Methods of Dispersal of Fruits and Seeds: 4 Methods

The following points highlight the four major methods of dispersal of fruits and seeds.

The four methods are: (1) Dispersal by Wind (2) Dispersal by Water (3) Dispersal by Animals and (4) Mechanical Dispersal.

Method # I. Dispersal by Wind:

For easy dispersal by wind seeds have to be light so that their buoyancy may enable them to float on air over long distances. Seeds specially adapted for wind dispersal are characterised by the following:

1. Very small, dry and dusty seeds as those of orchids (weighing as little as 0.004 mg in some cases—Fig. 460) are carried by wind like pollens. Seeds of Cinchona are also ex­tremely small and at the same time winged.

2. Parachute mechanism:

Certain seeds and a few fruits are provided with appendages which act like parachutes in helping them to float in air.

Among such app­endages one may consider:

(i) Pappus (Figs. 318 & 461) formed by the modification of the persistent calyx in Compositae and the allied families Dipsaceae and Valerianaceac.

This pappus is of different appearance in different species and in many cases it is large and light enough to keep the cypsela fruit floating in air for a long time.

(ii) Coma (Fig. 462) is a tuft of hair developed as a crown on the seeds of Calotropis, Holarrhena, Alstonia (two tufts) and most plants of Apocynaceae and Asclepiadaceae.

(iii) Hairy outgrowths on the testa (Fig. 463) completely cover cotton seeds. Such hair develops from the inner walls of the silk-cotton pod.

(iv) Persistent hairy styles (Fig. 464) of the achenes of Clematis, Naravelia (Fig. 455A), Anemone, etc., help them to float in air.

(v) Balloon type inflated persistent calyx of Physalis enables the entire fruit to float on air for some time. The same purpose is served by the inflated ovaries of the pod of Colutea arborescens (Leguminoseae) and the capsules of the balloon vine (Cardiospermum halicacabum) and also a few other plants of Sapindaceae (Fig. 465).

3. Wings are developed on fruits and seeds of different types (Fig. 466) rendering them buoyant:

(i) Seeds of Moringa oleifera, Oroxylum indicum, Lagerstroemia speciosa, Swietenia mahogoni, Cinchona, etc., are provided with wings developed from the testa.

(ii) Samaroid fruits of Shorea robusta and some other members of Dipterocarpaceae have persistent sepals forming wings.

(iii) The samara fruit of many plants like Dioscorea, Fraxinus (ash), Acer (maple), Hip- tagi, etc., have wings developed by the fruit pericarp.

4. Censer mechanism:

In certain fruits the seeds are discharged through small openings on the fruits. The outlets are so narrow that only a few seeds can escape at a time.

Thus, in Argemoru mexicana and the poppies (Fig. 467) of Papaveraoeae, after the apertures are opened by porous dehiscehcie, as the capsule swings in air, the minute seeds are dispersed through the pores.

A similar type of dehiscence may be seen in the peculiar pelican-shaped flowers of Aristolochia gigas. Seeds escape from the follicles of Aconitum in the same restricted manner.

The capitulum of sunflower, in which the seeds are heavy and not provided with pappus, shows a censer mechanism in the restricted escape of seeds when it is swung by wind.

5. A rather interesting type of wind dispersal is shown by certain weeds like Salsola kali tenuifolia which grow on sea coasts. When the plant matures, the branches become incurved giving the plant a spherical form.

The main stem now breaks and the plant laden with ripe fruits and seeds is rolled like a ball miles after miles by the strong wind. Seeds are dropped all along this route.

Method # II. Dispersal by Water:

Dispersal by water is of great convenience in fruit trees growing by water-side. This is of special importance in seashore plants like the coconut which has migrated thousa­nds of miles in the Pacific and Indian Oceans floating on the sea waves from island to island.

Such fruits must be pro­vided with a coat which is simultaneously waterproof, salt-resistant and buoyant. The dry fibrous mesocarp and the hard endocarp of coconuts render them specially suitable for this type of dispersal.

Similar properties are possessed by the fruits of some other palms like Areca catechu and Nypa fruticans and those of Cerbera odollam of Apocynaceae (Fig. 468), Pandanus (Pandanaceae) and also of a few leguminous plants. Gulf streams often carry seeds and fruits of a large number of species.

Among fresh-water plants, the lilies disperse as their spongy thalamuses bearing fruitlets float about in water. Seeds are deposited at different places as the tha­lamus rots.

Seeds of many aquatic plants like water-lily, Alisma, Sagittaria, etc., are very light and waterproof so that they can float easily. Many of these seeds are provided with spongy arils rendering them more buoyant.

Method # III. Dispersal by Animals:

Edible fruits, specially those that are brightly colo­ured, are devoured by various animals and, in many cases, their seeds pass through animal alimentary canals without any injury.

In this way, these seeds are disper­sed through animal excreta often many miles from the place of their origin and often to very inaccessible places like tree tops and building cornices.

Numerous herbi­vorous animals like birds, bats, squirrels, jackals, monkeys, etc., are thus responsible for such dispersal. Even ants play a role in disposal by dragging seeds from one place to another.

Human beings play-no minor role in this mode of dispersal. Various economic plants are distributed all over the world by them. A large number of fruit trees, vegetable and ornamental plants, etc., have been brought to India from foreign countries by the mankind.

Examples may be found in potato, tomato, tob­acco, water hyacinth, etc. Even coconut is an exotic plant according to our mythology (created’, i.e., probably, introduced by Viswamitra).

Sometimes, fruits and seeds are provided with appendages (Fig. 469) or sticky secre­tions which facilitate their mechanical dispersal by animals:

1. Fruits and seeds of many plants are provided with hooks, spines, barbs or stiff hairs (Fig. 469) so that if an animal grazes or brushes against them, these stick to the animal’s body or clothing.

Later on, these drop off somehow or other and are thereby dispersed. Thus, the fruits of Xanthium strumarium (cocklebur), Urena lobata, Martynia annua (tiger’s claw), etc., are provided with hook-like structures which arc effective in their dispersal.

Pupalia fruits are provided with bunches of spreading, hooked bristles. Fruits of the lovegrass (Andropogon aciculatus), spear grass (Aristida), etc., are provided with stiff hairs often curved backwards so that they stick easily.

Fruits of Tribulus are provided with sharp spines-enabling them to stick to cattle hoofs. Fruits of Achyranthes aspera (Fig. 264) are sticky with their scarious bracts and perianths.

2. Some fruits as those of Boerhaavia repens (Fig. 469E), Cleome viscosa, etc., possess sticky glands so that they stick to the faces and bodies of grazing animals and are thereby dispersed. Many seeds, as those of Aegle marmelos, Plumbago, mistletoe, etc., are sticky and are thereby benefited.

Method # IV. Mechanical Dispersal:

All dehiscent fruits scatter the seeds when they burst. This dehiscence is accompanied by the expression of great force in many fruits so that seeds are jerked a considerable distance away from the mother plant.

Such fruits are called explosive fruits. It has already been mentioned that the metre long lomentums of Entada gigas explode in the forests with the sound of crackers. Legumes of the mountain climber Bauhinia vahlii similarly explode with loud noise scattering the seeds yards away in all directions.

Ripe fruits of 0xalis corniculata and Impatiens balsamina (balsam) explode suddenly when touched. The valves are bent and the seeds are jerked off forcibly (Fig. 470A). Fruits of Geranium showing septicidal dehiscence similarly split into cocci scattering the seeds in this act. Here, the styles attaching the carpels to the carpophore curl upwards and outwards throwing out the seeds like slings (Fig. 470B).

The ripe pods of Abrus (Fig. 470C), peas and beans (Fig. 434), Clitoria, etc., suddenly twist on bursting and thus scatter the seeds.

Fruits of a number of plants belonging to the family Acanthaceae (viz., Ruellia tube- rosa, Andrographis paniculata, Barhria prionitis) suddenly burst into two valves ejecting the seeds (Fig. 471).

In these pods the seeds are arranged in four rows and the flat lens-shaped seeds have peculiar hood-like projections, called jaculators or retinacula, from their funicles which straighten out during dehiscence so that the four rows of seeds are ejected cross­wise in four different directions.

A very interesting type of seed ejection is seen in the mediterranean cucurbit Ecballium elaterium named the ‘squirting cucumber’ (Fig. 472). The internal content of the fruit remains under pressure and the stalk of the fruit acts like a stopper.

When the fruit ripens it gets detached from the stalk, the internal pressure is released and some of the contents, including the seeds, are literally ‘squirted’ out. Seeds may be carried as much as 20 ft. away by this squirting act.