Bottle Brush. The genus Callistemon was first formally described in 1814 by Robert Brown. In his description he noted that the genus includes “those species of Metrosideros that have inflorescence similar to that of Melaleuca, and distinct elongated filaments.” Carl Linnaeus had described the genus Melaleuca in 1767 and in 1867, George Bentham brought all the Metrosideros species into Melaleuca. Bentham described melaleucas as having stamens united in bundles opposite the (five) petals.
In his 1864 description of Callistemon salignus in Fragmenta phytographiae Australiae, Ferdinand von Mueller noted that the difference between the genera was “entirely artificial” (“omnino artificiale”). George Bentham also noted in Flora Australiensis that Callistemon “passes gradually into Melaleuca, with which F. Mueller proposes to unite it.” In 1876, Henri Ernest Baillon proposed in Histoire des Plantes that Callistemon, as well as Calothamnus and Lamarchea be merged into Melaleuca.
Nevertheless, most authors had preserved the distinction between the two genera Callistemon and Melaleuca until 1998. In that year, in recognition of the fact that the callistemons and melaleucas on New Caledonia were clearly related, Lyndley Craven and J.W. Dawson transferred the callistemons on that island to Melaleuca, even though some (e.g. Melaleuca pancheri) do not have stamens fused in 5 groups.
On the basis of DNA evidence, in 2006 and 2009 Craven moved all but four callistemons to melaleuca. Those four were Callistemon forresterae, Callistemon genofluvialis, Callistemon kenmorrisonii and Callistemon nyallingensis which were regarded as being hybrids.
The new description of Melaleuca has been accepted by some herbaria but not all. For example, the Queensland Herbarium accepts Melaleuca flammea (synonym Callistemon acuminatus) but the New South Wales Herbarium accepts Callistemon acuminatus. In 2012, Frank Udovicic and Roger Spencer transferred the newly described species of melaleuca with separate stamens (e.g. Melaleuca megalongensis and Melaleuca wimmerensis) to Callistemon (hence Callistemon megalongensis and Callistemon wimmerensis). Their argument is that using the DNA evidence is premature. They further argue that if all the genera Beaufortia, Callistemon, Calothamnus, Conothamnus, Eremaea, Melaleuca, Phymatocarpus were combined (as Craven has suggested), then there would be no characteristics that would define the group.