This chapter includes brief comments on the habit of acacias, the past and current state of acacia taxonomy, the origin and present occurrence of the genus and its ecology. Reference to the six species of special interest will be given in the text where appropriate. A short list of botanical texts is presented for the main countries or regions where acacias occur. Notes are given in the taxonomic section on the morphology of several acacia plant organs. The aim of the chapter is to broaden the readers appreciation of the genus, particularly in areas that directly or indirectly affect seed collection. A more comprehensive treatment of the taxonomy will be provided in a proposed FAO/IBPGR handbook on taxonomy of Acacia species.
Acacias vary widely in size. Some consist of woody vines, e.g. A. pennata some are small prostrate shrubs e.g. A. depressa, while others form large forest trees more than 35 m tall, e.g. A. melanoxylon. The height of the tree or shrub will influence methods used to collect seed.
The crowns of many mature acacias are distinctive. Many African species characteristically have flat tops whereas most Australian species tend to develop spherical crowns.
The foliage and leaf characteristics of acacias are diverse. Many African species (except A. albida) tend to shed their leaves during the dry season, whilst most Australian species are evergreen. Both deciduous and evergreen species occur in South America (Ross 1981). The leaves of some species e.g. A. elata (an Australian species from the fringes of rainforests), are held more or less horizontally, whilst the phyllodes of A. harpophylla (and many species from dry sites) are pendulous; this latter state is presumably an adaptation to avoid direct solar radiation. In addition some African species, particularly members of subgenus Acacia, have the ability to develop clusters of secondary leaves which arise at ther nodes from dwarf lateral shoot (Ross 1979).
Seed is the main source of natural regeneration but some species can regenerate from root-suckers (very common in A. harpophylla) and others from coppice e.g. A. albida.
According to Ross (1973) the genus Acacia was first described in 1754 by Philip Miller who based his description on ‘the Egyptian thorn’ (A. nilotica). The generic name is believed to have been derived from the Greek akazo (I sharpen) alluding to the spiny stipules of many African and Asiatic species (Ross 1973). The fascinating pre-Linnean history of Acacia taxonomy is documented by Ross (1980).
The reader may be confused as to the name of the plant family to which acacias belong. In Australia the genus has been placed in Mimosaceae by Jacobs and Pickard (1981) and in Leguminosae sub-family Mimosoidae by Green (1981) while the family name Fabaceae is preferred to Leguminosae in some African (Ross 1979) and North American (Shelter and Skog 1978) literature. The new ‘Flora of Australia’ (George 1981) follows Cronquist (1981) placing Acacia in the family Mimosaceae. Much of this variation can be attributed to personal preference.
The history of the classification of Acacia into groups of species having a similar evolutionary pathway has been summarised by Ross (1973). Of particular importance is the classification devised by Bentham (1842), with a later revision in (1875), in which the genus was divided into six series viz. Gummiferae, Vulgares, Filicinae, Phyllodineae, Botryocephalae and Pulchellae. The series were mainly circumscribed on foliage and stipule characters with less importance being attached to the inflorescence. Bentham's early synthesis, even though many species were then unknown to him, was remarkable and has basically stood the test of time (Pedley 1978).
The most recent important attempt to subdivide the genus was by Guinet and Vassal (1978). These authors took into account pollen characters, cytology (e.g. chromosome numbers) and seed characters as well as the character of the inflorescence, pod and vegetative systems (e.g. seedling morphology). Bentham's classification was re-evaluated and three subgenera were proposed. These were:-
|Subgenus Aculeiferum Vassal||(includes Bentham's Series Vulgares and Filicinae)|
|Subgenus Heterophyllum Vassal||(includes Bentham's Series Phyllodineae, Botryocephalae and Pulchellae)|
|Subgenus Acacia||(includes Bentham's Series Gummifera)|
These subgeneric divisions are gaining acceptance and were used by Pedley (1978, 1979) in his revision of the acacias of Queensland (Australia) and by Ross (1981) in his discussion of African and world acacias. Of the six species featured in this handbook, A. aneura is in the subgenus Heterophyllum, A. senegal in the subgenus Aculeiferum and the remaining four (A. albida, A. nilotica, A. tortilis and A. caven) are in the subgenus Acacia. It is of interest to note that Guinet (1969) suggested A. albida and A. caven should be removed from the genus Acacia and placed in the genera Faidherbia A. Chev. and Vachellia Wight and Arn. respectively.
In brief, species belonging to subgenus Acacia have bipinnate leaves and stipular spines, those in subgenus Aculeiferum often have spines of non-stipular origin, or prickles, and are woody vines or trees. Species in subgenus Heterophyllum usually have no spines or prickles and the leaves are often modified to phyllodes. While some species in the latter group, e.g. A. decurrens, have bipinnate leaves on adult trees, they are, when other characters are considered, more closely related to the phyllodinous group (Simmons 1981).
It is of importance to the seed collector when collecting provenances (see Chapter 4) to realise that variation also exists within species. This variation is amply demonstrated by the large number of sub-species and varieties that have been recognised and named in some acacias e.g. A. nilotica (9 sub-species) and A. senegal (at least 4 varieties). In addition, variation at the provenance level, although studies are rare, has been demonstrated in Australia for A. melanoxylon (Farrell and Ashton 1978) and in Africa for A. karroo (Robbertse et al. 1981). These studies have shown that considerable variation in several morphological characters may exist within a species and that variation in physiological response can also be expected. The seed collector should be aware of these considerations when making collections and choose his collection sites with care.
There are problems for the seed collector who wants to identify individual species in the field. No single, comprehensive botanical description of all acacias of the world has yet been produced. Botanical information and keys to local species can be usually obtained from country or regional floras, which include the following works - Africa: ‘Flora of West Tropical Africa’ (Hutchison and Dalziel 1958), Flora of Tropical East Africa - Leguminosae, Mimosoideae (Brenan 1959) ‘Flora Zambesiaca’ (Brenan 1970), ‘Trees of Southern Africa’ (Palmer and Pitman 1972), ‘Kenya Trees and Shrubs’ (Dale and Greenway 1961 and ‘A conspectus of African Acacia species’ (Ross 1979). Asia: ‘Flora of West Pakistan’ (Nasir and Ali 1971), ‘Flora of Java’ (Backer and Bakhuizen 1963) and Pedley's (1975) revision of some extra-Australian species. South America: ‘Flora of Guatemala’ (Standley and Steyermark 1946), ‘Flora of Peru’ (McBride 1943). Burkart (1946, 1947) published details on acacias in the Argentine. For North American acacias one can consult Britton and Rose (1928) but Isely (1969) is preferred for a more recent account. Although Acacia is an important genus in Australia, no single comprehensive treatment of Australian acacias is available. Most species descriptions are on a regional basis such as those of Whibley (1980) for South Australia, Armitage (1977) for New South Wales and Maslin (1981b) for Central Australia. Recent important revisions include those of Pedley (1978, 1979) for Queensland acacias and Maslin (1975) for some species in Western Australia. In conclusion, botanical information is scattered in the literature, and regional or country floras should be consulted for species descriptions and keys to species.
The seed collector should have a general appreciation of plant structures referred to in botanical keys so that such keys can be used efficiently. The importance of these organs in seed collecting or seed cleaning operations is mentioned where appropriate.
|Fig.1||Leaves and phyllodes of two species of Acacia -|
|(A)||A. baileyana showing four pairs of leaves and two glands on the rhachis.|
|(B)||A. baileyana showing a closer view of the glands, rhachis and four pinnae.|
|(C)||Phyllode of A. falciformis with a gland on the edge of the phyllode.|
|(D)||Closer view of the gland on the phyllode of A. falciformis.|
In the seedlings of all acacias the first few leaf pairs are pinnate or bipinnate. In all African species the bipinnate leaf condition persists e.g. A. nilotica. In many Australian species the leaves wither and disappear during the first year and their function is taken on by the petioles which develop into leaf-like structures termed phyllodes. Each phyllode may have a single vein or a series of more or less parallel veins. The phyllodes consist mostly of vertically flattened (a few species have horizontally flattened) petioles and both these ‘leaves’ and the rhachis of the true leaves often have small glands (also called extrafloral nectaries) on their upper sides (see Boughton 1981). These glands are apparently better developed in Central American than in Australian species. The glands may attract ants and so hamper seed collections. Phyllodes vary in size from about 1.5mm long, e.g. A. minutifolia, to 30cm long, e.g. A. dunnii. In some species the phyllodes may be flat and broad, e.g. A. mangium, or cylindrical, rigid and sharp, e.g. A. ulicifolia. Examples of a typical leaf and phyllode are shown in Fig. 1. Fragments of leaves or phyllodes should be removed from seedlots during cleaning operations.
Spines, which occur on many acacias and particularly African and American species, may make seed collecting from standing trees difficult. A spine has a vascular supply continuous with that of the stem whereas a prickle is merely an epidermal outgrowth without a vascular supply. All African species can be divided into two groups depending on whether the species have spinescent (spiny) stipules, e.g. A. nilotica (subgenus Acacia), or non-spinescent stipules. Most species in the non-spinescent stipules in subgenus Aculeiferum have prickles which occur either at the nodes (e.g. A. senegal) or scattered along the internodes (e.g. A. ataxacantha). The spines and prickles are of considerable taxonomic value with characters such as the number per node (2 or 3), whether the bases are inflated or constricted, or whether the tips are recurved, being of special use. The spines are up to 30cm long in some species e.g. A. karroo.
Acacia flowers are small, regular and are usually bisexual. Each flower contains basically 4 or 5 sepals and petals. The sepals may be free or united into a calyx and the petals free or united into a corolla. The stamens are numerous and arise from under or just above the base of the ovary. A threadlike style protrudes shortly beyond the stamens. The ovary is sessile or shortly stalked and its outer surface may be smooth or covered in minute hairs. Each flower is subtended by a small bracteole whose shape varies according to species.
Pedley (1981) stated that individual flowers are of limited value in the identification and classification of Australian acacias. By contrast, Ross (1979) noted an important difference in floral characters of African species belonging to the subgenera Aculeiferum and Acacia. Species in the former subgenus have stalked ovaries and a cup-shaped disc around the base of the ovary whilst species in the latter subgenus have no such disc and the ovary is sessile or subsessile.
Acacias are very conspicuous when flowering and heavy flowering indicates potentially large seed crops. The flower colour is mostly due to the colour of the projecting staminal filaments. In Australia, the range of floral colour is not great varying from almost white to orange-yellow (Pedley 1978, 1979). In African species the variation in floral colour is more significant ranging from white or pale yellowish-white to bright golden or orange-yellow and occasionally pale pink or rarely purple (Ross 1979).
The flowers of most acacias are arranged in either heads (compact spherical clusters of flowers) or in spikes (compact cylindrical clusters of flowers). Both Pedley (1978) and Ross (1979) have discussed the value of this character in subdividing the genus in Australian and African species respectively. The number of flowers in a head can also be of value in identification.
The arrangement of the heads and, to a lesser extent, spikes is useful in delineating groups of related species. The heads may be sessile or on stalks and inflorescences may be axillary or terminal. The axillary inflorescences may be solitary, paired, several or in racemes (which may also be paired). The terminal inflorescences may be in fascicles (clusters) or panicles (sprays). The arrangement of spikes is of little importance but the length of the spike and arrangement of flowers (sparse or otherwise) are of greater value (Pedley 1978). In addition, Ross (1979) noted that African species belonging to subgenus Acacia have an involucel (i.e. a ring of bracts) on the stalk of the inflorescence and that the involucel is absent in species belonging to subgenus Aculeiferum. Typical acacia inflorescences are illustrated in Fig. 2.
|Fig. 2||Three stylised inflorescal types in Acacia -|
|(A)||An axillary cluster of four heads on single stalks.|
|(B)||Two axillary racemes with heads.|
A feature of acacia is the great diversity of its pods. The pods are described by Whibley (1980) as linear to oblong, flat to cylindric, straight or curved or spirally twisted, papery to woody, and usually dehiscent. The features of the pods are often diagnostic of certain species e.g. whether the pods are straight, twisted or coiled, parallel-sided or moniliform (like a string of beads), or winged. The fibre pattern on the outside of a pod is also often diagnostic e.g. fibres maybe orientated latitudinally or longitudinally. While most species are dehiscent a few are indehiscent e.g. A. albida, A. nilotica and A. tortilis. In these the seed and pod parts usually break off together thus making seed cleaning difficult. Fig. 3 shows variation among pods of the six species covered in the handbook.
The arrangement of seeds within the pod is also often characteristic of particular species. In some species the seeds are aligned transversely while in others they are aligned longitudinally or obliquely. In most acacia pods the seeds are aligned in single rows but in a few species of subgenus Acacia e.g. A. farnesiana, they are in two or three series.
|Fig. 3||Pods or fruits of six species of Acacia -|
|(A)||A. albida||(B)||A. tortilis||(C)||A. nilotica|
|(D)||A. senegal||(E)||A. aneura||(F)||A. caven|
Because the seeds of many acacia species are distinctive, several attempts have been made to classify species by their seed, or to make use of seed characters in botanical keys. Seed descriptions are based on such characters as the type of funicle, the size and shape of the hilum and lens ( Fig. 4), the form, colour and dimensions of the seed and some aspects of internal morphology, such as the presence or absence of endosperm (Boelcke 1946; Vassal 1963, 1971; Gopal and Thapliyal 1971). Of particular interest is the shape and size of the areole (Fig. 4). Using this feature for African species, Ross (1973) found a good correlation between species whose seeds had small, horse-shoe shaped, centrally located areoles and spicate inflorescences (Ross 1973). By contrast the species with inflorescences in heads had large areoles whose boundaries followed roughly the outline of the seed. Many Australian species are notable, in contrast to those of Africa, for the bright colour of their funicles (see Ross 1981). Several Central American species have a distinctive sweet pulp around the seeds (Janzen 1969). Fig. 4 shows seeds of the six species covered in this handbook.
|Fig. 4||Outlines of seeds of six species indicating comparative sizes of seeds and varying shapes and sizes of areoles, and an end view of one species indicating various parts. The areole (a) is delimited by a thin line called the pleurogram (p).|
|(A)||A. albida||(B)||A. aneura||(C)||A. caven|
|(D)||A. nilotica||(E)||A. senegal||(F)||A. tortilis|
|(G)||End view of A. caven showing the lens of strophiole (1) the hilum (h) and the micropyle (m).|
There are more than 1200 species of acacia (Simmons 1981). The genus occurs naturally in all continents except Europe and Antarctica. The general nature of its distribution and the distribution of six individual species is shown in Fig. 5. There are 729 species currently recognised in Australia and an estimated 120 taxa as yet undescribed (Maslin 1981a). There are about 115 species in Africa (Ross 1973, 1981). The remainder occur in Asia (including China), and the Americas. In New Zealand the genus occurs only in the fossil record (Ross 1981).
The world distribution of the three subgenera of Acacia is shown in Fig. 6. Species in subgenus Acacia occur mainly in Africa but are also found in Asia, South America and to a limited extent (about 10 species only) in northern Australia (Tindale and Roux 1975; Simmons 1981). Subgenus Aculeiferum is spread widely in the tropics with only one species, A. albizioides (a rainforest liane), occurring in Australia. The majority of Australian species belong to subgenus Heterophyllum. Only about 18 species of this subgenus occur outside Australia and 10 of these are absent from Australia. The extra-Australian group extend from Madagascar, the Mascarenes, and Mauritius to New Guinea, Taiwan, and some Pacific islands to Hawaii (Pedley 1975; Ross 1981; Simmons 1981). Of the 115 African species, 52 belong to subgenus Aculeiferum and 63 to subgenus Acacia (Ross 1981). The distribution of the American species among the different subgenera is not known.
|Fig. 5||The natural distributions of six species of Acacia and the geographical limits (-.-.-) to the distribution of the whole genus.|
|Fig. 6||Approximate global distributions for the three subgenera of Acacia -|
|(A)||Subgenus Acacia (excluding A. farnesiana in Australia)|
|(B)||Subgenus Aculeiferum||(C)||Subgenus Heterophyllum|
It is intriguing to speculate on the origin and subsequent dispersal of such a wide-ranging genus. Biogeographical information is of interest to the seed collector and may influence his choice of collection sites when making provenance collections and his interpretation of the results of provenance trials. Raven and Axelrod (1974) and Beadle (1981a) suggested that the ancestral forms of angiosperms and acacias evolved in the tropical lowland forests of West Gondwanaland. Speculation has extended to dispersal routes within Africa (Ross 1981) and Australia (Beadle 1981a) after the break-up of Gondwanaland into separate continents. Beadle (1981a) suggested that a few species arrived in Australia before separation and that subsequent species evolved and spread southwards from tropical northern Australia. By contrast, Tindale and Roux (1974) suggested that eastern Australia was the centre of origin of the Australian acacias. There is no doubt that there has been a major proliferation of species within Australia with over a half of the Australian species (486 species, 80% of which are endemic) now occurring in the south-western area of Western Australia (Hopper and Maslin 1978).
Some species may have undergone long-distance dispersal. It has been suggested that some e.g. A. farnesiana, could have been transported by sea. Birds (Janzen 1969) are capable of eating and dispersing seeds of others e.g. A. caven. Man has in recent times carried seed of many acacias to other countries (Berg 1977).
Brief notes on the ecology of Australian acacias have been given by Pedley (1978), Johnson and Burrows (1981) and Beadle (1981a, 1981b). Ross (1979, 1981) has provided some general ecological characteristics of African acacias. In Australia, the genus (as typified by A. aneura) is characteristic of the arid and semi-arid climatic regions and is common also in much of the sub-humid region. There are fewer representatives in the humid region and the genus is represented rarely in tropical rainforest or in grassland. In general South African acacias will tolerate hot and arid, hot and wet, cold and arid, but not cold and wet conditions (Ross 1979).
In the arid zone of Australia acacias are dominant mainly on soils containing a high proportion of sand or gravel in the profile as on dunes, sandplains or rocky ridges, which they cover as woodlands or scrubs (Beadle 1981b). In the semi-arid areas they replace eucalypts in selected sites such as on shallow hilltop soils of fine texture. Of particular interest is the ability of acacias to colonise soils of low-fertility. They are able to do this due to their capacity to fix atmospheric nitrogen through their symbiotic association with Rhizobium, a bacterium in the root nodules (Beadle 1981b).
Finally, attention is drawn to the remarkable mutualistic association which exists between ants and some species of subgenus Acacia in Central America and Africa (Janzen 1969). The ants live in swollen, partially hollow, stipular spines. They obtain protein and oil from structures called Beltian bodies (modified leaflet tips) and sugar from greatly enlarged extrafloral nectaries (Ross 1981). The ants in turn protect the plant from leaf-eating insects. The main ant genera are Pseudomyrex in the Americas and Crematogaster in Africa. No such special relationship between acacias and ants exists in Australia although Australia is well endowed with ant genera. The presence of ants may make seed collection hazardous for the collector.