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Inflorescences, buds and flowers

Looking beyond the leaves, the researcher then seeks the floral structures. These traditionally hold the defining aspects of species. There are numerous characters associated with them. Basically there are two contrasting forms of floral architecture, the individual flower buds or flowers, and then their arrangement on the branchlets. In most species of eucalypts, the buds occur in clusters on single stalks in the axils of the leaves. The flowers are mostly small and whitish and are not conspicuous in the crown.

A very few species have the inflorescences in complex clusters in the axils, e.g. E. michaeliana or on elongated axillary shoots as in E. tessellaris. Four species from south-eastern Australia, E. fastigata, E. pachycalyx, E. regnans and E. squamosa, form their buds consistently in twin clusters in each axil of the leaf. In contrast, several large groups, the bloodwoods, some of the boxes and some of the ironbarks, form the individual bud clusters in large groups at the ends of the branchlets, with few or no leaves. These result in conspicuous sprays of flowers on the outside of the crown in certain seasons. A prominent example is the yellow bloodwood (E. eximia) of the sandstone regions of central eastern New South Wales. The creamy white flower clusters stand out in the forest. In the south-west of Western Australia the widespread marri (E. calophylla) exhibits the same prolific flowering effect, although the red-flowering gum (E. ficifolia), of very restricted distribution east of Walpole, is the most spectacular of the flowering eucalypts. One species, E. cladocalyx, has ramiflorus inflorescences, with the buds formed on the leafless part of the branchlets well inside the crown. Very useful diagnostic information can be derived from these inflorescence patterns, although the structures can be modified by various external factors including predation.

A common modification of the basic axillary inflorescence of the eucalypts can be seen in many "box" and "ironbark" species. In these, bud clusters are formed in the usual way in the axils of developing leaves towards the ends of the annual growth of a branchlet. The arrangement of these leaves and floral primordia is initially decussate and subsequent uneven elongation of the axis gives the appearance of alternation. Each branchlet terminates with a vegetative bud. In many "box" and "ironbark" species, this terminal vegetative bud aborts and the now apparently alternate leaf primordia cease their development. The floral primordia however, continues to develop, resulting in a "leafless" compound inflorescence, terminating the branchlet. A good example of this is E. paniculata, the common grey ironbark of south-eastern Australia.

The individual bud clusters in most eucalypts can be seen on close inspection to be in symmetrical patterns. A few species have a single bud in the inflorescence, e.g. E. globulus and E. macrocarpa but the basic numbers are 3 or 7. In a 3-budded inflorescence there is a central erect bud and two subtending side buds, all in a plane at right angles to the stem forming a "cross". A 7-budded inflorescence has a central erect bud, two subtending side buds plus two buds each subtending the side buds. Higher bud numbers form by the addition of further pairs of subtending buds, and the number of buds in an intact inflorescence is always odd (never an even number), although very high bud numbers may occur in an obscured pattern. Also, in inflorescences with high numbers, one of a pair of subtending buds may be suppressed, probably by compression in the very young inflorescence which is tightly held within bracts which are soon deciduous. When assessing bud numbers, it is important to take into account the fact that during inflorescence development, individual buds may be lost. This is particularly the case by the fruiting stage when the structures under examination have been exposed for a whole season and subject to various traumas including predation and simple death of individual buds.

Angophora flowers

Inner opercula

Operculum scars

Inner bud anatomy


The eucalypts as recognised in EUCLID comprise the traditional eucalypts and the genus Angophora. Angophora species are readily distinguished from Eucalyptus species in the flowers, by the presence of petals that have a green keel and white margin, and by persistent hard, woody, green sepals. Most of the traditional Eucalyptus species of south-eastern Australia do not have separate sepals (two exceptions are E. baileyana, which has very small calyx lobes fused to the petals near the top of the bud and E. microcorys, which has, in early bud developement, very small calyx lobes formed at the top of the hypanthium but which fall early and are seldom seen). In Western Australia the subgenus Eudesmia is widespread and consists of several species, the most famous of which is the glaucous, juvenile-leaved Tallerack (E. pleurocarpa). In this and related species, the calyx is formed of distinct separate sepals which are usually evident as four small teeth at the top of the hypanthium and usually persist to the fruiting stage. Another group of eudesmids have their sepals more or less fused to the corolla right at the apex of the bud. Some species of red bloodwood have the inner or petaline operculum partially or almost completely divided into overlapping petals, a feature that can only be seen in carefully dissected buds. A longitudinal section through an almost mature bud can reveal whether or not the inner operculum is divided at all. Similarly, removing the outer operculum but leaving the inner operculum intact can also show whether the inner operculum is partially divided or not. Some examples are E. ficifolia, E. zygophylla and E. deserticola.

Angophora species and some of the northern bloodwoods have hairs and bristle glands somewhere on the inflorescence, peduncle, pedicel, and often on the bud. The buds of most Eucalyptus species are glabrous for their whole life cycle.

The flower buds of Angophora are all very similar within the group and, apart from size, contain very few discernible characters that distinguish the species. The individual flower buds of the traditional eucalypts, however, contain a great deal of vital information, from the external superficial nature of the wall of the bud to the characters of much higher reliability contained within. One character of absolute reliability (no exceptions have ever been found) is the number of opercula, although this requires experience to assess.

The eucalypt flower lacks showy petals. The petals are in fact united very early in bud development to form a cap or a cone-shaped structure that covers the stamens and ovary during their development. This is the inner operculum, which sheds just before flowering when the stamens expand and are almost ready to shed their pollen. (There is a delay in pollen ripening and dispersal to lessen the chance of self-fertilisation and consequent inbreeding). The outer whorl of the floral parts is the sepals which, likewise, unite to form an operculum in most eucalypt species. In the majority of species, this, the outer operculum sheds early in bud development. In doing so the tissue around the approximate middle of the bud, i.e. where the outer operculum attaches to the base of the bud, dies and results in the detachment of the operculum. This leaves a scar around the middle of the bud which can sometimes be seen with the naked eye but is best seen with a lens. A few hundred species, comprising the subgenus Eucalyptus (the monocalypts), have lost the outer operculum altogether in the evolution of the group. Therefore, throughout the development of the bud in these species there is no scar, and the side of the bud is smooth. Some species have two opercula that are fused giving the superficial impression that only a single operculum is present, e.g. E. ochrophloia. The boxes and ironbarks show parallel development in operculum characters. Both groups divide into one in which the outer operculum sheds early leaving a scar, e.g. the box species, E. behriana and the ironbark species, E. paniculata, and another in which the outer operculum is held to bud maturity, e.g. the box species, E. microcarpa and the ironbark, E. sideroxylon. In Western Australia, the retention of the outer operculum in a box species occurs only in E. petraea. The double opercula and their retention to bud maturity is a diagnostic feature of all the red bloodwoods in both eastern Australia (e.g. E. gummifera) and Western Australia (e.g. E. calophylla, E. ficifolia and E. haematoxylon of south-western Western Australia and all the desert bloodwoods, e.g. E. lenziana).

Stamens have various forms of orientation in the unopened bud. Some species have their stamens wholly erect. Others have them uniformly inflexed, while others have irregular orientation. Again, the extremes of positioning, i.e. complete inflexion or complete erection, are easy to assess. There will be "in-between" species in which the character is difficult to categorise. The attachment of the anther on the summit of the staminal filament is useful diagnostically. Some anthers are basifixed, with the tip of the filament attached rigidly at the base of the anther. This character is seen in the boxes and ironbarks but at its most extreme in E. leptophylla, E. foecunda and related species. In the majority of eucalypts the anthers are dorsifixed, by attachment loosely to the back of the anther, such that it can swivel, i.e. versatile. Some eucalypts have flowers with staminodes, where the outer stamens lack anthers or have non-functional anthers, e.g. E. calycogona. The openings of the anther for pollen shed (dehiscence) is also an important diagnostic character. Most eucalypts have their anthers either opening by well separated longitudinal slits for the more or less cuboid anther, or, as in the monocalypts with their more or less kidney-shaped anthers, have the openings oblique and touching near the apex, finally forming confluent slits. The cuboid, freely dorsifixed anther occurs in many western species but the kidney-shaped anther with confluent slits is rare in western monocalypts but is seen in Jarrah (E. marginata) and a few related species. The butterfly-shaped anther in E. guilfoylei is unique in the genus. In a considerable number of species, particularly mallees, e.g. E. oleosa, the anthers are subversatile and open by small roundish pores, either at the sides or the top of the anther.

Within the base of the bud is the ovary and this contains characters of high diagnostic reliability. The most useful is the number of vertical rows of ovules. These can only be seen by dissection and is best done under a microscope but can be done in the field and seen with a 10× lens. Most eucalypts have ovule rows with 4 or 6 vertical rows. Another group has ovule rows consistently in 2s (the monocalypts), while others have rows of 3 or 5, or irregular patterns (bloodwoods and ghost gums).

The top of the ovary is surmounted by the style which terminates in the stigma. The style is usually erect in all but a few species but can be spiral in some e.g. E. albida, making it a useful diagnostic character. In the great majority of species the style arises from the narrowed summit of the ovary. In some bloodwoods, in series Melliodorae and some species of series Loxophlebae the style narrows at the base and is inserted into the roof of the ovary. The style is subsequently articulate, not rigid.

The pollen is transported to the stigma from another flower by wind, insects, small birds or mammals. On germination of the pollen grains, the contents including the vital nuclei migrate by means of a pollen tube down the stigma shaft to the ovary itself where several ovules at the base of the placentae are fertilised. The fertilised ovules mature into the seeds. The ovular structures on the upper part of the placentae are infertile or unfertilised and 'mature' into sterile particles smaller than the seeds known as the chaff.