View of the Wildflower
"Field Pennycress" ("Stinkweed")
by Brian Johnston (Canada)
Beginning in early April, this strange
looking plant begins to make its appearance in the “typical” locations
where wildflowers are found: waste areas, disturbed sites, and
roadsides. Many common names for Thlaspi arvense have been invented,
including Field Pennycress, Pennycress, Fanweed, Frenchweed, and the most used, Stinkweed. This last name was
given to the species because of the offensive odour released when young
plants are crushed. In fact, the milk and meat of grazing animals
can be tainted if large numbers of these young plants are eaten.
Although near my home, the plant is the exception, rather than the
rule, under ideal growing conditions, as many as 750 plants can exist
per square metre!
Thlaspi arvense is a member of
the Brassicaceae (mustard)
family, and it is therefore not surprising that its fruit has the
characteristic “cabbage” or “turnip” flavour when chewed. Note:
I’ll take the experts’ word for this. Knowing the location where
my plants were found discourages me from having “a taste”!
At the early stage shown below, the many clasping leaves completely
obscure the plant’s upper stem. These stalkless leaves are
lance-shaped (lanceolate), and have coarsely toothed edges.
The flowerhead–to–be is positioned at the very top of the stem, and
consists of a densely packed group of tiny, furry, beige buds.
When the inflorescence (flowerhead) begins to bloom, the small, 5 to 8
mm diameter flowers, extend out on relatively long, thin stalks.
Also notice that the leaves have angled away to reveal the strong,
Each of the flowers has four rounded, white petals, cupped beneath by
four green sepals (modified leaves). Outer flowers bloom before
inner ones. In those farthest from the flowerhead’s centre, the
dark brown colour of the developing ovaries is visible.
These growing seed-pods can be seen in several of the flowers shown
below. They will eventually look like those seen in the first
image in the article. Notice the reproductive structures visible
in the images. Each flower has a single pistil, and 6 stamens,
each of which is made up of an upper anther (male pollen producing
organ) and lower supporting filament. These stamens are grouped
in two sets of three on either side of the flattened ovary.
The flower’s anthers are extremely small.
Under the microscope, the many egg-shaped (ellipsoidal) pollen grains
that coat the anther’s surface, are clearly visible. Longitudinal
grooves are present on a grain’s surface.
The flower’s pistil, consisting of a light brown stigma, (female pollen
accepting organ), and green supporting style, can be seen in the
photomicrographs that follow.
Eventually a fertilized flower’s petals begin to close up around the
now much larger, brown seed-pod. The petals will disintegrate,
and fall off in time to reveal the flower’s fruit.
This striking transformation can be seen clearly in the two images that
follow, showing the upper portion of a stem. Notice the thick
coating of bubbles on the stem, produced by an insect, in order to
protect it while it “munches” on the cellular matter.
The oval, flattened, green seed-pods have a characteristic V-shaped
notch at the top, and a short stub of the flower’s style usually
remains in the notch of the V. It is the distinctive shape of
these pods that makes identification of the species an easy task.
The light-brown colouration, and unique structure of the mature fruit
make this the plant’s most photogenic stage. The inner, seed
containing section is divided into two compartments by a thin membrane
(septum). Each compartment contains 3 to 8 seeds.
Surrounding the seed compartment are two broad, flat “wings”.
Field Pennycress is a prolific seed producer. Under ideal
conditions, a single plant can produce
15 000 seeds! These seeds can live up to 6 years if they are
buried in shallow earth, or up to 20 years if they are buried
deeper. It is not a surprise then, to learn that the plant is
very difficult to eradicate from areas where it has become a botanical
The photographs in the article were taken with an eight megapixel Sony
CyberShot DSC-F 828 equipped with achromatic close-up lenses (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. 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 photomicrographs were taken with a Leitz SM-Pol microscope (using a
dark ground condenser), and the Coolpix 4500.
- 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.
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
January 2011 edition of Micscape.
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