Seed dispersal is the term coined for the movement of the seed from one place to another. This movement generally occurs with the help of dispersal vectors. Dispersal vectors can be biotic or abiotic components. Biotic dispersal vectors include animals, bees, and other insects whereas abiotic factors are wind, and water.
The need for dispersion arises because plants need to transport their seed far from the parental plant, this is done to reduce the intraspecific competition between the parent and progeny. The pattern of seed dispersion is greatly dependent on the dispersal vector that is being used. The following are the main types of dispersion mode – gravity, wind, water, animal, and animals. The dispersal unit of the plant is known as the diaspora, it includes seeds and additional tissue that assists in the seed dispersion.
Seed dispersal can be broadly classified into autochory and allochory. Autochory refers to the dispersion of the diaspore without any help from the external environment, whereas allochory refers to the dispersion of seed with the help of dispersal vectors. Ballistic and dispersion through gravity comes under the autochory mode of dispersion. Ballistic dispersion refers to the condition when a seed is ejected by the dehiscence of the fruit.
Seed Dispersal by Animals and Birds
Animals and birds disperse the seeds in different manners. Colourful fruits attract animals and birds. When they eat these fruits only the pulpy part is digested in their bodies and the seeds are excreted out by their systems in the form of their dropping, which eventually gives birth to new plants. Blackberry, cherry, tomato and apple seeds are usually dispersed in this way.
There are some species of squirrels that collect nuts from different plants like acorns and bury them under the ground as they store food for the winter season, these squirrels usually forget the place where they have previously buried the nuts and the seeds germinate into new plants.
Some plants bear seeds with hooks. A common example for such a type of plant is the Burdock plant. The seed of these plants gets attached on the fur of animals with the help of these hooks and are carried away to different places, far from their parent plants.
Examples of plants having seeds dispersed by animals are Dates, rambutan, sea grapes, sea holly, tamarind, raspberry, sunflower, tomatoes.
Dispersal by Animals
Seed dispersal by animals comes under the category of the allochory. Dispersal by the animal is termed as zoochory, zoochory. It can be further classified into saurochory, ornithochory, and, myrmecochory, which means dispersion by reptiles, birds, and ants respectively. Based on the way of carrying the diaspore zoochory can be classified as endozoochory, epizoochory, and synzoochory.
Endozoochory:
It is the condition when seeds are carried within the dispersal vectors, which are animals in this case. It is an example of mutualism between plants and animals. In this type seed is generally inside the fruit, animals that perform dispersion eat the fruit surrounding the seed and discard the seed at a distant location, thus performing dispersion. The plant provides fruit to meet the nutritional requirements of the animal whereas the animal helps in plant survival by allowing it to grow in various locations, thus both the species mutually benefited from the seed dispersal by animals.
Epizoochory:
It is the condition when an animal carries the diaspore outside its body. This condition arises in the case of accidental dispersion of the seed by animals. When animals or birds pass by the seed some of the seed might get attached to the body of the animal and move along with it, diaspore than generally falls at a distant location from the parental plant.
Synzoochory:
It is a condition when the diaspore seed has not completely entered the alimentary canal of animals rather they carry the seed in their mouth. Synzoochory is often seen in seed dispersal by squirrels. It is also done by rodents and birds. Synzoochory has a characteristic feature, the seed is generally a nut that is used as a food source by the animals.
Diplochory is also considered as a type of syn zoochory where it is indirect or two-step dispersion by an animal, this occurs when the nut is the food source of more than one species. It is a highly effective way of dispersion.
Seed Dispersal by Water
In the plant kingdom, water is natural and one of the fundamental sources of seed dispersal. In this mode of seed dispersal, seeds float away from their parent plant in rivers or ponds due to their light weight. Plants which grow near water bodies usually rely on water to transport their seeds to different places. The seeds of such plants are light in weight and thus float, or there may be fluff that helps buoyancy. This method of dispersal is mainly seen in those plants which are found in water or nearby the water bodies like sea shores, lakes, ponds etc. Some examples of plants whose seeds are dispersed by the water are Coconut, palm, mangroves, water lily, and water mint.
Characteristics of Seed Dispersal by Water:
There are some characteristic features of hydrochory they are as follows,
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Diaspore must be lightweight and hollow
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Diaspora can swim
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They generally have long filaments
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They generally do not produce nectar
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They are not usually very colorful
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They must have the property of hygrochasy, that is seed can germinate in moist weather.
Example of Seed Dispersal by Water
There are some of the following examples of water dispersal of seeds,
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Mangrove plants are the best example of hydrochory.
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Aster tripolium and Juncus species are some examples of hydrochory where seeds are transported to distant areas
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Mesembryanthemum, Sedum, and other plants show characteristic features of hygrochasy.
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Sea dispersal of the coconut palm is the best-studied common example of hydrochory.
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Some other examples of plants that use water for seed dispersal are water plantain, yellow flag, sea kale, sea rocket, sea beet.
Seed Dispersal is an adaptive mechanism that can be seen in all seed-bearing plants, it can be defined as a method by which seeds move
away from their parent plant in order to germinate and survive at a different place. There are many vectors/modes of transportation of the seed from one place to another.
Types of Seed Dispersal
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Seed Dispersal by Wind
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Seed Dispersal by Water
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Seed Dispersal by Animals and Birds
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Seed Dispersal by Gravity
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Seed Dispersal by Explosion