Resource: Sock Seeds

Millions of years of evolution have shaped the form and function of plants. One of the most effective evolutionary strategies of plants is seed dispersal, or the scattering of offspring (seeds) far away from the parent plant to a potentially better growing environment. Seed dispersal enables an essentially immobile organism to "move" its offspring to a new location. Doing so prevents the offspring from having to compete with the parent plant for light, water, and nutrients, thus giving the offspring a better chance for survival.

Wind and water are probably the two most effective transport systems used by plants to disperse their seeds. A single gust of wind can carry hundreds of thousands of dandelion seeds from one field to the next. Similarly, ocean currents can transport plants thousands of miles, from one continent to another. Some plants, however, use animals to transport their seeds to new locations. Berries, for example, entice animals to ingest seeds and later distribute them, undigested, in another location. Other seeds have hooks, barbs, and burs on their surfaces that can attach to an animal's hair or fur like Velcro®. These types of seeds, called "hitchhikers," are the most likely to attach to your clothing as you walk across your yard, a field, or through the woods.

Like many traits that prove to be advantageous to an organism, the structures on seeds that allow them to hitchhike on an animal's fur probably began as a fortunate accident. Perhaps one individual plant had a mutation that resulted in small, hook-shaped projections on its seeds. These otherwise functionless structures may have allowed some of that plant's seeds to attach briefly to passing animals and thus travel farther afield and do better than the seeds of other plants of the same species. Gradually, over thousands of plant generations, if the advantage of hitchhiking was great enough, plants with the stickiest or prickliest seeds would have outcompeted other plants of the same species and done a better job of passing their seed form on to future generations.

So the next time cockleburs stick to your socks or sandburs flatten your bicycle tires, you might feel a little less annoyed if you think of them as a magnificent evolutionary strategy instead of a bunch of pesky weed seeds.

Why is it best to do this activity in the late summer or fall?

Why is it best to do this activity on a sunny day?

What enables the seeds to stick to the sock?

Do some seeds stick better than others? Why?

How do you think the ability to stick "helps" the plant?