The Leaf | The Inflorescence | Pollination | Seeds | Growth

The inflorescence and flowers

By about three to four years, occasionally longer, most acacias mature and bear flowers. This is frequently in late winter to early spring time, although there are other species which have different flowering times. Species from arid regions tend to flower at irregular times, the time of flowering usually depending upon the fall of adequate rains.

In the floral arrangement of acacias, the individual flowers are aggregated into either ball-like (capitate) heads or rod-like spikes. A few species have oblongoid or ovoid heads which are very short spikes where the length is less than twice the width. The heads or spikes are borne in the leaf or phyllode axil on a peduncle which usually bears a bract at its base, although sometimes the peduncle is lacking and the heads or spikes are then sessile. The arrangement of all the capitate heads (capitula) or spikes and the peduncles on a flowering branchlet is referred to as the inflorescence. The inflorescence may be a raceme consisting of an axis (rachis) with the peduncles branching from it; or it may be simple, where the peduncles occur singly or in pairs or clusters in the phyllode axils or on a very short axis. A compound raceme is termed a panicle. Racemes and panicles which at the end of the branchlets are terminal.

In acacias each individual flower is sessile as they have no flower stalk (pedicel), and each has at their base a bracteole, which is a very small bract-like organ. The flowers are regular (radially symmetric) and are usually 5-merous, bearing 5 sepals or calyx lobes and 5 petals. Occasionally they may be 3-merous, 4-merous or 6-merous. Each flower consists of an outer whorl, the calyx, consisting of either separate or, more frequently, united sepals, and an inner whorl, the corolla, consisting of separate or united petals. The term perianth refers to both the calyx and the corolla. The calyx lobes are the apices of united sepals. Enclosed within the corolla is a ring of numerous stamens subtending the anthers, the pollen producing organs. Inside this ring of stamens, in the centre of the flower, is the ovary which, following pollination, develops into a pea-like pod or legume. The style is an extension from near the top of the ovary, with the stigma terminal on its outer end. The stigma is the receptive organ which receives the pollen from the anthers during pollination.

Fig. 2 Flower and inflorescence terms

Not all flowers have an ovary and style, that is, many are male only. Casual observation also suggests that there is a low number of fertile flowers in the heads or spikes of many rare and uncommon species, and this loss of reproductive ability is probably a contributing factors to their uncommonness.