A close-up view of the wildfkower 'Garlic Mustard'

In Northeastern Canada and the United States, the biennial, Garlic Mustard is becoming a major menace. Outcompeting most native plants that bloom in the very early spring, it forms dense colonies that can be tens of metres in diameter. Near where I live, the plants’ tall green stems and leaves are the first sign of spring in forested areas. (This was definitely not the case even five years ago.) The literal “sea” of Garlic Mustard is not limited to any particular location however. Anywhere with moist soil is a perfect place for these fast spreading plants to take root. Although the same prophesies of doom were given about the Purple Loosestrife, that menace has not become the scourge that was predicted. In reality, the Loosestrife plants in my area are very localized, and their area of infestation has not increased in size over the past decade.

Alliaria petiolata, native to Europe and Asia, is a member of the Mustard family (Brassicaceae). It may have been introduced to North America in the late 1800’s as a food, or simply by accident. The genus is named after Allium (garlic) because its species have a similar odour when leaves and stems are crushed.

Garlic Mustard’s upper stem leaves are triangular in shape, and deeply serrated. Newly opened leaves like the ones shown below have a brownish-green colour, while slightly older leaves are bright green.

The egg-shaped buds of the plant are tightly packed, and beige-tipped.

Occasionally, buds grow from the point of connection of a leaf to its own stalk The ones shown in the image are very immature.

Blooming flowers have four rounded white petals, and a diameter of about 0.6 cm.

At a flower’s centre, is a green or brown stalk – the style, that supports a rounded, pale green stigma (female pollen accepting organ). Surrounding this pistil are the six stamens.

Each stamen consists of a filament that supports a pale yellow, pollen encrusted anther (male pollen producing organ). Closer examination of the image on the right shows that the stamens are grouped in three pairs – two taller pairs, positioned opposite one another, and a single, separated pair that is shorter than the others.

While still very immature, the anthers are bright green, with no apparent pollen on their surfaces (left image). Later, the mature anther has the “typical” wildflower shape, and it is encrusted with pollen grains (right image).

Individual pollen grains are approximately egg-shaped, and lack any large scale surface features.

With the flower’s petals removed, it is easier to see the positions of the reproductive structures. The image on the right shows clearly the positioning, and relative size of the three pairs of stamens. Notice that the pistil is taller than any of the male reproductive structures.

When a flower first opens, the entire pistil has a light green colour. Notice the rounded top of what appears to be a style enclosing sheath.

As the flower matures, these style enclosing sheaths turn a reddish-brown colour.

Higher magnification photomicrographs showing the stigma can be seen below. Its surface is covered with spherically-tipped, glandular protuberances that help capture, and retain pollen grains.

In the immature stigma shown below, these protuberances have yet to grow out from the surface. Notice how different are the cells that compose the surface of the style.

When a large patch of Garlic Mustard appears in early spring, it is difficult not to admire the green lushness of its new leaves, and attractive white flowers. However, one must remember what might have bloomed instead – for example, the beautiful white Trillium, Trillium grandiflorum, (the provincial flower of Ontario since 1937).

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 dark-ground and phase-contrast condensers), and the Coolpix 4500.

References

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, NY.
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 Macroscopic Delights

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