Dispersal of seeds means how fruits and seeds disperse to distant places. Dispersal is very much essential for the new plants to grow. If they do not disperse, they will directly fall below the plant and germinate. New plants will grow below the mother plant. Due to this they will not get food and population of plants will be concentrated at one place.
There are four methods by which fruits and seeds disperse. They are—
1. Dispersal by wind
2. Dispersal by water
3. Dispersal by animals and
4. Dispersal by mechanical means.
1. Dispersal by Wind : The fruits and seeds dispersed by wind have basic adaptations. These adaptations help in their dispersal. These are—
(a) Size and weight
(b) Wings
(c) Hairs
(d) Pappus (parachute mechanism)
(e) Censor mechanism
(f) Baloon shaped appendages
(g) Persistent styles.
(a) Size and weight : Fruits and seeds light in weight and small in size are dispersed by even breeze. Light weight and thin fruits and seeds can be carried away by wind currents from one place to another, e.g. Orchids.
(b) Winged fruits : Seeds with wing structure and fruits are carried away to distant places by wind.
Three winged—Moringa
Two winged—Cinchona
One Winged—Oroxylum.
(c) Hairs : Some seeds have a dense coating of hairs for their dispersal by wind, e.g. Calotropis.
(d) Pappus : Some fruits have parachute mechanism. Cypsela fruits have persistent modified hairy calyx. This is called pappus. It opens like an umbrella, and so it is easily carried away by wind e.g. Tridax.
(e) Censor mechanism : Seeds of certain fruits come out from minute apertures of porous capsule. These pores are very small and the seeds can swing in the wind. These seeds are dispersed by wind.
(f) Balloon Shaped Appendages: Balloon shaped appendages make fruits and seeds to float on air for longer periods. This makes the dispersal by wind.
(g) Persistent Styles : Some fruits have feathery structure and make the fruit easily dispersable by wind, e.g. Clematis.
Dispersal by Water : Fruits and seeds dispersed by water are made buoyant by spongy structures, e.g. Cocos, Nymphaea, Nelumbium.
Dispersal by Animals : Dispersal of seeds and fruits by animals is aided by appendages like hooks, barbs, spines, bristles, stiffy hairs or sticky secretions.
Hooks : Fruits of Xanthium are covered with large number of curved hooks. They get themselves attached to the skin of animals or tails or clothes of human beings, and are carried to distances. Seed of Martynia has two very sharp pointed stiff and bent hooks.
Stiff hairs : In Aristida seeds have a cluster of stiff hairs pointed upwards.
Bristles : In Pupalia the perianth bears clusters of hooked bristles.
Sticky secretions : Some fruits have sticky glands. So they stick to the bodies of grazing animals and are carried away to long distances.
Fleshy and edible fruits : There are fruits eaten by animals and their seeds pass through their alimentary canal without any injury. These seeds are dispersed through animal excreta at far off distances away from their place of origin, e.g. Fruits of guava, papaya, mango, custard apple etc.
Explosive Mechanisms or mechanical devices : Many fruits burst with a sudden jerk and as a result the seeds get dispersed to a little distance.
Capsule of Abelmoschus esculentus, regma of Ricinus communis explode on maturity and disperse seeds. In Dolichos and Abrus mature fruits suddenly twist on bursting to scatter seeds. Legumes of Bauhinia vahlii explode with large noise scattering seeds in all directions. Fruits of Impatiens balsamina (balsam) explode.
The fruit of Ecballium (squirting cucumber) is turgid. The inner content of the fruit remains under pressure in elastic pericarp. When the fruit detaches from the stalk, a hole is formed and inner contents are dispersed to a distance.
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