It is well-known that the rainforest flora of the Australian tropics has a strong South East Asian element.
Various genera of laurels (Lauraceae), mahoganies (Meliaceae), tuckeroos (Sapindaceae) all appear to have origins in Southeast Asia and migrated into Australian from the north. Could there be genera of tropical rainforest trees that embody Gondwana, the memory of times when the great continents of the south were joined?
The Macadamia family (Proteaceae) might have had a good representation of a Gondwanan family, but the genera are species-poor. The genus of the lily pillies (Syzygium) is of course a prime example of a Gondwanan genus but next to this, the title of a pre-eminently Gondwanan genus goes to…Elaeocarpus!
With over twenty species, Elaeocarpus must be one of the largest Gondwanan genus in the Australian rainforest flora, and contain trees of immense cultural and ecological significance. A scientific interest in Elaeocarpus have seen a spike in the recent decades.
The director of Australian Tropical Herbarium (ATH), Prof. Darren Crayn, is a specialist in the family, and he and his students and co-researchers have published a number of papers on the family and is currently running numerous projects on Elaeocarps outside of Australia.
Elaeocarps are indeed an enchanting group of plants.
A number of features in combination can be very useful in identifying the genus Elaeocarpus when no flowers are available.
1. Members of Elaeocarpus are trees
2. Alternate leaves, arranged spirally around the stem
3. Leaves are often crowded at the tips of branches
4. The outline of the leaves or the leaf margin is typically toothed
5. Stipules or stipular scars are present where the leaf stalk meets the branch.
6. Swellings on the leaf stalk where it meets both the leaf base and the twig. This is called a double pulvinus.
7. Leaves often wither deep red or orange
8. Many species have foveoles or domatia (caverns or pits) where the nerve radiates from the midrib (best seen on the leaf underside)
Thou shalt know thy Elaeocarps when they flowers and fruit
The flowers often have frilled petals, are usually white, yellow, cream to pink, on a spike that arises from a leaf axil, and hang downwards.
The fruits are usually fleshy (often bluish) with a hard, and often ornamented “stone” Some species have fruits that mature deep purple, brown or greyish blue.
Elaeocarpus always have somewhat fleshy-gritty fruits with a stone. The stone often also reveals a rough surface when the flesh rots off.
In many tropical regions in Asia and also in Australia, the hard seeds have religious or spiritual significance, and are often used to make jewelry.
Just to make life interesting (or confusing), many arborescent members of the Spurge family (Euphorbiaceae) have these combination of features above as well. Same goes to the family of the Malvaceae.
However, many of the Euphorbiaceae have some kind of sap or raised glands on their leaves, while Malvaceae often have three-nerved leaves, or with star-like hairs on the leaf surface
Ultimately, flowers or fruits really help with distinguishing Elaeocarpus from genera of Euphorbs and Malvaceae as the fruits of the latter two families are typically capsules which open up along suture lines.
Crayn, DM, M Rossetto, DJ Maynard (2006) Molecular phylogeny and dating reveals an Oligo-Miocene radiation of dry-adapted shrubs (Tremandraceae) from rainforest tree progenitors (Elaeocarpaceae) in Australia. American Journal of Botany 93: 1168-1182.