Resource: Seeds Away

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 modes of transport used by plants to disperse their seeds. A single gust of wind can carry hundreds of thousands of dandelion seeds, for example, from one field to the next. Similarly, ocean currents can transport plants thousands of miles, from one continent to another.

Some plants 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 burrs on their surfaces that can attach to an animal's hair or fur -- or a person's clothing -- like Velcro®. These types of seeds, called "hitchhikers," may attach to an animal in one field and be transported to another field several hundred yards or many miles away.

Like many traits that prove to be advantageous to an organism, the attributes that allow seeds to float thousands of miles on ocean currents or to hitchhike on an animal's fur probably began as fortunate accidents. In the case of hitchhiker seeds, for example, perhaps one individual plant had a mutation that resulted in small changes to the seed coat, making it rougher or even producing small hooks. Or perhaps a seed that already had hairs for protection against drying out had a mutation that resulted in hooked hairs. These new 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 genes on to future generations.

Other seed dispersal adaptations undoubtedly came about in similar ways. Incremental changes over thousands of generations have produced great evolutionary leaps -- and plants that can travel from one field to another or around the globe.

Name some ways that seeds are dispersed.

How are the structure of the seeds related to how they disperse?