Close-up View of the Wildflower
"Small-Flowered White Aster"
by Brian Johnston (Canada)
This small but attractive wildflower
blooms during August and September in Eastern Canada. It tends to
grow in the same vicinity as its more flamboyant family member, the New
England Aster (the subject of an earlier article).
plant has flower-heads with a diameter of about 10 mm, and is typically
about a metre in height. As can be seen in the image above, there
are many flowers on the end of each stem. The term 'racemosus' in
the scientific name refers to a 'cluster' (as in a bunch of grapes),
and the flower-heads do seem to be attached to the stem in such an
A closer view shows several unopened flower-heads as well as mature
blooms. Asters have composite flower-heads composed of central
'disk' flowers and peripheral 'ray' flowers. What appears to be a
single flower is in fact a group of individual flowers. Most
flower-heads have from 15 to 30 white ray flowers and about the same
number of yellow disk flowers. Although the central disk flowers
do have petals, they are very small and inconspicuous. Beneath
each flower-head are several rings of green-tipped bracts
(modified leaves at the base of a flower). In the image, they can
be seen just below the unopened flower-heads.
As the flower-heads bloom, the central disk flowers do not open until
the outer ray flowers have unfurled. The image below shows this
The mature bloom in the following image shows how different the disk
flowers appear once they open up. Projecting out of each disk
flower is a brownish-yellow stalk that is made up of 5 stamens (the
male organ where pollen is formed), and a pistil (the female
organ). The 5 stamens are fused together, side by side in a
tubular structure, with the pistil running up through the centre.
Not all disk flowers show a stigma (where pollen lands) at a particular
time, but one bi-lobbed stigma can be seen projecting from the top of a
disk flower in the image below.
The following image shows one of the disk flowers under the microscope,
using dark-ground illumination. The petals of the disk flower are
fused at the base to form a tube, out of which projects the
brownish-yellow stamens, and bi-lobbed stigma. Beneath the petals
of the disk flower, one can see many hair-like bristles (the
calyx). These surround the petals and are attached at the base to
the ovary where the seeds develop.
In many of the disk flowers of this aster, the two lobes of the stigma
are fused at the top. Notice that the ends of the stigma are
covered with projections which may increase the surface area and
enhance the possibility that pollen grains will adhere to the surface.
A higher magnification shows these projections more clearly, as well as
the almost spherical, rough surfaced pollen grains.
In the following image, the rough surface of the lower section of the
stigma can be seen to be covered by many pollen grains.
As the disk flowers age, they tend to become brown in colour.
Eventually, the ray flowers also turn brown, resulting in the
appearance of the bloom at the bottom edge of the image.
The aster family contributes greatly to the diversity of the
ecosystem. With an almost unimaginable 20 000 species worldwide,
this family provides a wealth of beautiful flowers for amateur
naturalists to appreciate and study.
The low magnification photographs
in the article were taken using a Nikon Coolpix 4500 with a combination
of natural light and the Nikon Cool light SL-1. Higher
magnification images were taken with natural light using a Sony
CyberShot DSC-F 717 equipped with a combination of achromatic close-up
lenses (Nikon 5T, 6T and shorter focal length achromat) which screw
into the 58 mm filter threads of the camera lens. (These produce
a magnification of from 0.5X to 9X 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 717. (The magnification here is about 13X for a
4x6 inch image.) The photomicrographs were taken with a Leitz SM-Pol
microscope (using a dark ground condenser), 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 August
2009 edition of Micscape.
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