DISPERSAL OF FRUITS AND SEEDS | DON STEVE BLOG
March 28, 2024

After the maturation of fruits, an abscission layer is formed at the point of attachment of the fruits to the stems. This layer later dries up and the fruits drop from the plant. Some times the fruits split up to release the seeds before the fruit-walls finally drop. The pericarps of indehiscent fruits have to decay off before their seeds are exposed to conditions necessary for germination. Supposing that all the fruits and seeds formed in a plant dropped to the ground just below the parent plant,and that all the seeds germinated, the area would easily become overcrowded. Very serious competition will set in between the young plants and their parents. The plants compete for nutrients, water,light and space. The young plants are likely to die soon after. To avoid such adverse situations, the fruits and the seeds of many plants have got certain means by which they can be carried to different directions, some distance from the parent plants.

The scattering of fruits and seeds to different directions from the parent plants is known as dispersal.

 

Importance of Dispersal of Fruits and Seeds

The dispersal of fruits and seeds have the following importance or advantages:

  1. It helps to prevent overcrowding of plants.
  2. It prevents undesirable competition for space, light, water and nutrients.
  3. It prevents diseases from spreading among plant species.
  4. It results in colonization of new areas with new plants.
  5. It results in adaptation to adverse or harsh or unsuitable environment.

 

Agents of Dispersal of Fruits and Seeds

Agents of dispersal of seeds and fruits refer to any means by which seeds and fruits are removed from parent plants to other places.

Fruits and seeds can be dispersed:

  1. By wind.
  2. By animals (including man).
  3. By water.
  4. By explosive mechanism or explosion.

 

Fruits and seeds are especially adapted to suit their dispersal by any of these agents. It is either the fruits which are dispersed while the seeds are still enclosed in them, or for the dehiscent fruits, the fruits break up to liberate the seeds which are dispersed alone. Their various structural adaptations are designed to carry them over a short or long distance from the parent plants.

 

A) Dispersal by wind

For fruits and seeds to be dispersed by wind they must have the following characteristics:

  • They are generally small.
  • They are generally light.
  • Where they appear heavy they have some large and light membranous wings which make them buoyant in the air.
  • They may also possess some long hairy or thread-like structures which act as parachutes in the air.

 

Fruits dispersed by wind

These include Tridax, Emilia, goat-weed, Combretum, Obeche and Bougainvillea. In Tridax there is a tuft of hairy pappus formed from a persistent calyx. The hairy pappus and the one-seeded fruit formed from an inferior ovary,behave very much like a parachute in the air. The fruit can be parachuted to a very long distance from the parent place by the action of wind. In Combretum the pericarp has become extended into dry, light membranous wings which propel the one-seeded fruit in the air to some distance away from the plants.

 

Obeche has a single elongated wing with the fruit at one end, while in Bougainvillea, the wing is formed by a persistent bract.

 

Seeds dispersed by wind

These include cotton seeds, Tecoma, Plumeria and Neubouldia. In cotton, the seeds are surrounded by a mass of floss which makes them light and can easily be carried by wind. In Tecoma, Plumeria and Neuboldia, the seed testa become extended into membranous wing which can also be blown about by the wind and carried many meters away from the parent plants.

 

B) Dispersal by animals

There are four ways by which fruits or seeds can be dispersed by animals.

1) Fruits with hooks

The walls of certain fruits possess some hooks which can fasten them to the furs of animals or clothes of man passing through the bush. Such fruits will later fall off their bodies several meters away from the parent plants.

Examples of fruits with hooks are Urena lobata, Bidens, Triumfetta and Acanthospermum hispidium.

 

2) Fruits with sticky hairs

These are fruits whose walls possess some sticky hairs which can stick to the furs of animals or clothes of human being. Examples of fruits with sticky hairs are Boerhavia diffusa and Desmodium.

 

3) Seeds swallowed and passed out with faeces

Many birds and cattle eat certain fruits whose seeds are swallowed. Such seeds possess hard and thick seed coats which can resist enzyme digestion in the guts. The seeds are therefore passed out with faeces several kilometres away. They germinate if conditions are suitable. Examples are tomato, pepper and seeds of many grasses, such as Sporobolus species and Eluisine indica.

 

4) Fruits and seeds transported by man

Man is a very important agent of fruit and seed dispersal. The distribution of farm products to different parts of the world market is a kind of dispersal. These products are further dispersed to different homes by buyers and the fruits or seeds which are not immediately eaten may be re-cultivated. Sometimes the fruits are eaten after which their seeds are thrown away into a nearby bush. Examples are mango, orange, palm fruits, tomato, and pear. The fruits and seeds of many weeds are mistakenly collected and transported along with food crops from the farm. Such weeds then have a chance of reaching new areas where they are not naturally found. They may become easily established in their new environments. Amaranthus spinosus, goat weed, Eupatorium and many grasses belong to such group of weeds.

 

C) Dispersal by water

For fruits and seeds to be dispersed by water, they must possess a floating device to prevent them from sinking. For this reason not many fruits and seeds are dispersed by water. Seeds of water lilies and fruits of coconuts are few examples of fruits and seeds dispersed by water. Seeds of water lilies have air spaces which enable them to float for sometime in water until they are brought to the shore by waves. The fibrous mesocarp of coconut offers a good floating device to the fruits. They can float for up to one hundred days covering many kilometres away from the parent coastal plants, still retaining their ability to germinate if carried offshore by tidal waves. Tomato seeds are also carried away by run-off waters through gutters and gulleys to plain lands where they germinate.

 

D) Dispersal by explosive mechanism

When the fruit is getting ripe, it begins to dry up. The drying causes strains on the pericarp which eventually splits violently with sound along the two lines of weakness flicking out the seeds to a considerable distance from the parent tree. Explosive mechanism is common in leguminous fruits such as oil bean, Pride of Barbados and Flamboyant. Other plants include the para-rubber, mahogany and balsam.

In most of the leguminous fruits, explosive mechanism is accompanied with the twisting of the fruit-wall due to the high tension set up inside the dry walls. The explosion is particularly loud in the oil-bean fruits whose seeds are very large and heavy. The seeds can be thrown to so many metres away from the parent plant.

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