Acacia

Acacia maidenii. Most wattles look similar to the picture on the left soon after germination. The two seed leaves (cotyledons) soon drop off and the pinnate leaves fan out to catch the maximum light, just above ground level. In the case of Acacia maideni and A. prominens (Gosford wattle) the petioles (leaf stalk) start expanding while the leaflets get smaller, gradually taking over the function of the leaves. This transformation doesn’t occur with our other two wattles, A. schinoides and A. irrorata. They keep their pinnate leaves into adulthood.

The Maiden's wattle below shows the beginning this transformation underway, with the leaf stalk already quite leaflike.

And here the transformation is complete, showing how the adult plant will have quite different "leaves" to the seedling.

When plants substitute some other part of the plant, to replace the leaf as the main organ for photosynthesis, botanists call it a cladode.

More photos from the web:

Leaves

Flowers

Fruit