Oily and Salty Trees

The Annonaceae is another one of those plant families like Acanthaceae that, despite containing a high diversity of speceis, tend to be overlooked because that diversity is mostly tropical. A number of species in the type genus Annona produce commercially significant fruits: custard apples, cherimoyas, soursops and the like. However, these are just a few of the 2400+ species of trees and lianes assigned to this family.

Ylang-ylang flowers Cananga odorata, from here.

Taxonomically, the Annonaceae is well established as distinct, readily recognised by a number of distinctive features. Among these is a characteristic 'cobweb' appearance to the wood structure when seen in cross-section, resulting from prominent rays of xylem connected by narrow cross-bands of parenchyma (Chatrou et al. 2012). Relationships within the family have been much harder to work out, not becoming well established until the advent of the molecular era. Recently, Chatrou et al. (2012) have recognised four subfamilies within the Annonaceae. The majority of species are placed in the subfamilies Annonoideae and Malmeoideae (which together form a clade), but a handful of species are placed in two basal subfamilies: one for the single genus Anaxagorea, and the Ambavioideae. Anaxagorea and the ambavioids differ from the annonoid-malmeoid clade in the structure of their seeds. Seeds of Annonaceae have what is called ruminate endosperm: that is, the surface of the endosperm is not smooth, but divided by wrinkles and grooves (the term 'ruminate' literally means 'chewed'). In Annonoideae and Malmeoideae, the ruminations of the endosperm are shaped like spines or lamellae. In Anaxagorea and the Ambavioideae, the ruminations are irregular in appearance. Molecular analyses place Anaxagorea as the sister taxon to all other Annonaceae.

View into the canopy of a salt-and-oil tree Cleistopholis patens, copyright Marco Schmidt.>

The Ambavioideae, despite not being very diverse, are widespread, with species found in the tropics of Africa, Asia and the Americas. Perhaps the best known ambavioid is the ylang-ylang tree Cananga odorata, native to south-east Asia, whose flowers are used as a source of perfume. Other south-east Asian ambavioids belong to the genera Cyathocalyx, Drepananthus and Mezzettia. The type genus, Ambavia, is native to Madagascar; other ambavioids in the genera Meiocarpidium, Cleistopholis and Lettowianthus are found in continental Africa. Finally, a single genus Tetrameranthus is found in South America. Most species of ambavioid are not systematically economically exploited but a number are locally used as sources of wood. The wood is light and not suitable for structural uses, but can be shaped and finished for utensils and other small items. The West African species Cleistopholis patens, whose Ghanaian name has been translated as 'salt and oil tree' (in reference to the taste of the bark when chewed), provides a fibrous bark that is readily stripped from the tree and is used for such purposes as matting and carrying straps (see here).

REFERENCES

Chatrou, L W., M. D. Pirie, R. H. J. Erkens, T. L. P. Couvreur, K. M. Neubig, J. R. Abbott, J. B. Mols, J. W. Maas, R. M. K. Saunders & M. W. Chase. 2012. A new subfamilial and tribal classification of the pantropical flowering plant family Annonaceae informed by molecular phylogenetics. Botanical Journal of the Linnean Society 169: 5–40.