Close-up View of the
by Brian Johnston (Canada)
This very aggressive weed has many
names: corn thistle, field thistle, creeping thistle, perennial
thistle, Californian thistle and Canada thistle. Since it is
native to southeastern Eurasia, it seems strange that the last two
names were given to a plant that reached North America in the late 18th
century. In fact, most farmers consider it a very unpleasant, and
difficult to eradicate invader of their land. Having this
pernicious weed named after their country or state, may be a dubious
The examples photographed for this
article came from a typical infestation located at the edge of a park
near my home. An area of about five metres by three metres has so
many Canada thistle plants that it would be simply impossible for man
or beast to cross the patch. Each plant is chest high, and so
many have grown in the space that the area is a paradise for
butterflies - one of the few admirers of the species.
Canada thistle is different from
other thistles in that male and female flowers exist on separate plants. Such plants
are called dioecious.
Normally, all flowers in a particular patch are of one sex.
(Research has shown that the reality is not quite as simple as I have
indicated. Canada thistle is considered to be “imperfectly dioecious”. Some
of the flowers are self-fertile
hermaphrodites having both male stamens, and female pistils.)
The two images that follow show the
general appearance of the plant during the summer season. Notice
that although the leaf edges are ringed by sharp spines, there are none
on the stems.
The leaf spines are extremely
unpleasant, as they are very sharp, and thus easily penetrate skin and
Red buds are almost spherical in
early stages of development. Notice in the fourth image, the
spider-web like fine white filaments that enshroud the bud. The
bull thistle and nodding thistle discussed in an earlier article
share this characteristic.
As the buds develop, they become
longer and more ellipsoidal in shape.
The Canada thistle bloom (up to
about three centimetres in diameter), is quite striking in
appearance. The pinkish-purple composite flowerhead is composed
of either staminate (male)
flowers, pistillate (female)
flowers, or (unusually) hermaphrodite
flowers. In the bloom shown below, the flowers are all female
since no bright yellow pollen is present anywhere. The
greenish-purple layered bracts,
(modified leaves) making up the calyx of the flowerhead are sharply
pointed, but not needle-like as are those of other species.
The aster family (Asteraceae) to which the thistles
belong has flowerheads that may possess both ray and disk
flowers. In the case of thistles, only disk flowers are
present. The pink projections that can be seen below are the
individual pistils of the disk flowers.
Under the microscope, the bi-lobed stigma (female pollen accepting
organ) can be seen to be held aloft by the white style.
With higher magnification, stubby
projections can be seen that help the pollen grains adhere to the
At the base of the style, you can
see the tiny “petals” of the flower that are normally buried deep
within the flowerhead, and are thus not visible.
Canada thistle flowerheads are
extremely rich in nectar, and are clearly one of the favourite haunts
of butterflies, moths and bees. The patch from which these plants
were obtained has the highest density of butterflies on any given day,
that I have ever seen.
In the late summer and early fall,
the plants die, and dry out to an attractive light brown colour.
The feathery white pappus may help to carry the seed to other locations
with the aid of the wind. Usually, however, the pappus breaks
off, leaving the seed attached to the base of the dried flowerhead,
where it may fall to the ground, or be carried away by animals.
In a particular patch of plants, vegetative reproduction from the root
system accounts for most of the local growth.
Whenever I photograph thistles, I
have scars as trophies of my efforts! Even so, this unusual plant
is worth the amateur naturalist’s attention.
Two-thirds of the photographs in
the article 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.) 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 828. (The magnification here is about
14X for a 4x6 inch image.) The remainder of the photographs were taken
with an eight megapixel Canon 20D DSLR equipped with a Canon EF 100 mm
f 2.8 Macro lens. The photomicrographs were taken with a Leitz
SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.
The following references have been
found to be valuable in the identification of wildflowers, and they are
also a good source of information about them.
- Dickinson, Timothy, et al.
2004. The ROM Field Guide to Wildflowers of Ontario. Royal
Ontario Museum & McClelland and Stewart Ltd, Toronto, Canada.
- Thieret, John W. et al.
National Audubon Society Field Guide to North American Wildflowers -
Eastern Region. 2002. Alfred A. Knopf, Inc. (Chanticleer Press,
Inc. New York)
- Kershaw, Linda. 2002. Ontario
Wildflowers. Lone Pine Publishing, Edmonton, Alberta,Canada.
- Royer, France and Dickinson,
Richard. 1999. Weeds of Canada. University of Alberta
Press and Lone Pine Publishing, Edmonton, Alberta, Canada.
- Crockett, Lawrence, J.
2003. A Field Guide to Weeds (Based on Wildly Successful
Plants, 1977) Sterling Publishing Company, Inc. New York,
- Mathews, Schuyler F.
2003. A Field Guide to Wildflowers (Adapted from Field Book
of American Wildflowers, 1902), Sterling Publishing Company, Inc.
New York, NY.
- Barker, Joan.
2004. The Encyclopedia of North American Wildflowers.
Parragon Publishing, Bath, UK.
Microscopy UK or their contributors.
Published in the August
2006 edition of Micscape.
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