Bipinnate&phyll.JPG (50600 bytes)When is a Leaf not a Leaf: The Acacias (Wattles) are members of the Mimosaceae family and occur mainly in Australia and Africa. Australia has more than 750 species (all except one endemic). Australia Acacias may be divided into two major groups. Those with compound bipinnate leaves and those, which appear to have “conventional” simple leaves. A simple leaf has two parts: the stem or petiole and the expanded portion known as the blade. In the Acacias their simple leaf-like structures are modified petioles and are known as phyllodes. Australia is the world’s driest continent and phyllodes probably evolved to reduce water loss. Leaf blades have structures called stomata that are the interface between the atmosphere and the leaf interior. Stomata allow the interchange of gasses. Water vapour is released to the atmosphere and carbon dioxide is absorbed. Because phyllodes evolved from petioles they have fewer stomata per surface area than leaves therefore less openings for the release of water vapour. Phyllodes come in a bewildering range of shapes and sizes. Some Acacias have phyllodes with grey or blue-grey colouring. In Australia wattles with phyllodes far out number those with bipinnate foliage. For example in New South Wales there are about 230 indigenous species with only 17% having “conventional” leaves.
The first leaves of all germinating Acacias are bipinnate. Phyllodes appear as the plant develops. A few carry both bipinnate leaves and phyllodes into adulthood. The illustration shows a bipinnate leaf (Acacia baileyana) and a phyllode from Acacia rubida. Note the gland on the right hand margin of the phyllode. It is situated about 1.5 centimetres from the base. Glands are present on the phyllodes of many species. The number and position of phyllode glands is one of the features used in the identification of Acacia species.
Glands secrete nectar that is a food source for ants and other insects. A number of theories endeavour to explain the purpose of nectar secreting glands. One school of thought is that the presence of ants helps to protect young Acacia growth from foliage-eating insects. Another theory suggests that insects moving around the plant may help to disperse pollen and therefore assist fertilisation. Your guess is as good as mine. 

Research