In this essay we will discuss about the process of dispersal of fruits and seeds.
Dispersal is the phenomenon of transferring of plant parts to distant place. Dispersal of fruits and seeds are common in angiosperms. It has great importance in their life process.
The dispersal is essential because of the following facts:
1. If all the fruits and seeds fall under the parent plant, ail may not be able to germinate to develop seedling.
2. Seedlings will not get sufficient light and air for normal living.
3. Soil nutrition will be insufficient for them as these have been mostly used up by the parent plant, and there will be sharing of the existing nutrition present in the soil.
4. A group of same type of seedling will be developed under the parent plant.
For the above facts, there will be struggle for existence amongst themselves, thereby most of the seedlings will become very weak and liable to perish under such unfavourable condition. Still, the Natural hazards (floods, drought etc.) may destroy the total plant population.
To avoid such problem, nature has arranged wonderful provisions to scatter them in different localities far away from the parent plant. Thereby some or most of them may find favourable conditions for their growth.
Most of the plants have no power to disperse their seeds to distant place, thereby they depend on some agents for their dispersal. In some plants, the fruits are provided with some explosive mechanism to gain the goal.
The dispersal of fruits and seeds on the basis of these agents can be divided into the following groups:
A. Dispersal by Wind:
Wind plays an important role in the dispersal of some fruits and seeds. Plants which require wind as an agent are called anemochorous. For wind dispersal, the seeds should be light in weight, so that their buoyancy may help them to fly for long distance.
These fruits and seeds have the following adaptations which help them to disperse long distance away from the mother plant:
1. Minute and Light Seeds:
Seeds of some plants are very minute, dry and light in weight and are easily dispersed by wind, e.g., orchids (one seed of orchid weigh about 0.004 mg), Fig. 2.175A; grains (fruits and seeds) of grasses etc.
2. Winged Fruits and Seeds:
Wing-like structures of different types are developed on fruits and seeds of some plants — these help them to float in air.
i. Wings may develop from seed coat (testa), e.g., horse radish, Moringa olifera of Moringaceae (Fig. 2.175B); Oroxylun indicum of Bignoniaceae (Fig. 2.175C); mahogany, Swietenia mahogani of Meliaceae (Fig. 2.175D) etc.
ii. Wings may develop from persistent sepals, e.g., Dipterocarpus sp. (Fig. 2.175E), Shorea robusta (Fig. 2.175F) and many other plants of Dipterocarpaceae.
iii. Wings may develop from pericarp of fruit, e.g., Hiptage benghalensis (Fig. 2.175G) of Malpighiaceae; country almond, Terminalia catappa of Combretaceae; Dioscorea alata of Dioscoreaceae; maple, Acer pseudoplatanum (Fig. 2.175H) of Aceraceae etc.
3. Parachute Mechanism:
Appendages develop on some fruits and seeds, act as parachutes by which they remain in the air for long time and can disperse for long distance.
The appendages are:
Hair-like persistent structure develops on the fruit by the modification of calyx and called pappus. The pappus helps in the dispersal of fruits of many members of Asteraceae (Compositae), e.g., Vernonia cinerea, Blumea lacera, Ageratum conyzoides, Taraxacum officinale (Fig. 2. 176A, A’) Tridax procumbens (Fig. 2.176B) of Asteraceae etc.
These are tufts of hairy outgrowth of testa developed at one or both ends of the seeds and help to disperse for long distance, e.g., seeds with one comma, Calotropis procera (Fig. 2.176C) of Asclepiadaceae; seeds with double comma (one each at both the ends), Alstonia scholaris (Fig. 2.176D) of Apocynaceae.
iii. Hairy Outgrowth on Seeds:
This type of structure is present all over the seeds as an outgrowth of testa, e.g., Silk cotton, Bombax ceiba (Fig. 2.177A) of Bombaceae.
iv. Persistent Hairy Style:
The persistent hairy styles are seen in the achene fruits, by means of which fruits can float in the air and disperse for long distance, e.g., Clematis gouriana (Fig. 2.177B), Naravelia zeylanica of Ranunculaceae.
v. Balloon-Shaped Appendages:
The balloon-shaped appendages are formed by the modification of different parts of the plant; they help the fruits and seeds to remain in air for longer period. These are formed by inflated calyx in Physalis minima (Fig. 2.177C) of Solanaceae, inflated fruits (capsule) in Cardio- spermum helicacabum (Fig. 2.177D) of Sapindaceae, inflated ovaries of legume in Colutea arborescens (Fig. 2.177E).
4. Censer Mechanism:
It is the mechanism by which very small seeds are liberated slowly from the fruit through minute pores (apertures). These pores are so minute that when the fruits are shaken by wind only some seeds can come out through the pore at a time and disperse, e.g., snapdragon, Antirrhinum majus (Fig. 2.178A) of Scrophulariaceae; Papaver somniferum (Fig. 2.178B,C) of Papaveraceae.
B. Dispersal by Water:
The aquatic plants and the plants growing on the bank of river or sea-shore disperse their seeds and fruits by water current for a long distance. The seeds and the fruits have the floating devices in the form of fibrous or spongy water-proof coats. The plants requiring water as an agency for dispersal are called hydrochorous.
The fruitlets may lie embedded in the spongy and watertight thalamus and can be carried away by the water current, e.g., lotus, Nelumbo nucifera (Fig. 2.179A) of Nymphaeaceae etc.
The fruits may have watertight coverings of sufficient air spaces, by which they can easily float on water and drift away to distant places, by water current, e.g., coconut, Cocos nucifera (Fig. 2.179B,C); betel nut, Areca catechu of Arecaceae. Cerbera odollam of Apocynaceae etc.
In some cases, seeds have air spaces in their coverings, which enable them to float on water and can be dispersed easily by water current, e.g., Sagittaria sagittifolia, Alisma plantago and many others of Alismataceae.
C. Dispersal by Animals:
The fruits and seeds with their adhering structures are dispersed by animals. Plants having such devices are called zoochorous. The nature of dispersal is different in dry and fleshy fruits and seeds.
Dry Fruits and Seeds:
Many dry fruits and seeds have spines, barbs, bars, hooks or sticky glands by means of which they can attach themselves to the body of animals or human beings and thus be transported to a distant place.
Fruits may provide with:
The fruits provided with spines become attached to the body of animals including human beings and dispersed at distant places, e.g., Achyranthes aspera (Fig. 2.180A) of Amaranthaceae, Andropogon aciculatus of Poaceae etc.
ii. Hooked Bars:
The fruits provided with many hooked bars become attached to the body (tail etc.) of animals including the clothes of human beings when passing through the plant population. The fruits are removed from their adhering surfaces naturally or artificially, thereby dispersal takes place, e.g., spiny cockle- bur, Xanthium strumarium (Fig. 2.180B) of Asteraceae (Compositae); Urena lobata (Fig. 2.180C) of Malvaceae; Martynia diandra (have two hooks) (Fig. 2.180D), of Martyniaceae.
iii. Sticky Glands:
The fruits of some plants provided with sticky glands on pericarp, get attached with the body of animals and are dispersed to distant place, e.g., Polanisia icosandra of Capparidaceae, Boerhaavia repens (Fig. 2.180E) of Nyctaginaceae etc.
iv. Curved Hairs:
The fruits provided with large curved hairs, get attached with the animal body and are dispersed, e.g., Chrysopogon sp., Aristida sp. (Fig. 2.180F) of Poaceae.
Fleshy fruits possess beautiful colour or tastes or both; invite the animals specially the birds.
These animals and birds disperse fruits and seeds in various ways:
v. The red colour of the fruits of banyan, Ficus benghalensis of Moraceae, attracts the birds like crow etc. After eating the fruits they flow away to distant places. When the birds rub their beak to wipe of these seeds, the seeds are dispersed. Due to this, the plants are visible on the roof, cornices of old buildings, temples, on other places etc.
vi. Fruits like Litchi chinensis of Sapindaceae; mango, Mangifera indica of Anacardiaceae are collected by birds from the trees and then they fly away to distant places. After eating the sweet pulp, the seeds are left, thereby the seeds are dispersed.
vii. The tasty fruits of date palm, Phoenix sylvestris of Arecaceae, the jujube, Ziziphus jujuba of Rhamnaceae etc., are delicacies to dogs, jackal etc. When they devour the fruits, the well-protected seeds are not digested and are ejected along with droppings at distant places. These seeds develop into new plants on germination.
viii. Seeds and fruits of some aquatic plants are dispersed from one pond to the other, getting attached with the leg of aquatic birds like snipes, duck etc.
ix. The fruits and seeds are also dispersed by human beings according to their wish and need to distant place.
x. The dehisced pods of Abrus precatorius of Fabaceae (Leguminosae) look like caterpillar and are mistaken by birds. When they are able to understand their mistake, the pods are thrown away and thus the seeds are dispersed.
D. Dispersal by Explosive Mechanism:
In some cases, the pericarp of fruit — after ripening and drying — becomes stretched due to some external pressure in contact with air, water drops, animals or change of humidity. In this method, the fruits dehisce suddenly and the seeds are dispersed, but only for a short distance.
i. The mature fruits of Andrographis paniculata, Ruellia tuberosa (Fig. 2.181 A) of Acanthaceae, when in contact with moisture, suddenly burst with an explosive sound and the seeds are dispersed in the nearby regions. Sometimes the seeds are provided with curved hooks, the jaculators — these help in their dispersal.
ii. The mature fruits of Geranium palustre (Fig. 2.182A) of Geraniaceae and balsam, Impatiens balsamina (Fig. 2.182B) of Balsaminaceae burst suddenly and the valves twist upwards, thereby the seeds are dispersed in the adjacent region.
iii. The mature fruits of pea, Pisum sativum (Fig. 2.182C) and many other plants of Fabaceae, etc., suddenly burst into valves and the valves twist upwards. The seeds are then ejected with some force.
iv. The mature fruits of castor, Ricinus communis of Euphorbiaceae are normally broken up elastically and the seeds are dispersed.
v. The mature fruits of squirting cucumber, Ecballium elaterium (Fig. 2.183) of Cucurbitaceae (grows in the Mediterranean region); contain a mucilaginous substance. The fruits drop off from their stalk even by a slight touch and the mucilaginous substance comes out with seeds as a fountain through the holes left at one end. Similar situation is also found in Luffa aegyptiaca of Cucurbitaceae.