Tuesday, 23 December 2014

A merry bracteate Christmas


As a special Christmas treat, some colourful and showy flowers that aren't, entirely, flowers.

Around Christmas our thoughts turn to... Poinsettia. The bright red flowers seem to fit perfectly with our festive spirit. It's all a contrived of course, the poinsettia flowering. As I've explained before, it flowers naturally during winter so we manipulate it in glasshouses for this Christmas flush.

(That said, the plant photographed for this post sits on a window sill in the office of our Manager of Plant Sciences, Dr Frank Udovicic. Previously it has lived on other window sills in the National Herbarium of Victoria - outside the designated 'clean areas' where the preserved collections are held - but never subject to special lighting of any kind. So why it flowers in November, when this picture was taken, I'm not sure. Perhaps to do with the artificial lighting in Frank's office?)

In Poinsettia, it is not the flower we enjoy but the leaves around the flower, which we call bracts. They colour up when the plants becomes fertile, clearly have a role in attracting pollinators to the otherwise insignificant flowers, as in this Poinsettia, Euphorbia pulcherrima.


Bracts are special leaves, adapted in size, shape or colour to make the floral display even more attractive. Bougainvillea is another plant with showy bracts around insignificant flowers, in this case three or six bracts around a clutch of three flowers. These fine specimens are from the Xishuangbanna Tropical Flower and Plant Garden in Jinghong, which I visited in 2004.


There are lots of familiar plants with bracts around the infloresence, or group of flowers. Think daisies with their simple flowers (called florets) all grouped together in a head, often surrounded by a row of colourful bractsIf the colourful outer layer is soft and fleshy - for example, the Sunflower, Helianthus annuus - it is more likely made of real petals, each attached to a floret in the outer ring of the flowerhead. But in these pretty Everlasting Daisies (Xerochrysum bracteatum) from The Australian Botanic Garden, Mount Annan (out of Sydney), the 'flowers' are created by a clumps of florets surrounded by pink or white papery bracts. 


Another favourite of mine is Davidia involucra, the Handkerchief Tree. I saw a wonderful specimen of this in late October in Tieva Tara, the private garden of John and Judith Brand at Mount Macedon. Like in the daisies and Bougainvillea, the white 'handkerchiefs' are at the base of a cluster of flowers.


And then you have hellebores with their petals really being sepals, the layer usually outside the colourful petal layer. More on that in a previous post where I've given them enough photographic exposure!

Clearly the bracts, or specialised flowers, in these plants are doing what petals do in most single flowers: attracting pollinators. Given that flower parts are modified leaves anyway it's not much of a stretch to understand why sometimes evolution favours big showy leaves that just happen to be nearby instead of petals (or in the case of daisies, flowers clumped together with different structures and roles.) And we can be thankful for that. Say Merry Christmas this year with a bunch of bracts!

Credit: Thanks Frank Udovicic for the suggested topic of this post and Gerhard Prenner for reminding me that the coloured parts of a sunflower head are in fact real petals, not bracts (resulting in an update to this post).

4 comments:

Anonymous said...

Although not usually still flowering at Christmas time, the NSW Waratah is a favourite showy-bract-er of mine :)

Tim Entwisle said...

True, that almost sneaks in (perhaps high up on the mountains it might flower into December) and a great bracteate flower-head! Tim

Gerhard Prenner said...

Ups, in sunflowers and all other Compositae the showy part is NOT a bract, but the petals of showy ray flowers.

All the best,
Gerhard

Tim Entwisle said...

Very good point Gerard! I got over enthusiastic about my toppic... Will edit. Tim