11th Class Biology Morphology of Flowering Plants (Root, Stem And Leaf) Dispersal Of Fruits And Seeds

Dispersal by wind (Anemochory)

The wind is probably the most important agency of seed dispersal in nature. The fruits and seeds show following devices which help in dispersal by wind.

Light weight and minute seeds : Seeds of some plants (e.g., Orchids) are sufficiently light and minute in size to be easily carried away to great distances by air currents.

Winged seeds and fruits : Some seeds (e.g., Oroxylon, Cinchona, Moringa) or fruits (Acer, Hiptage, Terminalia, Dipterocarpus) develop one or more thin membranous wings to ensure their dispersal by wind.

Parachute mechanism : In members of the family Asteraceae (Compositae) e.g., Taraxacum, Sonchus, sepals are modified into tufts of hairs called pappus. The pappus is persistent and hence found attached to even small, single seeded fruits. It acts like a parachute that allows the wind to carry them to great distances. Seeds of many nasty weeds are also dispersed by this method.

Censer mechanism : In Antirrhinum (dog flower), Aristolochia, Papaver (poppy), Argemone mexicana (Prickly poppy), Nigella (love-in-a-mist), etc. the fruit is a capsule. At maturity it ruptures but the seeds do not come out. However, when the capsule is shaken violently by the wind, the seeds are scattered in all directions. In this process all the seeds do not escape together.

Rolling mechanism : In some species, like Amaranthus albus, Chenopodium album, etc., plants dry out after bearing fruits and seeds. Eventually the entire plant breaks off at the base of the stem due to the force of wind and rolls over the ground, shedding the seeds all along the way. Such rolling plants are collectively known as tumble weeds.

Hairs : In cotton, hairs are the outgrowth from the seed coat and occur all along its surface.

Persistent styles : Clematis, Naravelia, Geranium etc. have persistent and feathery styles which help the fruit to be easily carried by wind.

Balloon like appendages : In plants like Cardiospermum and Nicandra fruits develop balloon like appendages which make the fruits light to be easily carried by wind.

Dispersal by water (Hydrochory)

Fruits and seeds, specialized for dispersal by water, generally develop some kind of floating devices and a protective covering which makes them water resistant. e.g., fibrous mesocarp in Coconut, spongy thalamus in Lotus.

Dispersal by animals (Zoochory)

Fruit and seeds dispersed by animals can be divided into following three categories on the basis of their adaptive features :

Hooked fruits and seeds : The surface of many fruits is covered with hooks (e.g., Xanthium, Urena), barbs (e.g., Andropogon), spines (e.g., Tribulus), bristles (e.g., Pupalia), or stiff hairs (e.g., Aristida), by means of which they adhere to the body of animals or clothes of human beings and they are carried unwarily from one place to another.                

Sticky fruits and seeds : Some fruits like those of Boerhaavia, Cleome, and Plumbago have sticky glands by which they adhere to the fur of grazing animals and are thus dispersed. Seeds of Viscum (mistletoe), Loranthus, etc. have a viscid layer which adhere to the beak of the bird which eat them.

Edible fruits : Human beings, birds, squirrels, bats, etc. are of great help in the dispersal of edible fruits from one place to another.

Dispersal by explosive or Spring like mechanism (Autochory)

A less common method of seed dispersal is by means of explosive fruits. Such fruits open with force and scatter the seeds in all directions. e.g., Balsam fruit (Impatiers), Oxalis, night jasmine (Nyctanthus), castor (Ricinus), camel's foot climber (Bauhinia vahlii).