Close-up View of a Member of the
Protea Family -
a Grevillea (or
by Brian Johnston (Canada)
Documentaries about Australia usually
focus on the unique and fascinating animal species that inhabit the
landscape. The subject of this article is just as unusual, but it
belongs to the plant world. The genus Grevillea is one of the most
popular and widely cultivated of the country’s plant genera. Grevillea species of all shapes and
sizes are to be found in various natural locations throughout
Australia. They are all members of the Protea family
(Proteaceae). A colourful example of the same family, the
“Pincushion Protea” was the subject of an earlier
Since the various Grevillea species hybridize
readily, it is likely that the example studied here was either a chance
hybridization or a cultivar produced in a nursery. The plant was
imported to Canada from Australia for cut-flower use in exotic flower
As can be seen below, the flowers
of the plant are quite small, and occur in a cluster called an inflorescence at the tip of a
branch. The inflorescence can contain up to 100 individual
flowers. In some cultivated plants (like this one), the
flowers are clustered in a cylindrical brush shape (or raceme). The raceme in
this case is about 10 centimetres long, with a diameter of about 2.5
centimetres. Several racemes in bud stage are visible. I
was surprised to find that the stem was extremely strong and very
hard. An ordinary pair of scissors or common kitchen knife was no
match for this sturdy branch.
The leaves of the plant occur in
V-shaped “sprays” of leaflets, and are unusual in structure.
Closer views reveal that the brown
central vein of each leaflet is wrapped, sandwich-like, by green edging.
Higher magnification shows that
both the central, and outer structures are covered by very fine
hairs. The photomicrograph at right shows some of the hairs in
the central area of a leaflet.
In the bud stage, the flower looks
like a column composed of furry brown spheres, with the occasional tiny
green leaflet poking out here and there. As time passes, the
spheres move away from the stem and it is evident that they are held on
the end of short brown stalks.
Just how hairy the buds are can be
seen in the higher magnification images below.
Some plant inflorescences bloom
from top to bottom, others from bottom to top. In this species,
one side of the flower-head blooms before the other. As you can
see from the images, the concept of “blooming” as is normally
recognized in other plants has to be redefined for this flower!
Below, you can see the tip of the
inflorescence with a couple of flower stems off to the left, and a
circular dark ‘wound’ produced when one of the flowers fell off.
Notice the intense hairiness of the stem and flower stalks.
As the inflorescence begins to
‘bloom’, the stalks holding the spherical buds elongate and become
curved. Eventually, the upper, almost white surface of the stalk
splits, revealing the light green style
which supports the hidden female reproductive structure. The
image on the right shows a stalk before it splits longitudinally.
Three additional images show this
Over time, the flower’s stalk stops
lengthening, but the style continues to grow in length. This
forces the style to erupt out of the stem and form an arc above the
stem. As this process continues, the split in the style widens
(right hand image).
Eventually, the inflorescence
looks like the one shown below. The many looped green styles give
the flower-head a very strange appearance!
Notice that the style ends in a
green enlargement – the stigma
(female pollen accepting organ) – which is buried within the furry
The three light coloured ribbons
that are exposed by the split in the style are the surfaces of the
flower’s anthers (male pollen
A single flower, dissected from the
rest of the flower-head, can be seen below.
The back surface of an anther is
covered with numerous tangled, hair-like filaments as can be seen in
the photomicrographs below.
Eventually, the anthers open and
the stigma uncurls. Unfortunately, my cut-flower expired before
this occurred. Instead, I pulled the stigma from the anther
chamber to reveal the chamber’s interior.
The receptive surface of the stigma
can be seen in the image at left. On the right is a
photomicrograph of the flower’s triangular pollen adhering to the
surface. The knobby bump at each corner can be seen more clearly in the
higher magnification phase-contrast image (last).
One would expect that if the
flower’s stigma is in contact with the pollen covered anthers,
self-fertilization would occur. This is usually not the case.
Proteas are protandrous; the
male organs mature before the female ones. When the immature
stigma picks up pollen from the stamens, fertilization doesn’t
occur. The stigma simply serves as an organ of pollen transfer,
and is therefore called the “pollen
presenter”. A few hours after being exposed to the air,
the pollen falls from the structure. From 24 to 36 hours after
the flower blooms, grooves (called stigmatic
grooves) open in the tip of the stigma which can accept pollen
from another plant. Fertilization can then occur.
Almost all of the macro-
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
Canon 250D achromatic close-up lens was used to obtain higher
magnifications in several images.
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.)
The photomicrographs were taken
with a Leitz SM-Pol microscope (using dark ground and phase contrast
condensers), and the Coolpix 4500.
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
November 2008 edition of Micscape.
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