The Unique Pollination System of the Swamp Milkweed

Oct 25, 2018 | 5 min

Drake University Ecology Professor, Thomas Rosburg: Well here's the plant everyone maybe you can recognize as a milkweed this is the swamp milkweed. Milkweeds probably most people know are important for monarchs, is one interesting point.  Another one of course is that they are wind dispersed. Everyone that's probably played with the the follicles is the correct name for the type of fruit that they make but it's that pod like structure that we see late in the summer that you can actually splits open and that's all the seeds attached to some fluffy hairs that blow about and wind.

They have really cool flowers. The flowers are way different than most other plants in fact there's a number of terms that are used to help describe the milkweed flowers that only our apply to to these these plants. The petals are actually these little structures here and they go down you know again that's sort of against the the typical pattern that most flowers have petals that are upright. Well and milkweed the flowers are pointing down. And then up on top here there's a little structure called a corona or it means crown of course the term that means crown and that's a structure that you don't see on very many other plants.

It boils down to the milkweed having a pollination system that's very different than other plants as well having these unique flowers also means that they have a way of being pollinated that is pretty different than most other plants. They are similar to the orchid the orchids in one regard in that they produce pollen that's all in a group or cluster so unlike most plants that disperse pollen and by insects or by wind sort of just one grain at a time or a few grains at a time these plants have a package of pollen called a polinia. What has to happen is an insect has to be attracted to the flowers of course due to scent to the fact that there is nectar here but then it has to sort of accidentally get its foot pushed into or stuck into a kind of a narrow slit and when that happens it's when a leg or bottom of its leg goes into that narrow slit it will hopefully contact a structure there that will get caught on to the leg so when it pulls its leg back out it pulls out a couple of these polinia a couple of these packages of pollen. Well the insect it's not far then of course it has to fly to another plant someplace of the same species the same type of milkweed and again get attracted to the flowers and again land on the flowers and then its attempt to find and search for nectar now it has to somehow insert that leg that's carrying the polinia into another one of those slits. So it's got I just accidentally somehow position this leg and accidentally get that leg into that slit in order to get the pollen grains into that that little doorway we might say. And so it's it's a situation that is well pollination doesn't happen very easily. I guess there's one way of putting it so despite all of these flowers and there's probably maybe a couple hundred flowers here if we came back in the fall we might only see two or three actual fruits that means out of a couple hundred flowers only two or three of them actually got pollinated and went on to produce seeds.

So it's kind of kind of odd why the milkweed would evolve a strategy that makes it so hard to produce seed I guess. One thing it does do is it helps to ensure that only pollen from other milk weeds is is probably put into the little doorway and pollinate the plant so it's it's probably a way of making it more efficient in terms of making sure this pollen goes to another milkweed and doesn't just get wasted somewhere else on other plants. The fact that there's a polinia of a whole bunch of pollen helps ensure that if that does happen if it's you know all those things fall into place and that insect does push that polinia in through that little doorway and contact the stigma in there that at least will be a whole bunch of pollen grains that means a whole bunch of seeds then then can be can be formed. Milk weeds are cool. 

Gilchrist Foundation