As the seed develops, a new generation in the form of an embryo, begins within it. This beginning is a limited one, for the growth of the embryo is soon arrested. The seed is then detached from the parent plant, and dispersal or dissemination of the seeds (or the fruits that contain them) takes place.
The agents concerned in the dispersal of fruits and seeds include wind, water, animals such as birds, insects, and mammals, including humans, and even ballistic forces.
Wind as an Agent of Dispersal
Seeds dispersed by the wind are always small and light. The seeds of orchids are so small that more than a million may be found in a single capsule. In other seeds, and in some fruits, plumes or tufts of hair have developed. These hairy wind catchers are outgrowths of the seed coat, as in milkweed, and cattail, or are modified flower parts, as in the dandelion and thistle.
Dispersal by wind is also brought about by the movement of the entire plant, which, when mature, breaks loose and rolls along the ground peppering out seeds as it goes. The tumbleweeds, which include pigweed, Russian thistle, and certain species of grasses spread their seeds in this way.
Few plants wind disperse their seeds more successfully than does the dandelion. Over several weeks, their flower heads are transformed into gossamer globes, each awaiting the wind. And when the wind comes, the seeds sail away each on its own individual parachute, traveling to amazing heights and covering unbelievable distances.
The Leana tree of Borneo produces a seed that is a perfect replica of the most efficient glider ever produced by humans. Not only did plants predate our flight technology with gliders, but they also invented the helicopter. The Tri-Star plant produces a seed with six whirling blades.
Water as an Agent of Dispersal
The action of water in seed dissemination may almost as important as that of wind. Seeds and fruits, large and small, are conveyed over short distances by the washing of rain or over long distances by streams and floods. Modifications useful in dispersal by water include air bladders, light and spongy fruits, and water-tight seed coats.
The champion cruiser may be the Sea bean. Growing along tropical rivers, it drops its specially packaged seeds one-by-one into the passing water. Carried to the ocean, the fruit with seed safely sealed inside may wander the currents for over a year. Once the fruit becomes frayed and tattered, the seed is released to float by itself. Many are lost but a few will be washed up on distant tropical shores to begin the process again.
Animals as Agents of Dispersal
Dispersal by animals takes place in a variety of ways. Fruits may be eaten and the seeds, their vitality unimpaired, passed through the body of the mammal or bird. The ripening color of berries announces to bird and beast that they are ready to be plucked, eaten and eventually dispersed when the animal voids itself.
The survival of some plants is literally entwined with the animals that disperse them. The acacia tree produces seeds in pods. These seed pods are a favorite food of both insects and elephants. Those eaten and then defecated out by the elephants are not only dispersed but provided with a mound of high-quality fertilizer to jump start their life.
The seed pods not eaten by elephants are almost entirely destroyed by insects. Thus, as go the elephants may go the acacia trees. The Tilia tree of Nepal depends on the Indian rhino to carry its seeds from the shade of the forest where they cannot grow to grasslands and riversides where they can thrive. As the rhino drifts to extinction, it may take the Tilia tree with it.
Animals with fur or feathers are easily conscripted. Armed with hooks, bristles, or spines, many fruits and seeds adhere to the coat of animals, and the clothing of humans. Some species even use spikes to impale themselves into the flesh of the unlucky animal courier.
Internal Force as an Agent of Dispersal
Some plants, such as the oxalis, lupine, and viola, fling their seeds from the seed pods (known technically as ballistic dispersal). The seed pods and seeds of these gunslingers have evolved adaptations that create torsion as the seed pod dries.
When the seed pod becomes dry and brittle, the built-up torsion rips the pod apart the shooting seeds away from the parent plant often at amazing velocities. As the pods burst they often give off a loud pop that is easily heard.
For a plant species to thrive its members must be widely distributed within their habitat. Plants have evolved a number of remarkable adaptations to ensure that their seeds and thus the next generation are adequately dispersed.
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