A Close-up View of

Eucalyptus Tree Flowers

  Eucalyptus robusta

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 obtained it!

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 surface.

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 magnification.)

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 stamens.

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 other flowers.

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 valves”.)

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.

Photographic Equipment

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.

Further Information

The Eucalypt Page


Robusta Eucalyptus


An Introduction to the Eucalypts


 All comments to the author Brian Johnston are welcomed.

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