Most of us, I would surmise, associate the term milkweed with the large, elongated tear-drop shaped seed pods that the plant produces during late summer and early autumn.  This species however, is well worth observing during the rest of its life cycle, for it appears remarkably different than its neighbours.  Few wildflowers look like the one shown above!

The common name milkweed stems from the fact that when cut or bruised, all parts of the plant exude a milky white, sticky sap.  This liquid contains poisonous glycosides similar to those in Foxglove plants.  When Monarch butterfly caterpillars ingest this sap, they are not harmed, but their bodies become bitter tasting and poisonous to predators.  This protection remains even after their transformation to beautifully coloured butterflies.

Asclepias, the genus name, derives from the name of the Greek god of medicine, Asclepias, since milkweed was used historically in medical formulations.  Wart removal was one such application.  The species name syriaca refers to Syria.  At one time, it was hypothesized that milkweed originated in that country.  Today it is believed that the plant was transported to Europe from North America.

In early summer, buds begin to grow on long drooping stalks that are connected to the same point on the stem.  Each cluster of stalks and buds is called an umbel.  Notice in the image, that there are four umbels visible, each at a different stage of development.

Young buds are light green, with just a hint of pink at the tip.

As they develop, the buds take on a deep pink colour which becomes red just before blooming.  (One such umbel can be seen on the left of the second image above.)

Milkweed leaves, which can be up to ten inches long, grow in pairs and are positioned on opposite sides of the stem.  As can be seen in the close-up below, the leaves are prominently veined.  It is these leaves that provide the only food for Monarch caterpillars.  In fact, without milkweed foliage, it is not possible for larvae to complete their life cycle.

In Ontario, milkweed flowers bloom in July and August.  Each globular cluster is 5 to 7 cm in diameter and contains up to about thirty purplish flowers. 

The flowers tend to be from 8 to 12 mm across.

A closer view reveals the uniqueness of a milkweed bloom.

Notice in the left image below that the flower stalks are very woolly.  The image on the right shows the five pale green sepals, or modified leaves, that bisect each pair of petals.

In a milkweed bloom, the five petals are bent backwards.  The collective term for the petals of a flower is the corolla and each of the petals is called a lobe.  In proper scientific terminology, it is the lobes of the corolla that are bent backwards!

The corona, which looks like a five lobed crown in the photograph below, is formed by the fusing together of the five stamen filaments.  Each lobe of the corona consists of a tubular hood with a horn projecting from its centre.  Each of the hoods contacts its corresponding anther as can be seen in the diagram.

Between each hood and anther combination, if you look carefully, there is a tiny dark elliptically shaped glandular structure.

A photograph of the front of a flower shows the five hoods and horns clearly.  If you look closely,  a few of the dark glandular structures are just visible between the hoods.

The two photomicrographs below show that this gland forms the top of a slit-like opening between the anthers.  In the right hand image there are two orange brown structures hidden behind the walls forming the slit.  They are joined to, and part of the gland.  

These sticky orange structures can be seen in the image on the left below.  On the right is a higher magnification photomicrograph showing the point of attachment to the elliptical external feature.

In milkweed, pollen is produced in waxy masses called pollinia that dangle in pairs from the dark gland and are attached to the orange structures seen above.  Below you can see one of these pollinia.  On the right is a magnified section.

In the photomicrograph below, it is evident that the pollen grains are roughly elliptical and have a concave surface.

The pollination of a milkweed flower is a very specialized procedure.  (Use the image and diagram below to visualize what must happen as you read the following steps.)

• A visiting insect must accidentally slide one of its legs through the slit and into the interior of the flower.

• One of the sticky glandular structures with its two pollen containing pollinia  must become stuck to the insect’s leg.

• The insect must successfully remove both leg and attached gland and pollen sacs from the slit.  (If unsuccessful, the leg may be left behind, or the insect may die being permanently stuck to the flower.)

• The insect must then travel to another flower, and have the leg containing the pollinia slide through the slit of this second flower.

• The pollinia must come in contact with a very small area at the base of the stigma lobe.

• If all of these conditions are met, the flower has successfully been pollinated!

Since this procedure is unusually complex, very few of the flowers in each umbel produce fruit.  (This will be shown graphically later in the article.)

Milkweed flowers do not remain on the plant for very long.  After about a week or two, all of the unpollinated flowers drop off.  A long rough-textured green pod called a follicle begins to form at the location of the successfully pollinated flowers.  The stalk of each pod is strangely curled backwards as can be seen in the image below.

The surface of the approximately 12 cm long pod is covered in fine hairs and many soft protuberances.

At the point of attachment to the stalk, the pod is strangely lumpy.

It is interesting to study the top of the stalk holding the pod.  One can clearly see the depressions where the milkweed flowers were attached while blooming.  Of the approximately thirty flowers in this umbel, only one was successfully pollinated!  Clearly, chance does not favour insects as they investigate the blooms.

Within the pod, the approximately 150 overlapping seeds, each with its tuft of white hair, are growing.  The image on the right shows seeds with green centres at a later stage of development.  Note the thick walled shell of the seed pod.  The many fibres joining the two walls provide strong but rubbery-feeling protection for the growing seeds.

In the following plant, two flowers were successfully pollinated.  At this point the seeds are almost mature.

The details of both top and bottom of the seed assembly are shown below.

Each pod eventually opens along a fold on one side.  I was surprised at how long it took for the pods to dry out and open on their own.  Patience was rewarded when after more than a month from the stage shown above, the pods began spilling their seeds to the wind.  The leaves and stem had long since turned from green to yellow-brown at this stage.  In addition to propagation due to wind transport of the seeds, milkweed also grows underground rhizomes that give rise to new plants.  For this reason, the plant frequently forms large colonies.

The image below uses a polarizing microscope with crossed polars to show some of the white hairs attached to the seed.

Milkweed is not a particularly useful plant to humans, but try to imagine the world without colourful Monarch butterflies.  It is to be hoped that this plant long remains an interesting part of the flora of our planet!


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.  (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 photomicrographs were taken with a Leitz SM-Pol microscope (using dark ground and polarizing condensers), 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)

• 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.