A Close-up View of the Wildflower
"Small-Flowered White Aster"

(Aster racemosus)


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).  A typical 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 arrangement.
 
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 clearly.





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.
 

 
Photographic Equipment

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 Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.


The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

 All comments to the author Brian Johnston are welcomed.


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Published in the August 2009 edition of Micscape.
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