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.

Dighloti

Just a brief one today. The above picture, from Assam Plants.com, shows leaves of the dighloti, Litsea salicifolia, a tree of the laurel family Lauraceae found in sparse valley forests of southern Asia from India to Vietnam (Li et al. 2008). Litsea salicifolia is fairly variable in appearance, but general characters include long elliptic, alternate, leathery and evergreen leaves, glabrous branchlets and petioles, 4-6-flowered umbels and small oblong fruit.

Dighloti is cultivated in Assam, with two main uses. It is supposed to be a mosquito repellent, and this has been corroborated by laboratory studies (Phukan & Kalita 2005). It is also used as a secondary food source for the muga silkworm Antheraea assamensis (Bindroo et al. 2006). Muga silk, one of the so-called 'wild silks' produced from caterpillars other than the domesticated silkworm Bombyx mori, is raised primarily on two other Lauraceae species, the som Persea bombycina and the soalu Litsea polyantha. However, dighloti bushes are also grown in som plantations as a supplementary food for young or under-developed silkworms. Muga silk has a naturally golden-brown coloration (photo below from here).

REFERENCES

Bindroo, B. B., N. T. Singh, A. K. Sahu & R. Chakravorty. 2006. Muga silkworm host plants. Indian Silk, April 2006: 13-17.

Li X., Li J., Huang P., Wei F., Cui H. & H. van der Werff. 2008. Lauraceae. In: Flora of China Editorial Committee (eds) Flora of China vol. 7. Menispermaceae through Capparaceae. Science Press: Beijing, and Missouri Botanical Garden Press: St. Louis.

Phukan, S., & M. C. Kalita. 2005. Phytopesticidal and repellent efficacy of Litsea salicifolia (Lauraceae) against Aedes aegypti and Culex quinquefasciatus. Indian Journal of Experimental Biology 43: 472-474.

Taxon of the Week: Spices of Gondwana

The Canellales are a small order of flowering trees and shrubs, most of which are found in the southern continents though some species reach as far north as Florida and Indonesia. Two families are included in Canellales, the Canellaceae and Winteraceae. Canellaceae is found in the Ethiopian and Neotropical biozones, while Winteraceae is Australasian, Malesian and Neotropical (the image at left is of Drimys winteri [Winteraceae], and comes from Flora Chilena).

Canellales are members of the "magnoliids" - the paraphyletic assemblage of basal flowering plants that aren't eudicotyledons, aren't monocotyledons, and are still proving difficult to place phylogenetically. Recent analyses have all agreed that the so-called "ANITA grade" includes the basalmost flowering plants yet in existence (the name ANITA derives from the taxa included in this group: Amborella-Nymphaeales-Illiciaceae-Trimeniaceae-Austrobaileya). After that, it all goes a bit custard-shaped. As well as the well-established clades of eudicots and monocots, we have Chloranthaceae, Magnoliales, Laurales, Piperales, Aristolochiales, Canellales and Ceratophyllum (some authors, such as Thorne [2000], have included some or all of these orders in an expanded Magnoliales), with almost every combination imaginable of these nine clades turning up somewhere. Nevertheless, there is a reasonable amount of molecular support for a true "magnoliid" clade including everything listed above from Magnoliales to Canellales, with Canellales probably sister to Piperales (including Aristolochiales - Zanis et al., 2002; Angiosperm Phylogeny Group, 2003). Though an association between Canellaceae and Winteraceae was first recognised on molecular grounds only, morphological characters have since been identified in support of the Canellales.


Flowers of Drimys winteri - image from Plant of the Week

Previously, Winteraceae had been regarded as very primitive flowering plants because of their lack of vessels, specialised xylem cells for the transport of water that are characteristic of flowering plants (xylem is the central part of the plant stem that transports most of the plants water through no-longer-living cells). In vessels, the axial ends of the deceased xylem cells has become perforated or open, so instead of separate cells the plant has continuous tubes allowing for much easier water transport. The only other angiosperms to lack vessels are Amborella (possibly the basalmost of all living flowering plants) and the small eudicot order Trochodendrales.

However, the modern position for Winteraceae implies that its vessel-less condition is not primitive but derived from vesselled ancestors. What could have led the ancestors of the Winteraceae to abandon the seeming advantages of a vessel system? Field et al. (2002) compared the effectiveness of water transport in the vessel-less Winteraceae and the vessel-bearing Canellaceae. While Canellaceae had much more efficient water transport than Winteraceae, freezing and thawing had a much greater impact on water transport in Canellaceae than Winteraceae, leading Field et al. to suggest that the lack of vessels in Winteraceae might have been an adaptation to survive freezing cycles in temperate Gondwana. This seems to be the most likely explanation yet available, but as always with science, a whole host of other questions arise - if vessels are so problematic for cold-climate taxa, why have so few other angiosperms lost them? Field et al. do suggest that greater differences between vessels and standard xylem cells in more derived taxa may have put greater constraints on their loss, but I feel this is still very much an open question.

One feature that Canellaceae and Winteraceae share in common is the notable production of aromatic oils by members of both families. Canella alba (shown here in an illustration from Wikipedia) was transported from South America to Europe as "white cinnamon", and its bark is used in much the same way as cinnamon. The leaves of horopito (Pseudowintera colorata), a small tree found in New Zealand, were used as a condiment, as are those of Tasmanian pepper (Tasmannia lanceolata). I can personally vouch for the edibility of horopito - it has a strong peppery taste, similar to nasturtium but far stronger.

REFERENCES

Angiosperm Phylogeny Group. 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436.

Field, T. S., T. Brodribb & N. M. Holbrook. 2002. Hardly a relict: freezing and the evolution of vesselless wood in Winteraceae. Evolution 56 (3): 464-478.

Thorne, R. F. 2000. The classification and geography of the flowering plants: Dicotyledons of the class Angiospermae (subclasses Magnoliidae, Ranunculidae, Caryophyllidae, Dilleniidae, Rosidae, Asteridae and Lamiidae). Botanical Review 66: 441-647.

Zanis, M., D. E. Soltis, P. S. Soltis, S. Mathews & M. J. Donoghue. 2002. The root of the angiosperms revisited. Proceedings of the National Academy of Sciences of the USA 99: 6848-6853.