A close-up view of the single seed hawthorn.

From the "Forest Minstrel" by William Howitt

The hawthorn tree photographed for this article grows in the flood plain of a small river, and is located about ten metres from the bank. About three metres in height, it has branches almost to ground level. Strangely, if hawthorns grow in a forest environment, they tend to shed their lower branches and look more like “normal” trees. Since the trees’ branches possess many straight, stout spines, hawthorns have been a popular “hedgerow” or boundary tree, used to discourage entry into properties.

Crataegus monogyna is a member of the Rosaceae family. The genus name Crataegus derives from the ancient Greek for strength, and refers to the tough hardwood of the trunk and branches. (Walking sticks are sometimes made from hawthorn wood.) Monogyna, the species name, refers to the fruit of the plant, which has a single (mono) seed (gyna). There are several common names for this species other than “single-seed hawthorn”, including “whitethorn”, and “may”.

At the beginning of May, the hawthorn tree is liberally sprinkled with tiny, almost spherical white buds. The image on the right shows the five, pointed sepals, (modified leaves), that ring each bud’s base, and the swellings at the end of the stalks that indicate the presence of immature ovaries.

The blooming process occurs over the relatively short period of seven to ten days. The image that follows shows this development at an early stage.

Clusters of white, five-petaled flowers are so numerous as to almost obscure the tree’s leaves and branches. The flowers are often described in the literature as being “scented” or “heavily scented”, which in my opinion, gives the impression that their smell is pleasant. Perhaps, to some, but I find the scent to be vaguely “fishy” and not pleasant at all! (This may be due to the fact that I bring branches indoors in order to photograph them, and this concentrates the smell in a smaller volume than that of the out-of-doors.)

At any given time, a branch has blooms in many developmental stages. Notice below that when a flower first opens, the anthers are large and a bright red colour. In less than twenty four hours the anthers appear to shrink, and take on a dark brown colouration.

Hawthorn flowers have five, relatively thick, rounded petals with irregular edges. Notice the sharp spine in the lower left of the image.

Closer views of spines can be seen below. The tree is located in an area with no paths, and reaching it though long grass and vines, that are almost waist height, is a very difficult task. As if this wasn’t enough, every trip to the tree resulted in blood loss, due to the vicious spines.

The four images below show typical flower clusters. Notice the rough edges on petals, and the red tips of the sepals at the base of each flower.

Here are three closer views of several “just opened” flowers. Notice in the third image, that the change in the appearance of anthers has begun to occur.

The photomicrograph on the right, below, shows the cellular structure of a hawthorn flower petal.

A flower has multiple stamens consisting of reddish anthers, (the male pollen producing structures), supported by white filaments.

Most anthers appear to have a pointed horseshoe shape. If you look closely at the images below, you will see that the red structures do not have pollen on their surfaces – but that the smaller brown structures do! It may be that what I have referred to as red anthers, were in fact “anther caps”, that disintegrate to reveal the actual brown anthers beneath! (I could not, however, find any supporting evidence for this in the literature.)

Here then, are photomicrographs showing what I suspect to be an “anther cap”, and supporting filament.

Looking through a crack in the “anther cap” reveals pollen grains with an ellipsoidal shape. (I am more convinced of my hypothesis!)

Notice the difference in pigmentation of cells in various locations on an “anther cap”.

This species of hawthorn has a single pistil consisting of a bulbous, light green stigma, (the female pollen accepting organ), supported by a similarly coloured style. (Further to the earlier discussion, notice how the pollen seems to be pushing aside the red “anther caps” in these images.)

Under the microscope, the stigma appears to be covered with spherical, glandular protuberances.

These can be seen more clearly in the higher magnification photomicrograph at left. The cellular structure of the style is visible in the image at right.

When the tree’s leaves first appear, they are bright red, but this colour soon changes to pink, and then to the final green. The leaves are lobed, and have some serration.

The intricate structure of the underside of a hawthorn leaf can be seen below.

Cells missing green chlorophyll pigment appear at the intersection of two lobes.

Leaf vein structure can be seen in the two images that follow. In the image at left, the stoma and guard cells that control the entry of gas into the underside of a leaf are visible.

Fertilized flowers produce small (1.25 cm) red berries called pomes or haws which contain a single seed – hence the common name single-seed hawthorn. Fruit eating birds are the primary agents of seed dispersal in this species. Although germination of the seed is facilitated by passing through a bird’s digestive tract, this is not absolutely necessary for the process to occur!

Several species of hawthorn grow in my area, but this was the only example of the single-seed variety that I could find. The difficulty of getting to its location, and its many blood-letting spines, made this a memorable photo-taking experience!

Photographic Equipment

Most of the photographs in the article were taken with an eight megapixel Canon 20D DSLR and Canon EF 100 mm f 2.8 Macro lens. An eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 6T, and Sony VCL-M3358 used singly, or in combination), was used to take a few of the images.

The photomicrographs were taken with a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.

References

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)

Little, Elbert L. National Audubon Society Field Guide to North American Trees - Eastern Region. 2004. 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.

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