Melastomes and Pals

Princess flower Tibouchina heteromalla, copyright João Medeiros.

For most botanists currently working on flowering plants, the default taxonomic framework for their studies is the classification published by the Angiosperm Phylogeny Group. This is not the only classification for angiosperms proposed in recent years, but it is the most widely recognised, and it is the one that all its competitors are compared to. If there is one major deficiency of the Angiosperm Phylogeny Group classification, it is that it eschews the use of formal categories between 'orders' (which it tends to define broadly) and 'families'. As such, there are a number of well-supported clades that require one to turn to alternative classifications for suitable names.

Crypteronia paniculata, copyright Tony Rodd.

The Melastomatineae, as recognised by Reveal (2012), for instance, is a clade of mostly tropical and subtropical plants within the Myrtales commonly recovered by molecular phylogenetic analyses. Most of its members are included in the pantropical family Melastomataceae, but it also includes three much smaller and more localised families: the southeast Asian Crypteroniaceae, the western Neotropical Alzatea and the African Penaeaceae. Some authors also divide the Melastomataceae into two families Melastomataceae and Memecylaceae, but as the two groups are universally accepted as sister clades this is purely a matter of taste. Morphological characters uniting the families are few (see the Angiosperm Phylogeny Website). Many accumulate aluminium in the leaves, to the extent that the leaves of the small tree Memecylon edule were used in India as a mordant for fixing dyes to cloth. The flowers lack nectaries, and when nectar is produced in the Melastomataceae it exudes from locations such as the anthers or the corolla. Some Olisbeoideae (the Memecylaceae of other classifications) produce oil from glands on the anthers that is collected by pollinators in lieu of nectar. In the South American genus Axinaea, the anthers have a sugary appendage that is eaten by tanagers; as they attack it, a puff of pollen dusts their head. The anthers of Melastomataceae are often distinctly coloured from the rest of the flower, and arranged in a distinctive seried row on one side of the flower. The Melastomatoideae (i.e. Melastomataceae sensu stricto) also have distinctive leaves, with three or more strong longitudinal veins arising from the leaf base and connected by cross-veins.

Mountain hard pear Olinia emarginata, copyright H. Robertson. Olinia leaves smell of almonds when crushed, due to the presence of a cyanogenic compound.

Though Melastomataceae are often significant members of the tropical forest understorey, they tend not to have much direct economic significance for humans. Some, such as the princess flowers or glory trees of the genus Tibouchina, are grown as ornamentals. The hard pear Olinia ventosa of the Penaeaceae (no, I don't know why it's called that) is grown as a shade tree in South Africa. A few species of Melastomataceae are significant invasive weeds in warmer regions, including such luminaries as the Straits rhododendron Melastoma malabathricum and the evocatively-named Koster's curse Clidemia hirta. The velvet tree Miconia calvescens has earned itself the label of the 'purple plague' in Hawaii, where it over-runs native forest.

REFERENCE

Reveal, J. L. 2012. An outline of a classification scheme for extant flowering plants. Phytoneuron 37: 1–221.