Derivation and dispersal of Acacia (Leguminosae), with particular reference to Australia, and the recognition of Senegalia and Racosperma



The morphology of seedlings, leaves, flowers and inflorescences, anatomy of the pod, the occurrence of extra-floral nectaries, free amino acids of the seeds, flavonoid compounds in heartwoods, cyanogenic compounds and porate, colporate and extraporate pollen, and susceptibility to rusts, all indicate that three genera, Acacia Miller, Senegalia Raf. and Racosperma Martius, should be recognized. These correspond to currently accepted subgenera of Acacia. The size of these more narrowly circumscribed genera is in keeping with the size of genera of other tribes of low diversity in Leguminosae. Acacia and Senegalia arose independently from the Ingeae, with Racosperma being derived from Senegalia. Section Filicinae is more advanced than section Senegalia of Senegalia, and sections Racosperma and Pukhella, both with at least some species with bipinnate foliage, are the most advanced of Racosperma, while the other sections Pleurinervia and Lycopodiifolia have only phyllodinous species. Long-range dispersal of Racosperma from the Australian region has occurred, but the broad pattern of distribution is interpreted in terms of plate tectonics. Racosperma was present in Australia in the late Cretaceous but did not become widespread until the general drying of the continent in the Miocene. The flora of SW Australia has been isolated from the rest of the continent by climatic barriers since the late Tertiary and has a high proportion of endemic species. Barriers to plant migration in the east have operated only intermittently and there is no area comparable in endemism to the southwest.