Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Tuesday, September 02, 2008

More on Wattles

Here is a little more about Wattle "phyllodes" - those flat green things which pass for "leaves" on most of the Australian Wattle species.Firstly, all Wattles start out with true leaves, as they germinate. The local Blackwood Wattles (Acacia melanoxylon) which live a very long time, and grow into a large trees, keep their true leaves until the seedlings are about a metre high. Most Wattles with phyllodes convert their true leaves (into phyllodes) when they have developed only a inch or there abouts. I have this personal theory that Blackwoods, which are wet country wattles, have less pressure on them to convert to the more drought-adaptive form, and so are in no hurry to develop phyllodes.Here is the conversion process occurring. A pair of ferny leaves are being subsumed by a nice flat phyllode developing underneath them. This shows why "phyllodes" are described by botanists as "flattened stems". In fact the adaptation is part of a dry country survival technique, because leaves have far fewer "stomata" or "breathing holes", and thus a plant which has only stems (phyllodes) loses far less water by transpiration. Although in this example, true leaves developed first, and then the stem supporting the true leaves has gone flat, once the plant reaches a certain stage in its development, it goes directly to the stage of producing "phyllodes". That is what is seen in this image. The stem on the leaf is "converting" from true leaf to phyllode. The young stem in the centre, however, is developing directly as a phyllode. Once this stage is reached, all future "leaves" will develop as phyllodes. So, it is only at the young plant stage, that true leaves are formed which convert to the phyllode. Once that stage is reached, all future "leaves" will develop as "phyllodes".This plant is nearing the complete transformation, but it is about a metre tall already. Future development will be entirely via phyllode growth.This is a local Blackwood in flower. The colour is very pale, and the leaves (sorry, phyllodes) are quite a pale lime yellow. It does have a pleasant perfume, but not as sweet as some of the smaller, shrubby Wattles.This is a medium-sized Blackwood Wattle growing in the paddock below my house. It is the dense, rounded tree in the centre-left of the image. To the right of the image, you can see several smaller trees which are less "dense" in form, and which are greener in colour. They are Sassafras trees (Doryphora sassafras). In this area, really old Blackwood Wattles grow into round-headed trees, but with a more open structure than one would assume from this dense tree shape. Their trunks often reach more than a metre in diameter, and the old trees reach about 25 metres in height. I am advised that the Tasmanian form of the Blackwood, which is a famous furniture timber tree, is taller and straighter than the local form.


Mosura said...

I've seen some very broad spreading Blackwoods here in more open areas but in the steep wet gullies they seem to grow straight up.

Denis Wilson said...

Hi Mosura,
I am glad to read your comments.
Some people have planted Blackwoods here without considering their "provenance" and they look different from our trees. It seems most of the Blackwoods in cultivation come from seed collected in Tassie. So we get your tall ones.
Our old natural ones develop a very open crown, when they eventually mature. The ones in my paddock are probably what is regarded as "regrowth". The flatter parts of my paddock were cleared for potatoes, back in the early 20th century. Really old trees exist in the steep gullies, which were never cleared.

Junior Lepid said...

I have several Blackwoods here and they are just starting to flower, Denis. They are one of my favourite trees. Thank you for your informative post.