One of the key ways to identifying a wattle (acacia) is the form of the leaf. Some species have bipinnate (feathery) foliage (pictured left). These include Black Wattles (Acacia mearnsii), Silver Wattles (A. dealbata) and in our district, the environmental weed the Cootamundra Wattle (A. baileyana).
Other wattles such as Blackwoods (A. melanoxylon) and Lightwoods (A. implexa) have, instead of bipinnate foliage, leaf-like structures called phyllodes (pictured right). The development of phyllodes is a response to the harsh Australian conditions. Phyllodes can be oriented with their edges to the sun to reduce the amount of light hitting the leaf surface and hence reduce sun damage and loss of moisture.
I recently came across this young wattle (pictured left). It was obviously an acacia but it had both bipinnate foliage and phyllodes. It looked as if it couldn’t quite decide want it wanted to be when it grew up. Bipinnate foliage is the true form of wattle leaves. All wattles start life with bipinnate leaves. In some wattles these quickly transition into phyllodes. For species such as the Blackwood, bipinnate leaves can last until the plant is considerably developed. Such plants can exhibit both types of leaves simultaneously.
The acacia in the photo turns out to be a Blackwood, not a confused young plant, but a late transitioner. I can relate to that.