Thursday, August 13, 2009

Wattles - Evolution in Action

Wattles - Evolution in Action
Australian acacias had a problem. Their leaves were costing them too much. Plants lose water through their leaves, and as the continent dried out they were being pushed to the wall. A few million years ago, as the ice age advanced and climate change cooled and dried their home ground, the situation became serious. Something had to be done - but the trouble is that plants need those green leaves, to do the essential job of photosynthesis. They capture carbon dioxide and remove the oxygen from it, thus providing the plant with its “food” - a steady supply of carbon.

Typical wattle leaves.
The common local Cinnamon Greenwattle

Acacia irrorata

What the wattles needed was to develop a new way of photosynthesising, so they could get rid of those water-wasting leaves.
Changing the nature of the leaf-stems - the petioles - was the answer. In many species of wattle they became longer, broader, flatter, and full of green chlorophyll, the substance which is essential for photosynthesis. The wattles could then give up making leaves, and hey presto, here was a whole new suite of drought-hardy plants!

Leaf in Transition.
Oleander-leafed Wattle

Acacia neriifolia

Not all wattle species have done it. Some still have leaves, just as their ancestors have always done. We sometimes see them described as the “ferny-leafed” kind of wattle. The others are called “long-leafed”. In reality, they are completely leafless, but their modified leaf-stems, (which botanists call phyllodes), look so like leaves that the difference can be hard to detect.

Growing up. Three leaf-types on one branchlet, Acacia neriifolia

We can still see the evidence of their evolutionary history, though, in young plants. Wattles of the “long-leafed” type begin life with ferny leaves. Then, as they grow, leaves with exaggerated stems appear. The tendency to produce leaves soon disappears, and older plants have nothing but phyllodes.
The toughest, most drought-hardy wattles produce just one leaf, then get down to the leaf-free life as soon as possible. Others, such as Acacia neriifolia, go on producing the half-and-half model until they are more than shoulder-height.
They are a common local plant, so we can see plenty of examples in our local bush. If you want to see some, look for seedlings among the adults which are
looking so spectacular along the top of the range at the moment.

Some wattles evolve even further. Their phyllodes hang vertically, so as to present the least possible surface area to the sun - and their colour has changes to silvery grey, to reflect sunlight. These very tough plants waste little of their lives producing leaf - a single token leaf at birth is all they bother with, before getting on with producing phyllodes. This brigalow (Acacia harpophylla) at right is a typical example of these most drought-hardy wattles.

No comments: