Tuesday, 12 May 2015

Strange staminal swellings on Gold Medallion Tree

The Gold Medallion Tree, Cassia leptophylla, has large clusters of (appropriately enough) big, yellow flowers. These medallions were particularly dramatic in January this year, weighing down a specimen just opposite the Rose Pavillion in Melbourne's Royal Botanic Garden.

In California the Gold Medallion Tree is celebrated as one of the few 'tropical trees' that will flourish in LA and beyond: "Although it shares the large, showy flowers and tropical-looking foliage of its cousins, gold medallion tree is much hardier to frost, dry heat, and drought than the other two (Cassia fistula and Cassia nealiae) — and that’s what makes it special in California". Sounds like a good plant for gardens in southern Australia.

Take another look at that flower at the top of the post. What caught my eye - or to be fair Neville Walsh's then mine - was the capsule-shaped swelling on the three larger filaments (the stalks of the male part of the flower, the stamen, terminating in the pollen-bearing anther). It's true we were attracted by the in-your-face flowers to start with, but up close these golden tabules are rather odd.

Golden Medallion Tree is the only native American species with this kind of weird swelling on the filaments, although Cassia ferruginea, also from Brazil, has filaments with a gradually flattened expansion towards the end. Elsewhere, the pink- or purple-flowered Cassia javanica from central and southern Asia has a very similar swelling but this is thought to have arisen independently (that is, through what is called parallel evolution - similar pressures, perhaps, producing similar looking adaptations).

But why produce these swellings in the first place? Dunno. They don't seem to produce anything like nectar or act in an obvious way to make the flower more attractive to pollinating insect or bird. Perhaps they give a foothold to any visitor trying to get to the more lucrative female bits of the flower?

Gold Medallion Tree is said to have a 'massive four-angled pod', up to 70 cm long, but I gather they are rarely formed. There are definitely none on this particular tree, now four months after flowering. The leaves are still there, though, attractive but less mysterious.

Although...why so many (a dozen or so) pairs of 'leaflets', and why does the leaf end in a pair rather than a single leaflet? The terminal pair makes the botanical description of the leaf just a little simpler, being paripinnate (a equal number of pinnules or leaflets) rather than imparipinnate (an odd number of pinnules and leaflets, implying a single terminal one).

The Gleditsia japonica next to it, also paripinnate, has lost all its leaves. In full leaf they are difficult to tell apart (in the picture below Cassia, in flower, is on the left, Gleditsia on the right). In fact in it was a question of identity that drew Neville Walsh to the tree in January. Someone had commented on these flowers being rather odd for a Gleditsia, which tend to have small greenish flowers (as in the Honey Locust), thinking a branch of one was the other. The Gleditsia has the last laugh, though, with a number of fruit pods evident yesterday.

Note: There was supplementary question once the identity of the plant was sorted, as to whether this tree, a Cassia, might be an alternative source of cinnamon. The answer was no. Neville Walsh explained that cassia bark, sometimes used as a substitute for the bark of Cinnamomum verum ('true' cinnamon) and reputedly able to hold its flavour better during cooking (although reputedly more bitter and of inferior flavour), comes from Cinnamomum cassia

And for some better pictures of the Gold Medallion Tree in Melbourne's Royal Botanic Gardens, see the January 2013 blog post by Nick V. at Melbourne Fresh Daily.

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