A Close-up View of the Wildflower
"Dandelion"

(Taraxacum officinale)


by Brian Johnston   (Canada)



                           
The Dandelion's pallid tube
Astonishes the Grass,
And Winter instantly becomes
An infinite Alas-
 
The tube uplifts a signal Bud
And then a shouting Flower,-
The Proclamation of the Suns
That sepulture is o'er.
 
Emily Dickinson



Note: 

This was the first article that I wrote for Micscape.  At the time, I was not happy with the quality of the photographs that I had taken, so I put it in the "maybe someday" file.  Now, seven years later, I have decided to let it appear on Micscape, warts and all!



The dandelion is one of the most recognized perennial weeds in Europe and North America.  In early spring and fall, the ubiquitous carpets of yellow flowers are either considered a curse, or a thing of beauty, depending on whether they are in your lawn or in a field beside the road.  It is thought that the English name is a corruption of the French "Dent de Lion", and Latin "Dens leonis", meaning teeth of the lion.  This may be due to the roughly toothed appearance of the leaves.
 
The scientific name for the dandelion is Taraxacum officinale and it is a member of the Compositae family.  Strangely, the bloom is not a single flower!  It is actually a composite of many individual flowers, called florets.  ( Each petal then, is a single flower! )  These florets are held above the ground by a tubular upright stem that can be up to 45 cm in length.  The bloom, florets and stem can be seen in the images below.




 
At the base of  each flower-head is a ring of narrow green bracts called the involucre.  Some of these stand upright to support the immature florets before the flower opens up (left image), and some hang down to form a barricade against small insects that might crawl up the stem and injure the bloom (right image).




 
Each of the florets mentioned earlier is notched at the edge into five teeth, each tooth representing a petal, and lower down towards the stem, is narrowed into a tube which rests on an ovary containing a single ovule.  In the tiny tube is the nectar so sought after by bees and many other flying insects.  The stigma grows up through the tube formed by the anthers and is covered with pollen.  As can be seen in the two images below, the stigma is split at the end into two curling lobes.




 
This photograph shows a bloom at an early stage, in which the florets have not fully opened by moving away from the central axis of the plant.  Tiny immature light green seeds can be seen at the bases of the florets.  The fine white fibers that will eventually form the white ball (at a later stage) are visible between the yellow florets.





If a single floret is carefully removed from the bloom, it is possible to see clearly all of the parts of a single dandelion flower.  At the base is the growing seed.  Above this is the stalk surrounded by the tuft of white fiber that will become part of the white ball that carries the mature seed to its destination.  Higher is the yellow floret.  Crowning the flower is the pollen covered stigma.





When viewed under the microscope, the structure of the stigma can be seen more clearly.  On the left is an image of the stigma before  it branches into two lobes.  Notice the sharp spikes and the pollen which clings to the surface.  The photomicrograph on the right shows the two lobes.  Handling has dislodged much of the pollen that originally covered the surfaces.





The bi-lobed nature of the stigma is more evident in the image below.  The pollen remains in the protected areas within the loops of the two lobes.





A higher magnification shows clearly the very large number of pollen that cling to the inner surface of each lobe.





By adjusting the focus of the dark-ground condenser on the microscope, it is possible to increase the contrast in the final image.  The spikes on the surface in the more highly magnified right image seem to help in the attachment of the pollen.





As can be seen below, most of the surface area of the bloom is covered by these pollen covered stigmas.  Insects drinking the nectar, smear themselves with pollen grains and carry them to other flowers, insuring cross-fertilization.




 
When the entire head of the bloom has matured, all of the florets close up within the green bracts that lie beneath, and the bloom returns almost to the appearance it had as a bud.  The withered yellow florets are blown away by the wind, leaving the seeds connected by a stalk to tufts of hair.  In the images below, some of the bracts have been removed to show the structure beneath.  Notice the milky juice that exudes from the broken bracts.  This juice causes difficult to remove brown spots, when it comes in contact with human skin.




 
As time passes, this structure opens up into a gossamer ball, the whiteness of which is striking.




 
This ball is made up of myriads of plumed seeds or pappus, ready to be blown off when completely ripe, by the slightest breeze.




 
A closer view reveals that each seed is covered by tiny spikes.  These projections may help the seed to attach to the soil when it finally touches down from its aerial journey.







 
When all the seeds have been blown off by the wind, the disk to which the were attached remains bare, surrounded by the sheathing bracts.  In the middle ages the dandelion received the name 'Priest's Crown', when a priest's shorn head was a familiar object.





Although considered a botanical pest, the common dandelion is an amazingly complex and beautiful object.





 
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 June 2010 edition of Micscape.
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