Seed dispersal

Plant seeds are the product of sexual reproduction, and their dispersal determines where succeeding generations will grow. Seed dispersal is the process that enables a species to expand its territory and helps to reduce the competition for resources between the parent plants and the seedlings.

The seed is often contained in a unit of dispersal, such as a fruit. The embryo inside the seed lies dormant while dispersal takes place, a factor that is as important to the embryo’s survival as the protective structures of the seed or fruit. In many cases, the seed, fruit, partial, or whole plant is structurally adapted for dispersal by animals, wind, or water, or for self-dispersal.

The structure of seeds determines the way in which they will be dispersed. Winged seeds, such as those of the maple, are light and can be carried by the wind. So too can clematis and milkweed seeds, aided by their light hairy tufts, which act as a parachute mechanism. Other seeds are contained in edible fruits and rely on animal consumption for them to be released and dispersed. The barbs and spikes on some seeds also depend on animals for their dissemination, becoming attached to fur or clothing. Some fruits, such as the squirting cucumber, blow their seeds out when they release internal turgor pressure.

Animal dispersal

The dispersal of plant seeds by insects, birds, and mammals may be carried out in various ways, all equally haphazard. The fruits or seeds may be eaten for their nutritional content and the undigested seeds may subsequently be voided elsewhere. Many fruits and seeds have some edible part, whether it is the fleshy outer part of a seed, as in the peony (Paeonia sp.), or more commonly of a fleshy fruit, such as a tomato.

Birds feed largely on fleshy fruits retained on the plant. They are attracted primarily by color (they are particularly sensitive to red) because they have little or no sense of smell. The different colorations of immature fruit, reinforced by an unpleasant taste, warn the birds of unripe fruit. This allows the young seed embryo more time to develop. The red currant (Ribes rubrum) is an example of this type, as is the mistletoe (Phoradendron flavescens), the seeds of which stick to birds’ bills and are later scraped off.

The fruit of wild mangoes (Mangifera spp.) are eaten by the biggest fruit-eating bats, and the trees are adapted for dispersal by them. The mangoes hang away from the dense foliage and often have a strong rancid smell. These adaptations are necessary because the bats are less maneuverable than birds, are nocturnal and color-blind, and have relatively unsophisticated sonar systems for their species. They do, however, have teeth and a good sense of smell. They suck the fruit dry, spitting out the hard seeds.

Fruit enclosing seeds dispersed primarily by terrestrial mammals are often shed and are easily eaten on the ground. An example is the spiny durian (Durio zibethinus), which also has an exceptionally powerful smell. Its oily fruit is eaten not only by humans but also by rhinoceroses, orangutans, elephants, and even carnivorous cats. Its seeds, however, are toxic, which encourages the animals to excrete them as quickly as possible.

Some birds and some of the smaller mammals collect and store nuts for consumption at a later stage. Squirrels and rats store beechnuts and hazelnuts underground, and jays cache acorns in tree hollows. Where hoards are forgotten, seeds are left to grow.

Ants disperse seeds of some Datura, Euphorbia, Cyclamen, and Primula species and are drawn to them by attractant oils. Small structures (elaiosomes) impregnated with these oil food-substances are appended to the seeds and are easily detached. Alternatively, the oils may be generally distributed in the outer layers of the seeds, which are borne close to the ground. The ants eat the oil substance and leave the rest.

Another way in which seeds are dispersed is when seeds and fruits become attached to animals. They may be transported in pieces of mud that cling to the animal, as are the seeds of sweet grass (Glyceria sp.), which stick to the feet of passing waterbirds. Some plants produce a sticky mucilage that helps them to adhere to passing animals. Mistletoe berries eaten by birds also stick to their beaks and plumage, eventually falling off or being removed when the bird preens itself some distance away from the parent plant. Other plants produce fruits and seeds that bear hooks by which they become attached to animals. Stick-seed (Hacheiia sp.) has a small nutlike fruit with barbed prickles that catch in fur. Burdock (Arctium sp.) has hooked bracts below the flower, which remain on the fruit.

Wind, water, and self-dispersal

Wind is another important way in which seeds are scattered; it operates best on the outer edges of plant communities and in fairly open environments. The dustlike seeds of orchids can weigh as little as 0.001 mg and are released in quantities of hundreds of thousands; a single fruit capsule of the Cynorchis orchid may release 4 million seeds of similar weight.

Some of the heavier fruits and seeds have specific adaptations to increase their surface area relative to their weight and assist in flight.

Some sedges rely on the wind to disperse their seeds. When airborne, the feathery seedcases can travel for many miles.

Such adaptations include the “parachute” of the dandelion (Taraxacum officinaiej— actually . a ring of fine hairs, called a pappus, attached to the seed—the downy fruit and style of virgin s-bower (Clematis sp.), and the hairy tufts of poplar (Popuius sp.) and willow (Salix

sp.) seeds. The fruits of ash (Fraxinus excelsior) and maple [Acer sp.) have wings that slow their fall and help to widen their distribution. Tumbleweeds (Salsola kali) are more radical seed dispersers. The whole plant is blown about, scattering its seeds as it rolls. The poppy [Papaver sp.) ripens its fruit on long stalks; when these dry out they are caught by the wind, and the seeds are catapulted out through holes in the fruit.

Some plants rely on water to disperse their seeds. The coco-de-mer Hodoicea maldivica) produces seeds that can weigh up to 45 pounds (20 kilograms) and that float with the help of trapped air. Water-repellent oils and chemicals also enhance buoyancy. But water can also assist a plant’s self-dispersal of seeds. The impact of a raindrop on the calyx of the sage, Salvia lyrata, flexes the delicate flower stalk, which, on regaining its undisturbed position, throws off the attached nutlets.

The bright colors of fleshy fruits attract birds to them. Their seeds have a hard casing that may need the animal’s digestive juices to act on them and change the structure of the seed coat so that germination is induced.

Specially weakened tissues in plants can be ruptured by high local water pressures, throwing the seeds in all directions. For example, the touch-me-not (Impatiens biflora) has pods that burst open at the slightest touch, scattering seeds everywhere. Excessive water absorption can also create torsions sufficient to release seeds, as in Vicia and Lupinus species. Torsions in the awns of storkbills (Erodium spp.) provide enough force for the attached seeds to bore themselves down into the ground. Certain plants have explosive fruits that, when ripe, rupture and release their seeds in a high-pressure jet of fruit pulp, as does the squirting cucumber (Ecballium elaterium).

Coconuts are the seed carriers of the tropical seashore palm tree Cocos nu-cifera. These fruits can weigh several pounds but are dispersed by the sea, floating on the water by means of the air trapped inside the shell. The seed’s germination may be delayed for up to two years, allowing enough time for the coconut to reach land.