The dispersal of seeds as well as fruits takes place by wind, water and animals. Sometimes there may be some specialized mechanism of spore dispersal. Now we are going to have a brief description about them:
DISPERSAL BY WIND
The dispersal of seeds by the wind takes place in the following way:
The orchids and grasses seeds are minute in size and weigh around 0.004 g, hence they can easily be dispersed by wind.
The seeds are provided with wing like projections, which are easily carried by wind. e.g. Jacaranda, Moringa, Oroxylum etc. Similarly the fruits of Acer, Hiptage, Terminalia, Shorea also have wing like projections. So dispersal of seeds are easily made by those wings.
The single seeded fruits of members of family Asteraceae hold hair like persistant sepals called pappus. They carry the seed to great distance in the form of a parachute, e.g. Taraxacum.
In these plants the fruits are represented by capsule, e.g. Antirrhinum, Aristolochia, Argemone Mexicana. This capsule ruptures on maturity, but the seeds do not come out. When the fruit is shaken forcefully with the help of wind, the seeds come out in small aggregates and this mechanism is called censer mechanism.
In certain weed plants like Amaranthus and Chenopodium, the entire plant gets dried up and due to the force of the wind rolls over the ground in the form of tumble weeds and thus the dispersal of seeds and fruits are made.
The hairs are specialized epidermal trichomes arising from the seed coat, which carry the seed through longer distances. . In cotton, the hairs surround the entire seed surface, in Asclepias, it remains as a tuft of hairs on one end of the seed.
In plants like Clematis and Naravelia, the style remains, persistent, it is hairy and helps in dispersal of seeds.
Balloon like appendages
In certain plants, the floral parts surrounding the ovary become swollen. In Cardiospermum, the capsular fruit becomes swollen. In Tilia americana, the fruit is attached to a thin leafy bract, which keeps the fruit floating in air.
DISPERSAL OF SEEDS BY WATER
This method of dispersal of seeds is observed in aquatic plants and they may be of the following types:
The fibrous mesocarp of coconut and double coconut act as a layer impermeable to water, which helps the fruit to float in water.
The seeds of Nelumbo are rooted in the spongy thalamus, which floats in water and helps in the dispersal of seed over long distance.
The plants like water-lily contain seeds with air-filled aril. These seeds are small and light and can float in water or even dispersal of seeds may be done by rain water.
DISPERSAL OF SEEDS BY ANIMALS
The dispersal of seeds by animals are brought about by specialized devices like hooked fruits and seeds, sticky fruits and edible fruits.
Hooked fruit and seed
Some fruits and seeds have hook-like projections, bristles, hairs, spines, which helps them to get attached to the animal skin, hoofs or body parts and after being dispersed, they are dropped off. They include Bidens, Xanthium, Martynia, Aristida etc.
Sticky fruits and seeds
The plants like Cleome, Boerhavea etc. produce sticky fruits or their seeds are surrounded by a sticky pulp. These fruits adhere to the beaks of birds or hoofs of animals and after being transported for some distance, and then germinate at a different place.
These edible fruits are fleshy and juicy in nature. They pass through the intestine of birds and animals. These plants include oak, chestnut etc.
Transfer by human agency
Human agency can also be liable for the transfer of certain crops from one country to other.
SPECIAL TYPE DISPERSAL OF SEEDS
Sometimes the fruits perform specialized dispersal methods, such as the explosive mechanism. In balsam and Geranium, the fruit bursts open with a great force and seeds are dispersed at a greater distance. In Barleria, the fruit bursts open into two valves and seed are projected with the help of a hook like structure called jaculator. It opens with a jerk and the seed is dispersed at a great speed.