Melaleuca and Callistemon are two of the best known Australian members of the Myrtle family. All of the Callistemons and many of the Melaleucas have flowers arranged in "Bottlebrush" fashion clustered together in cylindrically shaped spikes. But only Callistemons are commonly called "Bottlebrushes" ; Melaleucas are usually called "Paperbarks" or "Honey Myrtles" or sometimes "Tea Trees" although that name is more appropriate to another related genus, Leptospermum.
So what makes Melaleuca and Callistemon different?
The main difference has to do with the way in which the stamens (the male parts of the flowers) join to the floral tube. Figure 1 shows a cross section of a single Callistemon flower. This shows that each stamen joins the floral tube independently of every other stamen (this is referred to as the stamens being "free").
In Figure 2, however, which is a cross section of a single Melaleuca flower, the stamens are joined together into groups with each group joining the floral tube as a unit (this is referred to as the stamens being "united"). Each Melaleuca flower contains five of these groups or "staminal bundles".
In most cases this difference can be easily seen by examining the flowers with the naked eye. However, the problem with the current classification on the basis of the arrangement of the stamens is that this supposed difference is not clear cut and Callistemon tends to merge into Melaleuca rather than being unambiguously distinct. The well known Callistemon viminalis is one that has often been discussed as not easily fitting the accepted definition of Callistemon.
Over the years there have been suggestions that the differences between species of the two genera are not sufficient to warrant them being kept distinct. A paper by Lyn Craven of the Australian National Herbarium (Novon 16 468-475; December 2006 "New Combinations in Melaleuca for Australian Species of Callistemon (Myrtaceae)") argues that the differences between the two genera are insufficient to warrant them being retained separately and that they should be combined. As Melaleuca has precedence, adoption of Craven's work would transfer all species of Callistemon into Melaleuca but at this stage the re-classification has not been taken up by the Council of Heads of Australasian Herbaria (although the Queensland herbarium has accepted the change).
Craven's work is the basis of a forthcoming volume of the "Flora of Australia", so the possibilitity remains that Callistemon will eventually be subsumed into Melaleuca.
While all Callistemons have their flowers arranged in a "bottlebrush" shape the inflorescences of Melaleuca may also have a globular or irregular shape. It should also be remembered that there are other genera in the myrtle family which may have free or united stamens combined with "bottlebrush" flowers. Botany was never meant to be easy! The other common genus with free stamens is Kunzea which differs from Callistemon in having seed capsules which are not woody and which shed seed annually. Apart from Melaleuca there are several genera which also have united stamens. These include Calothamnus, Beaufortia, Eremaea and Regelia. The distinction between these and Melaleuca requires examination of the arrangement of the anthers and other floral structures.