The Question: I’m trying to identify the burr/seed in this photo. I think it’s from a weed I’ve been pulling from my flower garden. I pulled off about 100 from my shirt. They grip like iron!
Submitted by: Jane, southwestern Connecticut, USA
(click on photos and graphics to expand)
The Short Answer: This appears to be the fruit of a wildflower in the genus Geum, possibly Geum urbanum, also known as the wood aven. The avens are in the rose family and are related to cinquefoils and strawberries. Many Geum species have burrs that readily catch in the fur of animals or the clothing of people.
More Information: Because “adult” plants don’t generally move, they have evolved ways to move their pollen, seeds and spores. Botanists generally classify methods of moving seeds as dispersal by wind (anemochory), dispersal by water (hydrochory), and dispersal by animals (zoochory). Dispersal by animals is further divided into internal seed transportation (endozoochory), which is what happens when animals swallow fruit and seeds, and external seed transportation (epizoochory), when plant burrs use hooks, barbs, or spines to stick to the outsides of their animal transportation.
The trick for a plant that wants to use a human gardener or other animal to transport its seeds is that its seeds need to hang on long enough to go somewhere, but not so long that they never get dropped onto soil so they can grow. The other problem is that the plant has very little control over where the seed lands. But there are possibilities for how a plant could evolve some control over the location where its seeds get sowed. If you imagine, for example, a plant that puts its burrs in a location where mice are likely to pick them up, that could result in a different pattern of sowing than that of another plant whose burrs are located where deer are likely to pick them up.
You could also imagine that different kinds of burrs might hang on more or less tightly, which would have an impact on how far a seed travels from the parental location before it is likely to fall off. The optimum for each plant species will probably be different. If, for example, a plant requires a very specific soil type, it might be best if seeds don’t travel very far from parents because that is likely to result in them being dropped in a soil type different from that where the parent is thriving. On the other hand, a plant that can grow in almost any soil might spread more effectively if its seeds travel a longer way from the parent, as long as it doesn’t disperse so far that it finds itself alone, without a nearby mate for pollination. Species that can self-pollinate might be able to disperse even further, since a single plant can colonize a new location.
One scientist who studies this phenomenon is Dr. Mason W. Kulbaba, a postdoctoral fellow at the University of Calgary. Dr. Kulbaba’s larger area of study is about the evolution of flowers, but as part of that interest, he published a study on the properties of eight species of plants that carry burrs, including common avens (Geum aleppicum). Dr. Kulbaba and his co-authors tested the burrs and their ability to stick to the fur of five different animals (bear, raccoon, deer, mouse, bison) and one set of cotton pants. One of the things he was looking for was whether it seemed like burrs were specialized to specific types of animals. In this study, the only strong association he found was that common avens burrs stuck to mouse fur much better than to any other animal in the study. This is a very limited survey of animals, and limited evidence, but it would be interesting if the burrs of avens are adapted specifically to stick to the fur of rodents.
Interestingly, Dr. Kulbaba found that the stickiest burrs were those of two invasive species, common burdock (Arctium minus) and bluebur (Lappula echinata). They were also the two that stuck to pants the best. No wonder they’ve been able to hitch a ride around the world. And if those avens burrs in Jane’s Connecticut garden seemed to “grip like iron,” she should be thankful her garden isn’t full of burdock and bluebur!
Kulbaba, M W, Tardif, J C, & Staniforth, R J. (2009). Morphological and ecological relationships between burrs and furs. The American Midland Naturalist, 161(2), 380-391.
Photo of Geum urbanum by Badlydrawnboy22 – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Geum_urbanum.jpg#/media/File:Geum_urbanum.jpg[cite]