Dispersal of Fruits and Seeds

 

The seeds or fruits where seeds are enclosed must be dispersed a certain distance from the “mother plant” so that they can produce effective seedlings. The reason for doing this is to avoid competing for the necessary resources such as water, nutrients or sunlight. 

Self dispersal mechanism (Autochory)

 Some fruits at maturity burst with a jerk and their fruit wall opens suddenly. Then the seeds are scattered here and there. This mechanism is also called explosive mechanism. In legumes, e.g. Abrus, pea (Pisum sativum), the fruit breaks into two valves, which twist spirally to throw the seeds. In Geranium, the ripe fruit breaks into five one seeded parts (cocci), each with one style. The styles bend upwardly and throw the seeds away. In case of balsam, when the fruit matures it bursts suddenly and the fruit wall splits into two valves, which roll up inwards, as a result the seeds are ejected with a great force and are dispersed to distant places. Dry and capsular fruits of Ruellia also burst suddenly, when they come in contact with water, i.e. after a shower of rain. The seeds are provided with jaculators or curved hooks, which are hygroscopic in nature. Getting water they straighten out suddenly and seeds come out in a jerk with a loud noise. 

Dispersal by Wind (Anemochory) Wind is one of the best carriers of seeds to distant places. Such seeds and fruits are having some morphological adaptations like wings, hairs, pappus etc., which are helping in dispersal mechanism. They are described as follows:

 (a) Light weight seeds: Very small, dry seeds of orchids and grains of grasses are easily carried by wind because they arc very light in weight. 

(b) Wings: The wing like membranous outgrowth from the pericarp of fruits and seeds help them to float in air and are carried to distant places, e.g. Acer, Dipterocarpus, Hopea, Shorea (fruits), Tecoma, Moringa (seeds). 

(c) Parachute mechanism: The fruits of family Asteraceae have persistent sepals modified into hairy structures called pappus. The fruit is known as cypsella. When the fruit matures, it opens in an umbrella like fashion and becomes very light due to pappus and float in air current easily. The hairy pappus is hygroscopic in nature. Whenever the fruits with pappus happen to pass through humid areas, they come down just like parachutist landing on the ground. Here, this mechanism is called parachute mechanism, found in sunflower (Helianthus), marigold (Tagets), etc. 

(d) Hairs: In Alstonia, madar (Calotropis) and cotton (Gossypium) a dense coating of hairs covers the testa of seeds. These hairy outgrowths help the seeds to float in air.

 (e) Censor mechanism: It is a common method of dispersal in the many seeded fruits like legumes, siliquas and capsules. After maturity when the fruits are violently shaken by strong wind, they burst and the seeds are thrown to a longer distance from the mother plant, this mechanism of dispersal is known as censor mechanism, e.g. prickly poppy (Argemone), thorn apple (Datura), mustard (Brassica), Aristolochia, etc.

 (f) Persistent styles: In Clematis, the fruit has persistent feathery style. It helps in dispersal by air. 

Dispersal by Water (Hydrochory) The fruits and seeds of aquatic plants or plants growing by waterside, need to be carried by water current to a long distance. In such plants the seeds and fruits develop floating devices in the form of spongy or fibrous coats, water-proof or buoyant nature of coats etc. In lotus (Nelumbium nucifera) the etaerio of achenes is embedded in spongy thalamus helps the fruits to float on water. Seeds of many aquatic plants like water lily (Nymphaea), Alisma, etc. are small and light and they can float on water due to the presence of aril, which contain air between the seed coat and embryo. The seed coat is impervious to water, hence protects the embryo inside. Then the latter germinates in favourable conditions at proper place. The fruit of coastal plants like coconut (Cocos nucifera), betel nut (Areca catechu), Devices for animal dispersa.

Dispersal by water etc. has a fibrous mesocarp that is a floating device. The hard endocarp protects the embryo from external injury; so the fibrous fruits are capable of floating long distances. The double coconut (Lodoicoa maldivica) is native to the Seychelles Islands, an island nation off the east coast of Africa in the Indian Ocean. Its fruits reach even to the coastal regions of India.

 Dispersal by Animals (Zoochory) The seeds and fruits, which are carried by the animals including human being, either develop certain structural peculiarities to attract the animals or carried by them unknowingly or they stick to the body of the animals automatically and are thus dispersed. 

(a) Forced zoochory: In this case, the fruit and seeds which are non-edible produce some special devices, which help them to stick to the clothes of men or the body of various animals

(i) Hooks and spines: The fruits and seeds fall on the ground are carried by birds and animals visiting these places. The presence of special devices like hooks, spines on the seed and fruit help them to cling to the fur of birds and animals and are carried to distant places. For instance, fruits of Xanthium and Urena bear curved hooks; Tribulus has sharp and rigid spines.

 (ii) Sticky glands of fruits and seeds: The sticky glands present on the surface of the fruits and seeds, help them to stick to the body of animals and are thus dispersed. Examples are Boerhaavia, Cleome, Datura, etc. 

(b) Compensated zoochory: The fruits have edible pulp. The seeds, if bigger, are thrown away, e.g. mango, apricot. If smaller, the seeds are eaten along with fruits, e.g. tomato (Lycopersicon esculentum), mulberry (Morus alba), guava (Psidium), fig, etc. The small seeds usually come out unharmed from the alimentary canals of animals. As the animals move from place to place, the seeds are also dispersed. It is the top method of dispersal as it is very sure and specific while other modes of dispersal are indiscriminate and less sure. 

(c) Dispersal by man: Man is the most important agent in the dispersal of seeds and his role can be seen in agriculture, horticulture and forestry. He is an agent of dispersal of new varieties of seeds into new areas, which are of economically