, can be found in A. melanoxylon. A recent hypothesis on the evolution of phyllodes in Acacia is given by Pedley (1986).
Stipular spines are found in all species of subg. Acacia and in only 15 species of subg. Phyllodineae (Pedley 1986). In subg. Aculeiferum the species are armed with prickles scattered along the internodes or paired, or ternate, just below the nodes (not in Malesian species).
Stipules spinescent or not. Extensively reviewed by Pedley (1986). In subg. Acacia a ring of bracts is found on the peduncle; in A. leucophloea and A. tomentosa such a ring is situated in the middle of the peduncle (Medibracteatae Benth.), in A. farnesiana it is found just below the flowers (Summibracteatae Benth.); in subg. Phyllodineae and subg. Aculeiferum no ring of bracts is found, but in the latter a single bract is often present.
Inflorescences consisting of pedunculate glomerules or spikes b >rne in axillary clusters or aggregated into terminal panicles. The small flowers, which are aggregated in either pedunculate glomerules or spikes, do not furnish many characters for the distinction at species level. Pedley (1986) noted that the species of subg. Aculeiferum have a disc on which the stamens are inserted; this can also be seen in most Asian species of this subgenus. Discs are absent from the members of subg. Acacia and Phyllodineae and a few American species of subg. Aculeiferum. Most species of subg. Aculeiferum have the ovary placed on a gynophore (but e.g. in A. vietnamensis a gynophore is almost absent). The species of subg. Acacia and Phyllodineae have sessile or subsessile ovaries. The glandular appendages of the anthers, found in several genera of the Mimoseae but absent from the Ingeae, are found in most species of subg. Aculeiferum and in several species of subg. Acacia (e.g. in A. leucophloea and A. tomentosa), but are absent from other members of subg. Acacia (e.g. A. farnesiana and related American species) and from all species of subg. Phyllodineae. The stamens are free in all Asian and Australian Acacia species; a few African species of subg. Acacia, viz. A. (Faidherbia) albida, A. ogadensis and A. eriocarpa (Wickens 1969) have the stamens shortly connate at the base. Because of this character and for palynological reasons A. albida has been referred to a separate genus, Faidherbia A. Chev.
Flowers bisexual, or male and bisexual, tetra- or pentamerous, uniform. Stamens numerous (15 or more), free; Seeds usually elliptic to oblong, ± flattened, with a hard black-brown testa with pleurogram, wingless; In a recent account on the anatomy of the seed-coat of 123 Acacia species, Maumont (1990) found that subgenera Aculeiferum and Phyllodineae have a thinner seed- coat than subg. Acacia, that both the external and internal layers of hour-glass cells are more developed in subgenera Aculeiferum and Phyllodineae, and, moreover, that these have a mucilaginous layer on the Malpighian cells, a layer that is absent from subg. Acacia.

Distribution

Africa: present America: present Asia: present Asia-Tropical: Australasia: Madagascar: present Pacific area: present southern hemisphere: present
Tropical and subtropical areas of the world, mainly in the Southern Hemisphere, Africa> 130 species, Madagascar c. 100, America c. 270, Australia and the Pacific area> 900, and Asia> 55 species, more than 1300 species in all; in Malesia 29 species.

Morphology

The genus consists of trees, shrubs and climbers, the climbers being restricted to subg. Aculeiferum. Members of subg. Acacia in Africa display the characteristic flat-crowned habit and are deciduous during the dry season. Members of subg. Aculeiferum and subg. Phyllodineae are, as far as known, evergreen. In many species of the latter subgenus (sect. Phyllodineae) the mature leaves are reduced to xeromorphic phyllodes, formed by a dilatation of petiole and proximal part of leaf rachis. Acacia concinna, A. megaladena, A. palawanensis, A. pennata, A. pluricapitata, A. pseudointsia, and A. sulitii, all members of subg. Aculeiferum, display a plasticity in habit, being climbers in dense vegetation and erect shrubs (or trees) with scandent branches in open vegetation types. A similar potential for change of habit has been observed in the African A. ataxa- cantha (Ross 1981) and perhaps this can be regarded as necessary for forest-dwelling climbers to adapt to and exploit new habitats in savannas and woodland as the forest retreats, or vice versa.

Taxonomy

All modern classifications are based on the monograph of Bentham (1875), who noted (I.e.: 444): "This genus, which still appears to me to be naturally as well as definitely characterized by the stamens, neglecting the various forms assumed by the pod in ripening, remains the largest among Mimoseae, and, next to Astragalus, the largest among Leguminosae. I have not either been able, in this my third careful revision of the species, to divide it into sections founded upon any character derived from the flowers or fruits." Since then the genus has not been revised on a worldwide basis. The works of Guinet, Pedley, Ross, and Vassal deal mainly with African and Australian species and give sufficient evidence for elevating the rank of series proposed by Bentham. Pedley (1986) proposed that the species of ser. Vulgares Benth. (= subg. Aculeiferum Vassal) should be referred to the genus Senegalia Raf. and that the species of subg. Phyllodineae (DC.) Seringe (= subg. Heterophyllum Vassal) should be referred to the genus Racosperma Mart., thus leaving the genus Acacia sensu stricto to comprise species with stipular spines, involucellate peduncles, and with colporate pollen with columellar exine structure.

The only characters investigated throughout the range of the genus Acacia were those of leaves, flowers, inflorescence, pollen and, to a lesser degree, seedlings. Some characters give indications about other affinities between the three subgenera and point toward a common ancestor. They are: the occurrence of stipular spines in 15 species of subgenera Phyllodineae and Acacia; the simply porate pollen grains and granular exines of subgenera Aculeiferum and Phyllodineae; the extra-Australian species A. confusa (Taiwan, Philippines), A. kauaiensis (Hawai) and A. simplex (New Caledonia) share non-protein amino- acid patterns with members of subg. Aculeiferum; the flowers of subgenera Acacia and Phyllodineae have a sessile (or subsessile) ovary and lack a disc; most members of subg. Aculeiferum have glandular appendages on the anthers as have several species of subg. Acacia; glandular appendages are absent from all species of subg. Phyllodineae; the presence of internodal prickles in subg. Aculeiferum and the absence of such prickles in the two other subgenera.

The results of the studies by Guinet, Maumont, Pedley, and Vassal suggest that there are two different evolutionary lines within the original genus Acacia. One consists of Acacia and the other of the two subgenera A culeiferum and Phyllodineae, with an interesting convergence in the characters of fully or partly extra-Australian species of subg. Phyllodineae (e.g., A. mangium, A. wetarensis) to those of subg. Aculeiferum. Even affinities to both the tribes Mimoseae and Ingeae suggesting a biphyletic origin of the genus Acacia have been postulated. No evidence has been given that any of the three subgenera are more closely related to either genera of the Mimoseae or of the Ingeae than they are to each other. Therefore Acacia is treated as one genus here.

Citation

Pedley - in J. Linn. Soc. Bot. 1986: 238
Pedley - in J. Linn. Soc. Bot. 1986: 239
Nielsen - in Opera Bot. 1985: 7
Mill. - in J. Linn. Soc. Bot. 1986: 219
Ross - in Mem. Bot. Surv. S. Afr. 1979: 1
Mill. - in Trans. Linn. Soc. 1875: 444
Pedley - in Contr. Qld. Herb. 1975: 1
Vassal - in Bull. Soc. Hist. Nat. Toulouse. 1972: 125
Mill. - in Bothalia. 1981: 389
Mill. - in Austrobaileya. 1979: 235
Hutch. - in Gen. Fl. Pl. 1964: 280
Isely - in Mem. N.Y. Bot. Gard. 1973: 10
Mill. - in Austrobaileya. 1978: 75
Willd. - in Sp. Pl., ed. 4. 1806: 1049
Benth. - in Lond. J. Bot. 1842: 318