How Fruits and Seeds are Dispersed
Dispersal of fruits and seeds can defined as the process by which the fruits and seeds are distributed or spread over a wide area.
When flowering is over and the seeds are mature the whole ovary or the individual seeds fall from the parent plant to the ground, where, if conditions are suitable, germination will subsequently take place. In many plants the fruits and seeds are adapted in such a way that they are distributed over considerable distances from the parent plant. This helps to reduce overcrowding among, and competition between members of the same species and results in the colonization of new areas.
The principal adaptations are those which favour dispersal by wind and animals. In addition, some plants have “explosive” pods or capsules that scatter the seeds, and others have fruits that are adapted to dispersal by water.
(a) Censer Mechanism:
Examples are the white campion, poppy and antirrhinum. The flower capsule with one or more openings in the capsule.
(b)”Parachute” Fruits and Seeds:
Clematis, thistles, Willow herb and dandelion have seeds or fruits of this kind. Feathery hairs projecting from the fruit or seed increase it’s surface area so much that air resistance to it’s movement is very great. In consequence, it sinks to the ground very slowly and is likely to be carried great distances from the parent plants by slight air currents.
(c) Winged Fruits:
Fruits of the lime, sycamore, ash and elm trees have extensions from the ovary wall, or leaf-like bracts on the flower stalk which make wing-like structures. These cause the fruit to spin as it falls from the tree and so prolong it’s fall, increasing it’s chances of being carried away in air currents.
Animals disperse seeds in different ways. First, some plants have barbs or other structures that get twisted together in animal fur or feathers, and are then moved to new sites. Other plants manufacture their seeds inside fleshy fruits that then get eaten be an animal.
(a) Mammals Hooked Fruits:
In herb bennet agrimony, burdock and goose-grass, hooks develop from the style, the receptacle, from the bracts round the inflorescence, or on the ovary wall. These hooks catch in the fur of passing mammals or in the clothing of people, and later, at some distance from the parent, they fall off or are brushed or scratched off. The mud adhering to hooves or feet may also carry seeds which are so dispersed by wandering birds or mammals, including man.
(b)Birds. Succulent Fruits:
Fruits like blackberry and elderberry are eaten by birds. The hard pips containing the seed inside are undigested and pass out with the faeces of the bird away from the parent plant.
Even if the seeds are not swallowed, the fruits is often carried away before the seeds are dropped, e.g. rose hip. Some seeds with a fleshy, sticky covering, e.g yew and mistletoe, stick to the birds beak and are discarded some distance from the parent plant, in the latter case often being wiped off on to a branch of the tree on which it will grow as a parasite.
The succulent texture and conspicuous color of these fruits may be regarded as an adaptation to this method of dispersal.
The pods of flowers in the pea family, e.g. gorse, broom, lupin and vetches, dry in the sun shrivel. The tough, diagonal fibres in the pericarp shrink and set up a tension. When the carpel splits in half down two lines of weakness, the two halves curl back suddenly and flick out the seeds.
Some types of exploding fruits, like the squirting cucumber,make use of built-up water pressure to expel the seeds, while others, such as violets, make use of the tension from the drying fruits to fling their offspring away.
A thick outer coat is present which traps air and makes the fruit or seed buoyant. Examples are the coconut and water lily.
Seeds which are dispersed by water are held in light and buoyant fruit, giving them the ability to float. Coconuts are also known for their ability to float on water to reach land where they then germinate. Similarly, willow and silver birches manufacture lightweight fruit that can float on water.
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