Close-up View of
Eucalyptus Tree Flowers
by Brian Johnston (Canada)
There are many strange flowers in the
plant kingdom. One of the most unusual that I have had the
opportunity to photograph, is the bloom of the eucalyptus tree.
Unfortunately, tropical plants have an impossibly difficult task
growing in my area, where frigid winter temperatures often kill native
plants. Fortunately, exotic plant dealers supply eucalyptus
branches in bud stage to florists for use in floral arrangements.
My specimen was flown to Toronto from Peru several days before I
Eucalyptus robusta (Myrtaceae family) is a moderately
large tree, which grows up to 53 metres in height, and up to 1.5 metres
in diameter. In older trees, the wood is dark mahogany red, and
very hard. The wood is amazingly dense ~ 0.8 g/cm3.
(The density of water is 1.0 g/cm3.)
The tree is found in the tropics,
sub-tropics, and temperate Australia. In Australia it is called swamp mahogany; robusta eucalyptus is the name in
the United States, and in Puerto Rico it is called the beakpod eucalyptus. The wood
is ideal for construction in wet areas, and has the advantage of being
resistant to marine borers.
The image above, and the two that
follow, show the end of an eucalyptus branch. Both the unopened
buds, and blooming flowers are very strange! The buds are
vase-shaped and possess a “cap” that falls off during the blooming
process. The flowers are remarkable in that they have no visible
petals. Instead, the very
large number of male stamens constitute most of the visible part of the
flower. (“Normal” non-eucalyptus flowers have up to about 20
stamens.) A central green female pistil extends out from this
mass of stamens. The flowers are about 3 cm in diameter.
When the flowers first open, (more
about this process later), the bloom appears white due to the colour of
the many white filaments supporting tiny yellow anthers. After
about a day, the filaments slowly begin to take on a beige or very
light brown colour.
A typical eucalyptus leaf has a
broad lanceolate shape, and a very prominent central vein.
Higher magnification reveals less
prominent secondary veining. The second image shows an older leaf
with a red band along its edge.
Under the microscope, the intricate
pattern on the leaf’s upper surface is revealed. The leaf appears
darker green (compared to the earlier images), because the light passes
through the entire structure, whereas before, it reflected from the
The greenish-gray underside of a
leaf has a finer, more reticulated pattern.
The underside has many
reddish-brown spots on its surface. The right-hand image is a
photomicrograph (low power) of two of the spots.
A high power objective shows more
detail. Each pore (called a stoma),
that allows gas to enter the underside of the leaf can be seen to have
two crescent-shaped guard cells
surrounding it that control whether the pore is open or closed.
I am always struck by the
sculptural beauty of botanical specimens. Even the nodes on the
stem are remarkable! (Several views follow that show increasing
Unopened vase-shaped eucalyptus
buds, 25 to 30 mm long, grow out radially from the end of a
branch. At this point, the surface appearance of both bottom and
top of a bud is identical.
Over about a day, the top part or
“cap”, called the operculum,
turns brown and begins to shrink. The surface takes on a deeply
grooved, shriveled appearance. This operculum gives the
eucalyptus its name. Eu
translates to “well” and calyptos translates to “covered”.
The two images that follow show the
operculum’s surface detail.
Notice in the image below, the ring
to which the operculum was attached. Remnants of the darker brown
cap can be seen clinging to the lower edge of the ring. The white
ribbon-like structures are the tightly packed filaments of the flower’s
During anthesis the stamens beneath the cap
swell, and the cap is forced upward. (Botanists refer to anthesis
as the time and process of budding and unfolding of blossoms.)
Eventually, wind or vibrations
cause the cap to fall off, revealing the tightly packed parallel
stamens and the long green pistil. (Note that the point on the
cap is elongated to allow for the long pistil!)
The two images below show the
reproductive structures immediately after the cap has fallen off.
Within twelve hours, the many
stamens have straightened into a roughly radial pattern. A single
group of flowers is quite striking with its dense masses of beige
stamens and protruding green pistils.
Several views of individual flowers
can be seen below.
The head-on view of a bloom shows a
strange green cross pattern beneath the pistil. This ‘cross
within a ring’ is actually the top of the flower’s ovary (seed producing organ).
The side-view of a flower shows no
sepals or bracts (modified leaves) at its base.
There is some time lag in the
blooming of flowers in any particular group. The bud shown in the
image below is particularly tardy!
Close-ups of a pistil show the light green style which supports the darker
green stigma (female pollen
accepting organ). The eucalyptus flower is protandrous - the anthers
release their pollen before the stigma of the same flower is
receptive. This helps prevent self-fertilization.
The following photomicrograph shows
the cellular structure of the style.
At the end of each white supporting
filament, there is a single
light brown anther (male
pollen producing organ).
Under the microscope, both anther
and filament are irregularly shaped when compared to those of many
Eucalyptus pollen grains are very
tiny, but a few can be seen clinging to the anther’s surface.
Photomicrographs using different
objectives reveal the orderly cellular structure of a filament which is
interrupted by frequent yellowish “bumps”.
For comparison, the image below
shows a similar filament which has begun to dry out as the stamens age
and fall from the flower.
Eventually, all of the stamens are
gone from the flowers, and all that remains are the ripening
fruit. Each fruit is a vase-shaped greenish-brown capsule
containing many small seeds. The fruit ripen 5 to 7 months after
flowering, and are retained on the tree until stimulated to be shed by
heat from a fire, or by the death of the plant.
Two views follow that show the top
of a fruit. The second image shows a quantity of viscous nectar
that still remains in the depressions below the pistil. (The
depressions are referred to as “sunken
The orange pulpy interior of a
fruit can be seen in the images below. The “top” of the fruit is
shown at left, with the pistil facing away from the observer. The
“bottom” is shown on the right, with the stem facing away.
Most of us associate the word
eucalyptus with commercial products that ease nasal congestion and
coughing. The active ingredient in these mixtures is a compound
called eucalyptol, (or cineole), that occurs in the oil
obtained from the leaves and branch tips of two eucalyptus species E. globulus and E. fructicetorum.
Most of the photographs were taken
with an eight megapixel Canon 20D DSLR equipped with a Canon EF 100 mm
f 2.8 Macro lens which focuses to 1:1. A few were taken with an
eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic
close-up lenses (Canon 250D, Nikon 5T, 6T, Sony VCL-M3358, and shorter
focal length achromat) used singly or in combination. The lenses screw
into the 58 mm filter threads of the camera lens. (These produce
a magnification of from 0.5X to 10X for a 4x6 inch image.) Still
higher magnifications were obtained by using a macro coupler (which has
two male threads) to attach a reversed
50 mm focal length f 1.4 Olympus SLR lens to the F 828. (The
magnification here is about 14X for a 4x6 inch image.) The
photomicrographs were taken with a Leitz SM-Pol microscope (using a
dark ground condenser), and the Coolpix 4500.
Introduction to the Eucalypts
Microscopy UK or their contributors.
Published in the
October 2007 edition of Micscape.
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