A Close-up View of the Wildflower

"Field Pennycress"  ("Stinkweed")

Thlaspi arvense

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, ribbed stalk.

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 “pest”!

Photographic Equipment

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.  


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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