In this tutorial, we have discussed ‘why seed dispersal is important?’ ‘Different agents of seed dispersal’ etc.
TABLE OF CONTENTS
What is Seed Dispersal?
Seed is the final outcome of the sexual reproductive process in flowering plants. It represents the beginning of a new generation. Kozlowski and Gunn (1972) have defined a seed as a ‘fertilised mature ovule‘ that possesses an embryonic plant, stored food material, and protective coat.
The process of carrying away the seeds and fruits away from the parent plant for providing them with better chances of obtaining water, food and light to enable them to germinate is called seed dispersal.
The evolutionary success of the flowering plants has been due to the development of a mechanism that protects the new generation within the old generation. Seed represents a miniature plant with an adequate supply of reserve food meterial for nourishment at the time of germination. Seed is a means of perennation. During this phase, all the life activities are temporarily suspended to tide over the unfavourable and injurious climatic conditions. With the arrival of favourable conditions the seed resumes active life, germinates and grows into a new plant.
Why Seed Dispersal is Important?
The importance of seed dispersal is as follows:
1. In the absence of dispersal, all the fruits and seeds might fall just near the parent plant and germinate to form the seedlings.
2. Young seedlings of the same kind may not be able to compete with the grown up plants for light, water and minerals.
3. Due to overcrowding, many seedlings may perish due to a tough competition.
Agents of Seed Dispersal
Air, water, animals and even the self explosive mechanisms help in the dispersal of seeds. On the basis of dispersing agencies, the dispersal of seeds and fruits can be of the following types:
1. Fruit and Seed Dispersal by Wind (or Anemochory)
Seeds and fruits dispersed by wind have any one of the following specialities:
(i) Light and Minute Seeds
Seeds of many grasses and orchids are very minute and light. They are just like particles of dust and are easily blown by the wind. One seed of some orchids weighs only 0.004 mg. Seeds of Cinchona
Many seeds develop wings for their dispersal. The winged seeds are found in Jacaranda, Oroxylum, Moringa, Cinchona, Lagerstroemia, Pinus, etc.
Many fruits are also winged and dispersed by wind, e.g., Acer, Chilbil (Holoptelea), Hiptage, Dioscroea, Shorea, Hopea, etc.
(iii) Parachute Mechanism
Appendages of some fruits and seeds act as a parachute which keep the fruits and seed in the air for a longer period and are dispersed at longer distances. These appendages are:
(a) Pappus: Fruits of family Composite like Sonchus and Sunflower have a crown of persistent, hairy calyx known as pappus.
(b) Coma: These are one or more tufts of hair attached to the seeds, e.g., Calotropis, Alstonia seed has two commas.
(c) Hairy Outgrowth on Seeds: Long, hairy outgrowths are borne on the Cotton seeds.
(d) Persistent Hairy Style: Some seeds have a persistent hairy style, e.g., Narvelia, Clematis.
(e) Balloon-like Appendages: Swollen capsule of Cardiospermum, swollen calyx of Physalis and the ovary of Colutea develop into balloon-like structures. These help the fruits and seeds to remain in air for a longer period and are dispersed to far off distances.
(iv) Censer Mechanism
Certain fruits have minute openings by which seeds are dispersed by the currents of wind. These pores are so minute that only some seeds can come out of the pores at a time.
Examples: Aristolochia and Antirrhinum.
2. Fruit and Seed Dispersal by Water (or Hydrochory)
Fruits and seeds dispersed by water are adapted for floating so that they are carried to distant places by water currents.
(i) The Coconut fruit floats because its fibrous mesocarp encloses air, thus making it buoyant. These fruits float in the sea and are carried to long distances.
(ii) Lotus has spongy thalamus with fruitlets embedded in it. The thalamus floats in water streams. After some time thalamus decays and fruitlets settle down at the bottom of the pond and germinate.
(iii) Seeds of certain Water lilies have air spaces.
3. Fruit and Seed Dispersal by Explosive Mechanism (or Autochory)
Certain fruits when ripe burst open with a jerk so that their seeds are thrown apart.
Examples: Ecballium, Ruellia, Geranium, Balsam.
In Ecballium, a mucilaginous substance is present in the ripe fruit. It dehisces even by a slight touch and the viscous substance comes out with seeds as a fountain.
In Ruellia, Jaculators or curved hooks are present which help in the dispersal of seeds when they become straight.
Geranium and Castor fruits open with a sudden blast and the seeds are dispersed. In the bright sun, dry fruits of Phlox burst liberating the seeds.
4. Fruit and Seed Dispersal by Animals (or Zoochory)
Many fruits and seeds are dispersed by animals including Man. Such fruits are provided with hooks, barbs, spines, stiff hair and are carried away to long distances, for example by hooks in Xanthium and Urena and by stiff hair in Aristida.
In Tribulus, the spines are stiff and pointed. In Boerhaavia, sticky glands are present on the fruits. Seeds of Aegle are also sticky.
Seeds of certain edible fruits such as Tomato, Brinjal, Pear, Mango, Guava, Pomegranate, Plum, Apple, Peaches, Papaya are dispersed by man to various places. Certain animals like Squirrel, Crows, Monkey, Cattle, Birds, etc., eat various fruits and vegetables and help in seed dispersal.