Wow! what a weird, spectacular mutation!
That was my first thought when I noticed an amazing expansion and deformation of the growing tip of a young casuarina at the top of Red Hill in Canberra.
I had just begun the December 2010 quarterly survey of Red Hill as part of the Canberra Ornithologists Group Woodland Bird Survey when I came across it on the side of the track. And luckily I had my camera with me on that occasion. I hadn’t seen anything quite like it before; nor since.
|The fasciated growing tip of a casuarina (Allocasuarina verticillata) in Canberra.|
|A less extreme (or earlier in development?) example of fasciation on the same casuarina.|
For a size estimate, the ant (a common Sugar Ant Camponotus consobrinus) is about 15 mm long.
These exceptional growths are the result of a process called fasciation which, although rare, is common enough that the condition is well known in a wide range of plants. Indeed some plant cultivars have been developed, especially of certain cacti, daisies, and the cockscomb (Celosia argentea), specifically for this ‘feature’ (see for example http://wimastergardener.org/?q=fasciation ).
The word comes from the Latin fascia meaning ‘a band’, and alludes to the typical broad flat ribbon-like structures that commonly result. Although this broad banding is perhaps the most common pattern, fasciation can also manifest as radial or adventitious additional flowers growing out of the main flower, sometimes referred to as ‘hen and chicks’. I wish I’d known this earlier this year when I had several orange daisies in the front garden with this condition – I thought at the time it was just how they grew, but since then they’ve all been quite normal!
In simple terms, fasciation results from the abnormal growth of the meristem tissue (i.e. the growing tip of the plant, be it stems, flowers, roots etc). The underlying reason for this may be physical damage (including frost damage), insect damage, bacterial, viral or fungal infections, hormonal imbalances, or genetic mutation.
It seems that casuarinas, and particularly the Drooping She-oak Allocasuarina verticillata, may be prone to such fasciation – see for example http://www2b.abc.net.au/science/scribblygum-old/posts/topic9591.shtm , and especially http://medlarcomfits.blogspot.com.au/2007/08/more-views-of-that-casuarina.html for some incredible photos.
In fact, Casuarinas seem to be prone to quite a range of growth abnormalities other than fasciation. Below are some examples of a range of ‘defects’ I’ve noticed in several different casuarina species over the past couple of years. These generally have been small and unobtrusive, and presumably result from some similar perturbation to the growing tip of the plant (or might they be insect-induced galls?).
|Abnormal growth in a casuarina, Harrington NSW, July 2012.|
|Abnormal shoot growth (or gall?) in a casuarina, Harrington NSW, July 2012.|
|Abnormal shoot growth (or gall?) in a casuarina, Cessnock NSW, February 2014.|
Another well-known growth abnormality in trees is what is known as ‘witches’ brooms’. These show up as a dense cluster of shoots arising from the one spot, often resembling a clump of mistletoe. Again, the causes for these growths may be varied, and may relate to infection by rust and other fungi, viruses, bacteria, mite or insect damage, or even mistletoes.
So to come full circle, on the latest Red Hill woodland survey, done just last Friday morning, I photographed the example below. Clearly, it had been growing in this tree (again an Allocasuarina verticillata) for quite a while – the witches’ broom is a bit over a metre in length – but, despite hanging right over the main access road, this was the first time I had really noticed it. Perhaps because I had this blog post in mind!
|A pendulous witches' broom in an Allocasuarina verticillata on Red Hill, Canberra, ACT, September 2014.|
|The witches' broom is a dense clustering of twisted, shortened, casuarina needles.|
So I’ll keep my eye out for further growth abnormalities in casuarinas (and other plants). And while I’m at it, perhaps I should get myself a copy of a recent book by Rosalind Blanche that sounds really interesting called Life in a Gall: the Biology and Ecology of Insects that live in plant galls. Galls really are another story entirely…