When it comes to evolution, reproduction is more important than life itself. Even if an organism lives a very long life, it contributes nothing to the evolution or the long-term survival of its species unless it passes its genes on to the next generation.

Many plants reproduce through the process of sexual reproduction. Pollen grains from the flowers of one plant fertilize the eggs of another plant's flowers. These fertilized eggs, or zygotes, then develop into embryos, the nascent offspring inside a plant's seeds.

Unlike animals, plants can't just get up and move to a better location. If conditions are crowded or the soil is poor, individual plants must simply endure. The same is not necessarily true for their seeds, however. Plants can disperse their seeds far and wide, a process that prevents offspring from having to compete with the parent plant for light, water, and nutrients, and one that in some cases helps to establish new populations of the species elsewhere in the world.

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, sometimes painfully, to an animal's hair or fur -- or to a person's clothing. These types of seeds, called "hitchhikers," may become stuck to an animal in one field and get 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 hitchhike on an animal's fur probably began as fortunate accidents. 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 spikes. Or perhaps a seed that already had hairs for protection against drying out had a mutation that stiffened the hairs and gave them barbs. 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.