Seed Dispersal

Seed Dispersal

Seed dispersal refers to the processes by which mature seeds disperse from the parent plant. Dispersal decreases competition with the parent and increases the likelihood of finding a suitable environment for growth. Sexual reproduction generally results in the production of fruits whose sole purpose is to enable the species to disperse and multiply. The part of the plant that acts in the dispersal is the diaspore (a term incorporating both fruit and seed). Although diaspore dispersal is the obvious end of reproduction, some plants rarely flower or set fruit and instead have evolved a very efficient system of vegetative reproduction by means of sucker shoots. Vegetative reproduction is very common in herbaceous plants that may spread by stolons , bulbils , or stem suckering. Still, most plants that reproduce vegetatively also reproduce sexually since this enables them to remain genetically variable and more adaptable to changes in the environment.

Wind Dispersal

The simplest form of seed dispersal is by wind and, not surprisingly, wind-dispersed fruits in temperate areas usually develop in breezy spring months. The same species that are wind-pollinated in temperate areas often bear wind-dispersed seed such as maple (Acer in Aceraceae), willow and poplar (Salix and Populus in Salicaceae), and ash (Fraxinus in Oleaceae). Typically the wind-dispersed seeds are developed quickly and dispersed in the same season. Wind-dispersed tree species are numerous in the warm, moist forests of the tropics—especially for tall trees in areas where there is a slight to prominent dry season. The height of the tree is important to enable the diaspore to catch the wind currents. The shape of wind-dispersed diaspores is often critical to their dispersal as well. Maple seeds have a samara and set up a whirling pattern as they fall, which may assist them in implantation. Poplar and willow seed are borne in a loose, cottony mass, which is extremely buoyant even in weak air currents. Although different morphological structures have evolved to disperse tree seed, the most common form of seed dispersal is wind-dispersed.

Typically, wind-dispersed species in tropical areas with seasonally dry periods lose their fruits late in the dry season or in the early rainy season that follows. This ensures adequate moisture for germinating seeds and adequate establishment before the next dry season. Wind-dispersed seeds have the distinct disadvantage of being at the peril of the elements. Most do not get carried very far away from the mother plant, and the population of insects that feed on the particular plant increases greatly at the time of flowering and can often destroy much of the crop. Some tropical species successfully avoid this by fruiting irregularly or even by what is known as mass fruiting, in which hundreds of individuals somehow manage to flower all at one time, literally swamping the predator population with more food than it can eat and thus preventing the insects from eating all the fruit.

The manner in which wind-dispersed diaspores are released is often critical to their dispersal. Because wet seeds do not float well on the air, most do not disperse except when the capsule is dry. Seeds are often contained within the capsule walls, and the valves of the capsule open increasingly further with only the uppermost seeds being capable of being blown free.

The same capsules may release seeds not from wind alone, but in part by mechanical motions and the inertia built up by movements of animals passing through a population. Each time a plant is bumped more seeds are cast away from it. This is a short distance but common type of seed dispersal in many prairie and forest edge plant populations.

Animal Dispersal

Animal-dispersed fruits are more common than wind-dispersed fruits and occur in species with a wider variety of life forms, including herbs, many vines, a modest number of tropical lianas, and shrubs as well as some trees. The morphology of animal-dispersed fruits varies depending on the organism doing the dispersal. The animals vary from those as small as ants to as large as horses or elephants.

Both birds and mammals are very effective dispersers. Birds are particularly effective dispersers since they can move the diaspore the farthest and the fastest. Diaspores dispersed by birds are usually colorful and lack any obvious scent (birds have keen vision but a poor sense of smell). Often the fruits feature contrasting colors so they are more easily seen. Frequently the outer covering of such fruit might be green or brown, but when the fruit opens the inner surface is bright red with a black seed. Often birds eat only the sweet portion of the diaspore and spit out the seed. If eaten, most seeds pass rather quickly through the bird's system and are ejected. Many times tiny colorful berries, such as those of Anthurium (Araceae), are initially quite sweet but quickly turn bitter after being eaten to encourage rejection. Anthurium also produces seeds with a sticky appendage that causes the seeds to stick to the bird's bill.

Mammal dispersers are common in both temperate and tropical areas. Mammal-dispersed diaspores are usually not particularly colorful but may be tasty and even have a distinct aroma when mature. (Mammals have only average sight compared to birds but typically have a good sense of smell.) Squirrels and rodents in temperate regions gather and hoard oak and hickory fruits while tropical agoutis "scatter hoard" fruits by burying them seemingly at random on the forest floor. Those fruits that are not found later in the late rainy season—when fruit is rare—are already planted and ready to grow. Monkeys in the tropics have a diverse diet with a broad array of fruits that are dispersed by them. They are amazingly adept at dispersal, but seemingly wasteful since they gather many fruits, eat part of them, and then discard the remainder along with the seeds. Some less common animal dispersers are horses, which are known to eat and disperse the seeds of calabash (Cresentia cujete in Bignoniaceae) in Central America.

In the case of epizoochorous fruits, animals are responsible for dispersing fruits without actually consuming them. These are diaspores that attach themselves to fur or clothing. Among the most effective types are beggar's ticks (Bidens in Asteraceae), tick-trefoil (Desmodium in Fabaceae), and Queen Anne's lace (Daucus carota in Apiaceae). These fruits are difficult to avoid and are difficult to remove, so they are usually picked off and discarded far from where they were first encountered.

Mechanical Dispersal

Mechanically dispersed seeds are common in both temperate and tropical areas. Many legumes (Fabaceae) have fruits that dry under torsion, and are suddenly released when the two halves of the fruits fall apart. In this instant the two halves of the valve twist laterally and sometimes also longitudinally, which causes the dry seeds along their length to be thrown for considerable distances. One of the most remarkable mechanically dispersed seeds is that of Hura crepitans (Euphorbiaceae), which is made up of a series of pie-shaped segments that burst open with such force that it sounds like a rifle shot. Its small flat seeds are carried for great distances.

Water Dispersal

Water dispersal is quite effective in estuarian populations of plants. The nature of water-dispersed fruits is important since a seed that lacks buoyancy would sink to the bottom near the mother plant and have to compete with it. A diaspore that was too buoyant would perhaps never sink at all and thus might never be implanted. Urospatha, a tropical aroid, has fruits with seeds that are embedded in a thick, buoyant, gelatinous mass, which allows them to float for a period and then sink into the water. The seeds of some tropical trees that occur along water courses are known to be consumed by fish. It is not yet known, however, whether the movement of the fish are important to the dispersal of the seeds.

Seed germination and the establishment of the young plant is, of course, the only true sign of reproductive success. Dispersal without establishment is to no avail. In every case the rate of germination is critical. Many dias-pores do not fall into the proper situation for germination. Often large numbers of seeds are killed by a wide variety of beetles or weevils that specialize on seeds. Different species have developed various methods of survival. Some, such as orchids, produce thousands of minute seeds per capsule, giving some a good chance of success. Other species use the opposite strategy of producing large and heavy fruits with a lot of stored food material to ensure survival after germination. Some species, such as the seeds of the Beilschmiedia in the Lauraceae, have an increased chance of survival by having the seeds begin the germination process while still on the trees, where they are less susceptible to attacks. The red mangrove Rhizophora mangle (Rhizophoraceae) goes even further by actually establishing a young plant on the tree that has a pointed base that actually implants in the soil when it falls.

see also Germination; Interactions, Plant-Vertebrate; Seeds.

Thomas B. Croat

Bibliography

Van der Pijl, L. Principles of Dispersal in Higher Plants. New York: Springer-Verlag, 1969.