All acacia seedlings start life with one or two pinnate leaves (small leaflets arranged on both sides of a central stalk). These eventually develop into bipinnate leaves (pairs of leaflets are attached to secondary stalks). Many acacias continue to have bipinnate leaves when mature.
Some Australian acacias, however, lose their leaves and develop phyllodes instead. A phyllode is a flattened or cylindrical leaf stalk (petiole) that serves the same function as a leaf. They often are mistaken for leaves, as they look quite similar! Phyllodes have a higher photosynthetic rate, and better water use efficiency. Contrary to what many people believe, it is not a sign of a sick/damaged plant, but rather a clever feature that acacias have in order to grow in tough environments.
Phyllodes can take on a variety of forms such as thin and needle-like, elongate or oval. In some species, the phyllodes are reduced to spines; in others, they look like wings branching off the stem.
Some acacias have small nectary glands along the midrib. Ants are attracted to the glands for nectar. The ants also protect the plant by attacking small herbivores. It’s a symbiotic relationship. Acacia baileyana and Acacia mollifolia are examples of acacias that only have leaves (no phyllodes), and both have glands present.
Next time you are looking at an acacia, ask yourself “does it have leaves or phyllodes?”
This can help you to determine the “hardier” varieties.