Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Sunday, February 05, 2006

Raging hormones - Wattle I do?

Raging hormones are not the sole prerogative of teenagers and menopausal women. We see the effect of hormones in animals, of course. Anyone who has ever shared a house with a bitch "in heat" will be familiar with that. We understand animals fairly well, anyway, for we are animals ourselves.

But what about plants - are they subject to hormonal flushes?

Have a look at the photo of the leaves taken from a young Blackwood (Acacia melanoxylon) sapling. It is showing the confusion about what it really is, as clearly as a young boy whose voice is breaking, or a girl child with budding breasts.

Blackwoods belong to a class of Wattles which have "phyllodes" instead of true leaves. However, at their juvenile stage, they have real leaves - "ferny leaves" is a term commonly applied to them. Plants breathe through their leaves. They absorb and release gases, and transpire moisture, through tiny pores in their leaves. In a dry continent, it can be an advantage to reduce moisture loss (through leaves) by having fewer leaves (or none at all). Many Wattles have developed the ability to transform their stems into flattened structures (phyllodes) which perform the functions of true leaves (especially photosynthesis).

This transformation can be seen in the photo - the ferny leaf is supported by a flattened stem - a phyllode in the making. Other phyllodes are forming, but without any of the ferny "true leaf" structure at all. They will continue to grow as mature stage phyllodes.

As plant stems (and hence phyllodes) have fewer tiny pores than true leaves do, presumably this is an evolutionary advantage for Wattles with phyllodes - as adapatation to life in a dry climate. Plants with phyllodes will be better protected against moisture loss than plants with true leaves. So in a dry climate (I am talking about the continent of Australia, not Robertson specifically) that is a system with obvious benefits to these plants.


But what about the hormones? Well, after they have developed from the seed, these Wattles grow true (ferny) leaves, until they are about 2 years old, sometimes more. Then the stems underneath the true leaves start to flatten out, until the phyllode takes over the function of the true leaf, and renders the ferny part of the leaf redundant. The ferny leaves shrivel and fall away. Eventually, the whole plant then will adapt to the presence of the phyllodes, and it will stop producing true leaves.

How does the plant know what is going on in its leaves? The answer is a system of chemical messengers - which we call hormones.

The photo above shows a single leaf in its transitional state - part ferny"true leaf" and part flattened stem "phyllode". The photo also shows phyllodes developing without any ferny parts at all. So even the leaves of this tree "know" (and can show us) how sexually mature the plant is.

Clearly this plant is "in transition" to sexual maturity. It is as much a victim of its own raging hormones as a teenage boy whose voice keeps breaking - one moment deep, next high and squeaky. It can be described as experiencing "hormonal flushes" - the poor thing is a bit confused.

But it will settle down, grow only phyllodes, and next year it will flower as a mature Blackwood Wattle. In years to come it will grow into a tree like the 2 largest trees in this photo - on the left and in the centre.

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