Tuesday, 29 August 2017

Butterflies pollinating a milkweed risk arm and leg


Asclepias syriaca, the Common Milkweed, is most often grown in the Northern Hemisphere to encourage butterflies but it manages to also attract the wrath of farmers and environmentalists.

In North America and Europe the Common Milkweed is weedy and toxic to livestock, but the flowers do provide nectar for various butterflies and the leaves are particularly attractive to Monarchs. (Depending on the species of Monarch, they may favour this species or another Asclepias: see for example my post about butterflies on the Tropical species in the botanic garden of Buenos Aires, Argentina.)

Although the species name suggests it might be from Syria it is native to the USA and a bit of Canada. It was described first in Paris, in 1635 by French Physician Jacques Philippe Cornut, from seeds sent from Canada. Cornut compared this New World species to one from Syria and through some error, part of that descriptor became the species name applied by Carl Linnaeus in 1753.

In any case, this American plant has now spread considerably through cultivated land on that continent and also become established in central and southern Europe, through to the Middle East. Declared prohibited and noxious in Australia is hasn't yet arrived, but has the potential to enter the country inadvertently as a contaminant in crop seed.


The genus name provides a bigger hint as to why this plant is featured in a useful plant collection in Musée et jardins botaniques cantonaux, the botanic garden of Lausanne, Switzerland (where all my pictures are taken). Asklepios was the Greek god of medicine.


However the milky sap (and this sign) is a warning to take care. As is the promulgation of the species as a diuretic, emetic and purgative. Clearly this plant contains things your body might want to expunge.

Monarch caterpillars, however, can ingest the sap and not be harmed but the bitter and poisonous residue in their bodies does help protect them from predators throughout their life.

Still, flower buds and young shoots are (apparently) cooked and eaten by some humans. To what merit, I don't know. I'd suggest the structure of the flower and its mechanism of pollination are far more edifying so you might be better advised to watch the buds bloom rather than eat them.


First you'll see the buds, or flowers, in a cluster, all arising from the end of a common stalk. The buds blush pink at their tip, eventually turning red or deep pink when ready to open and reveal this...


In this picture the five outer pale green flaps, the sepals, are hidden but you can see the streaky petals, bent backwards. The inner parts of the flower are the most interesting. Those five hoods with an elongate tube project out of each and into the centre of the flower are modified 'stamen filaments', usually the stalks supporting anthers full of pollen. The hoods contain nectar, attractive to butterflies and other insect visitors.

As to the pollen, in this flower it sits in the green patches between the hoods. There are two connected sacks of pollen in each gap, together called a pollenium. It's tricky to see in my picture but the sacks like on either side of a narrow slot or slit. The pollen here hasn't yet become yellow or orange-brown, as it will do when ready to be released.

This pollenium is not unlike what you find in orchid flowers, where it is often thwacked onto the back of a visiting insect. In the case of the Milkweed, it's said that the leg of a visiting insect - a Monarch butterfly perhaps - slips into one of the slits forcing a sticky pollen sack to become attached to its upper leg.

The insect then extracts its leg, with the pollen, and flies to a new flower, hopefully (for the plant) repeating the awkward slip, this time bringing the pollen in contact with a receptive part of the female organ of the flower, which I gather is inside those slits.

Your butterfly or other pollinator might on occasion unintentionally leave a leg behind or even remain attached to the flower, and die in situ, but generally a 'happy' insect visitor leads to a happy 'plant'. A happy plant produces a fruit that starts off like this.


While this pollination system is intriguing for botanists it is rather difficult in practice and only a few flowers in a cluster will be successfully pollinated (there are also barriers to successful pollination but let's stick with the butterfly story). Typically only one or two fruits form, eventually producing the silky coated seeds responsible for the French/Swiss common name on the sign reproduced in this post (Herbe à l'ouate, or perhaps Herbe à la ouate, the Silkweed).

Note: If you want to see and learn more of the floral (and fruit) detail see the Microscopy-UK site, where Brian Johnston from Canada, home to our Common Milkweed, has posted some gorgeous close-up pictures. You can also read more about the pollination in various learned articles.