Tuesday, 31 January 2017

Such beautifully draped foliage but no leaves


Most wattles (Acacia) don't have leaves. Instead, the leaf stalk expands into a flat blade - which to all intents and purposes acts like a leaf - and any juvenile feather-like leaves at the end of the stalk are lost. We call this expanded stalk a phyllode.

Take a look at a local Blackwood, Golden Wattle or Coastal Wattle to see what a phyllode looks like. But not, for example, a Silver Wattle which has actual real feather-like leaves!


The species I've illustrated here (in the right of the picture above) has phyllodes that are particularly long, up to 20 cm long, but only a millimetre or two wide. It's as if the evolutionary instruction didn't quite get through. Instead of forming something like a standard leaf, it's become more like a conifer or she-oak.

There are advantages in having long, thin phyllodes, particularly where this plant grows, between Litchfield National Park and Darwin. When it's hot and dry it isn't always an advantage to expose lots of leaf(-like) surface. Far better to keep some of your foliage partly shaded, with less surface area exposed to the sun all day.


This may be a successful adaptation but as a species Acacia praelongata isn't doing that well. Its restricted a scattered localities in the top bit of the Northern Territory. So while up that way with my fellow heads of Australian Botanic Gardens in October last year we helped (I think) Ben Wirf collect some seed for the George Brown Darwin Botanic Gardens, part of the Australian Seed Bank Partnership.


There seem to be about 40 species of Acacia with leaves at least this long and sometimes this narrow, but the habit and habitat of Acacia praelongata are definitely unusual. A species called Acacia murrayana has similarly long phyllodes but grows in arid areas outside the tropics and those long phyllodes have a single nerve/vein, rather than three in our species (although I'm not sure what quite to count in this photograph - one is certainly more prominent).


Unfortunately we didn't see it in flower, but the pale yellow heads stick out from a dangling stem, presumably a few weeks before we arrived to find the seed.

1 comment:

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