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.
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.
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