2 MIMOSACEAE Acacia 
numerous useful products including timber, tannin, perfume, food and animal fodder. [New, 1984; Pinyopusarerk, 
1990; House & Harwood, 1992]. Numerous species have also been recorded with a variety of ethnobotanical 
uses, including food, timber, fish poisons and medicine [Miller er al., 1993; Smith er al., 1993]. 
In a genus as large as Acacia it is not unexpected that one would find a great deal of morphological diversity 
and differences in opinion concerning the classification of the group. The following notes are intended as a 
brief introduction to the extensive literature available on Acacia. The morphological characters important in 
using the keys are: 
Stipules. In a few species the stipules are modified into spines and therefore present as a pair subtending 
each leaf. However, many of the north Australian taxa that produce stipular spines do so only in the juvenile 
growth stage and spines are not consistently found on the adult plants. In most species of Acacia the stipules 
are minute and often falling early. 
Leaves. The most characteristic feature of the majority of Australian Acacias is the production of a 
modified leaf known as a phyllode (Fig. 1). Typically the phyllode is an expanded and flattened petiole. Other 
species of Acacia have bipinnate leaves (Fig. 1). Seedlings of all species in the genus initially have compound 
leaves and in most groups the first leaves are once-pinnate. A developmental sequence from seedling to adult 
foliage can be found at the earliest growth stages or on reversion shoots. The phyllodes can also be highly 
reduced. Arrangement of the phyllodes is typically alternate but fasciculate, whorled or rarely distichous 
arrangements are found. 
Venation. The venation pattern is a highly useful character in identification. Phyllodes may have a single 
longitudinal nerve and in broader phyllodes be markedly penniveined (Fig. 1) or have several to numerous 
longitudinal nerves. For these plurinerved species the presence or absence of anastomosing veins is 
important. 
Glands. Extrafloral nectaries (“glands”) occur on both true leaves and phyllodes. The function of these glands 
in the Australian species is not well understood, although interactions with ants has been suggested (New, 1984). 
However, the number, form and position of these glands is of some taxonomic value. 
Inflorescence. Although the individual flowers are of limited value in identification, the arrangement of the 
flowers in the inflorescence has been attributed considerable significance. The flowers of most species are 
arranged in either cylindrical spikes or globular heads, although there are some species which show an 
intermediate state of slightly elongated or “obloid” heads. The inflorescences themselves can be arranged singly 
or in pairs in the axils or grouped into racemes which may be variously compound. 
Pods. The pods show a great diversity and although often diagnostic for identification are often ignored by 
collectors. The shape, texture, surface features, dehiscence mechanism and orientation of seeds in the pod are 
important features. 
Classification 
Acacia, as traditionally recognised, is now generally accepted to comprise at least 2 or possibly 3 groups. 
The ranking of these groups has been the subject of some controversy with major implications for Australia. If 
Acacia is split into a number of genera then the species that remain in Acacia are mostly non-Australian. The 
predominantly Australian phyllodinous species would be placed in another genus. Such a classification 
was that of Pedley (1986) in which the majority of Australian species were placed in Racosperma (DC.) (CF 
Martius. This classification has not been widely accepted, partly due to the magnitude of the change but also 
partly due to uncertainties in the number of generic groups to be recognised, appropriate ranking of the groups 
and the nomenclatural validity of some of the available generic names. For detailed discussion of the 
arguments for and against splitting of Acacia see Pedley (1986, 1989) and Maslin (1989). 
For the purposes of this treatment and to enable some discussion of the subgeneric groups an earlier 
classification by Pedley (1978) is followed. This classification divides Acacia into the following 3 subgenera: 
Subgenus Acacia - Trees with stipular spines and bipinnate foliage. Best represented and known as the 
flat-topped trees of Africa. Poorly represented in Australia but with a number of species in the Top End. In the 
DR are *A. catechu var. sundra, A. farnesiana, A. pachyphloia, A. pallidifolia and A. ‘Douglas River’. 
Subgenus Aculeiferum - Prickly woody vines and small trees which are widespread in the tropics. There is 
only 1 species in Australia, a woody vine found on Cape York. 
