Distinctive Features of Acacia

Marion Simmons

Marion is a former leader of the Society's Acacia Study Group and author of several books on the genus. The following is part of a longer article which appeared in the March 1988 issue of the Society's journal "Australian Plants".



Phyllodes and Leaves

The majority of Australian Acacias produces "leaves" or phyllodes of great variety. These phyllodes are not really leaves but are flattened leaf stalks which have adapted to look like and function as leaves. Size is extremely variable and some of the phyllodes are huge - up to 30 cm long. Some are so small they appear to be absent as they are often reduced to small scales or spines. Occasionally the stems have distinctive wide wings (as in A.glaucoptera and A.alata) with tiny phyllodes present.

A number of the acacias have bipinnate (fern-like) leaves, made up of a large number of small leaflets (pinnules) along a central stalk. The tree species with this leaf type are usually found in the eastern states and another group about 1 m tall with similar leaves occurs in Western Australia, with the exception of A.mitchellii, which occurs in South Australia, Victoria and New South Wales.

Acacia Foliage  
Figure 1 - Phyllodes and Leaves
  • 1 and 2: Phyllodes of different shapes
  • 3: Fern-like (bipinnate foliage)
  • 4: Winged stems, the only foliage on some species
  • 5; A phyllode showing net-veins
  • 6: Phyllodes are reduced to spines on some species

Phyllode details are important as they are used as "key" characters in identification. Some important things to note are size and shape of the phyllodes, the type and number of veins, whether penni- or net-veins, length of leaf stalk and gland position.

The gland is an indented or raised part usually occurring at the base of or on the top margin of the phyllode or on the leaf stalk of the fern-like leaves. These are extra-floral nectaries which produce very small, varying quantities of nectar to which insects and occasionally birds are attracted.


Flowers

Some acacias flower in their first year, others take longer. Flower buds may be produced at any time of the year, depending on the species; plants usually flower in great profusion once a year. Flowering time may vary in length from a few weeks to many months, once again depending on the species and often the season. the availability of soil moisture, particularly in the arid zones, determines whether some of these species flower at all in any one year.

Acacia flowers vary considerably in size and occur in the form of balls or spikes. The flowers are usually bi-sexual (having male and female parts on the same flower) with 4-5 or occasionally more sepals and petals. The flower heads are made up of a few or many tiny individual flowers with numerous free stamens, as illustrated in the diagram.

Acacia Flowers  
Figure 2 - Flowers
  • 1. A ball flower head enlarged to show individual flowers
  • 2. One single flower enlarged 15 times
  • 3. Ball flower heads in racemes (spikes of flower)
  • 4. Ball flower heads, solitary, from a phyllode axil
  • 5. Rod flower head and a phyllode with 3 veins or nerves

Flowers are attached to the stem in different ways. Many ball and spike flowers occur on simple stalks and many on racemes (branched stalks) which sometimes extend into panicles (branched racemes).

Flowers vary in colour from almost white through shades of cream and yellow to orange. At least one species, A.purpureapetala, differs from the usual colour range, and produces mauve or purple flower heads.


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