Seeds And Dispersal Mechanisms - Angiosperm - Britannica

Mechanisms of dispersal

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Fruits and seeds are the primary means by which angiosperms are dispersed. The chief agents of dispersal are wind, water, and animals. Some fruits and seeds have modifications that aid in wind dispersal. Fruit modifications include samaras, samaroid schizocarps, and the feathery calyx lobes (e.g., dandelion). Seeds may be modified in various ways to promote dispersal: they may be extremely small and light (e.g., orchids, Orchidaceae), winged (e.g., common catalpa, Catalpa bignonioides; Bignoniaceae), plumed (e.g., milkweed), covered with woolly hairs (e.g., willows), or surrounded by explosive capsules that forcefully eject them into the air, as, for example, the touch-me-not (Impatiens; Balsaminaceae) and the witch hazel (Hamamelis; Hamamelidaceae). The fruits or seeds of many aquatic and shore plants are adapted to float on water as a means of dispersal; for this reason, coconuts (Cocos nucifera; Arecaceae) are readily transported across oceans to neighboring islands. Adaptations for water dispersal include aerenchyma in fruits or seeds and light weight (e.g., water chestnut, Trapa natans; Lythraceae).

Animals disperse fruits and seeds either by ingesting and subsequently excreting them or by passively transporting them once they have adhered to an external part of the body, such as the fur or a claw. The evolution of fleshy fruits and seeds exemplifies the coevolution of plants and their animal agents of dispersal. An animal diet often consists solely of fruits and seeds that are designed to be eaten and dispersed, and in many cases these seeds require full or partial digestion to stimulate germination. Most fruits with a fleshy pericarp are eaten whole by vertebrates, including the stony endocarp or the stony seed coat. The seeds then either are regurgitated by the animal or pass through the alimentary canal and are excreted, often some distance from the original site. Seeds with an aril that encloses a stony seed coat or seeds with a sclerotesta and a fleshy, colored sarcotesta are found in dehiscent fruits. They are eaten by animals after the fruit has ripened and split open. Often these seeds dangle from the fruit by long stalks (e.g., the follicles of Magnolia). The fleshy portion, whether originally a fruit or seed, is brightly colored and sweet so as to attract vertebrates, particularly birds and mammals. Many fruits and seeds in the Amazon, however, are actually eaten and dispersed by fish during times of high water.