A seed can be regarded as a vessel in which lies a partially developed young plant in a condition of arrested growth, waiting for the correct conditions for growth to resume. Successful reproduction depends on seed dispersal to places appropriate for germination to occur. During the evolutionary history of plants, seeds and fruits have developed a great variety of specialized structures that enhance seed dispersal.
Wind is one major means of seed distribution. Very small seeds, such as the dust like seeds of orchids, heathers, and some rushes and grasses, are dispersed by wind. Such seeds have been recovered from the atmosphere by airplanes at elevations up to 1,000 meters. Heavier seeds have evolved a variety of structures to ensure wind dispersal. For example, some members of the daisy family, such as dandelions, bear numerous one seeded fruits to which are attached a feathery, tuft like structure that acts as a parachute. Similar structures aid in the dispersal of the seeds of many other plants, such as cattails and milkweeds. Heavier seeds, such as those of ash, maple, and pine trees, have developed large, flat wings that allow the seeds to fly in a propeller-like manner for a considerable distance from the parent plant.
Often adaptations for wind dispersal of seeds can be seen not only in the seed's structure but also in structures of the parent plant. Many plants offer their seeds to the wind by bearing them on long flower stalk that tower above the surrounding vegetation. Tumbleweed bushes are small and almost spherical. When mature, they develop a weakness of the main stem at soil level. Wind can break the stem of the bush from which, as it rolls over the ground, the seeds are shaken loose and are scattered.
The seeds or fruits of many plants are dispersed by sticking to the outsides of birds or land animals. Seeds are transported in mud that sticks to the feet of animals. The large numbers of species whose seeds show no obvious special dispersal adaptation are probably spread in this manner. Many seeds, however, can attach themselves to a passerby by means of adhesive substances, hooks (such as the fruit of bedstraw), or burrs (such as the seeds of the burdock plant).
Some plant species have seeds that are adapted for dispersal by animals and birds that transport them internally. The attractive and tasty fleshy fruits and berries of plants can be considered an adaptation to aid in seed dispersal. The seeds of most fruits eaten by animals and birds have a digestionresistant coat. The animal deposits excrement containing seeds at a location at some distance from the parent plant, where the seeds grow into new plants. In some species, germination will not even occur unless the seed has passed through an animal's digestive tract. The presence of seeds in bird droppings is responsible for the appearance of some plants on remote, barren, volcanic islands. Various animal behaviors, including the collecting behavior of ants and the seed-burying activities of mice, squirrels, and jays, also aid in seed dispersal.
Several plants have evolved mechanisms that expel seeds explosively away from the parent plant. The pods of Impatiens, for example, develop strongly unbalanced tension forces as they ripen. When the pod is fully mature, the tension is so intense that the slightest disturbance causes the pod to split open, violently expelling the seeds.
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