A close-up view of the Campanula


 

A Close-up View of the

"Campanula"

 
Campanula x hybrida



by Brian Johnston   (Canada)


The genus Campanula contains hundreds of species, of which about ten are grown as houseplants.  The examples photographed in this article were obtained as cut-flowers.  Campanulas are commonly referred to as “bellflowers” because of their bell-like shape.  In fact, the name campanula translates to “bell”. Some are long and tubular, while others are shorter and more rotund.  All of these flowers belong to the family campanulaceae.

Although most family members are native to the Mediterranean and mountainous Balkan countries, my examples were flown to Canada from a greenhouse in Columbia.  Air transport has certainly changed the availability of flowers for household display!  As you will see, the stems contain flowers of different colours - pink, pale violet, white, and white with pink lobes.

The image above shows the main characteristics of the bellflower.  A fused tubular corolla ends in six flared, pointed lobes.  The surfaces of stems, and the narrow leaves, are covered in fine hairs.  My samples had stems approximately 40 centimetres in length.

Beneath each flower and bud, there is an unusual bulbous, pumpkin-shaped swelling.  This is the ovary in which the flower’s seeds will develop.  Note the short green bracts (modified leaves), located above the swelling, that surround the base of each bud and flower.  Also notice that the early buds show no sign of the flower’s final colour.





A close-up of the stem shows the many hairs growing from its surface.



The images that follow reveal the overall shape of a mature bellflower.  If you look very closely at the corolla’s surface in the right-hand image, you can see that it is covered in a myriad of tiny downward pointing hairs.  These hairs discourage insects like ants from climbing to the top of the flower.  (Such insects are not efficient pollinators, and this effectively prevents the possibility.)



If the tip of one of the corolla lobes is examined under the microscope, its cellular structure is visible.



When the corolla is viewed from the front, the flower’s reproductive structures can be seen.  Two additional adaptations to prevent the entry of small insects into the flower’s mouth are also visible.  Note that the corolla lobes curl backwards, forming a barrier to climbing insects.  If you have good visual acuity, you may also be able to see a number of long hairs that grow from the top of the corolla and crisscross the flower’s mouth.  These too help discourage the entry of small insects.  Large ones, of course, have no trouble pushing them aside.





The photomicrograph below shows a couple of these long hairs and their points of connection to the corolla.



Cutting away a section of the corolla allows the reproductive structures to be seen more clearly. The flower’s pistil is composed of a long pollen encrusted style which supports a lobed stigma (the female pollen accepting organ).  Clustered at the base of the style are a number of curled stamens with their yellow anthers (male pollen producing organs) supported by short filaments.



A close-up of the style reveals that it is covered with hairs which retain copious quantities of pollen.



An even higher magnification of the base of the style shows these hairs more clearly.



The microscope provides an even better view of the hairs and the spherical pollen grains that cling to them.



The images that follow show the stigma itself.  Notice that it is composed of five rather long lobes which curl back to be perpendicular to the stigma’s base.  (The third image provides another look at the long hairs at the corolla’s edge.)





The bellflower seems to erect many barriers to prevent insects from obtaining the flower’s nectar. Here is another one.  At the base of the corolla, the pool of nectar seen in the image at right is covered by a number of curled, white, petal-like structures that form a protective dome.  In order to get at the nectar, the insect’s proboscis must pass between the petal segments.  In doing so the insect (most likely a bee), must be close enough to brush against the anthers that are positioned just above the dome. How amazing are the schemes that the plant kingdom has evolved in order to facilitate fertilization!



Now let’s look more closely at these stamens.  As can be seen in the three images below, the yellow anthers are tightly coiled, and held in position by relatively short white filaments. The bellflower is protandrous; the stamens become mature before the stigma.  This helps prevent self-fertilization.





A close-up shows the large number of pollen grains clumped on the anther’s surface.



Under the microscope, the under-surface of the anther appears to be relatively smooth, while the upper surface is covered with the hair-like protuberances that can be seen in the right-hand image.



Several photomicrographs follow that show the accumulation of pollen grains on an anther’s surface.





Here is the stem containing pale violet flowers that was mentioned earlier.



Closer views of the flower’s base, and detail in the corolla tube itself, can be seen below.



Notice the hairy edge of one of the plant’s leaves.



If the edge of one of the bracts immediately below the base of the corolla is examined under the microscope, hairs are clearly visible.



Higher magnification reveals still more details.  Notice that the base of each hair is ringed by light green spherical cells.  Also note the circular dimples that cover each hair’s surface.



Adjacent to each hair is a small lumpy protuberance.  (I have been unable to determine the function.)



The photomicrograph below show hairs growing from the central rib of the bract.  The black specks are carbon soot.  (Amazing - Columbia must have air pollution!)



Finally, here are several images showing the colouration of other bellflowers.







Photographic Equipment

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.

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 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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