After reading this article you will learn about: 1. Methods of Dispersal of Fruits and Seeds 2. Dehiscence of Fruits.
Methods of Dispersal of Fruits and Seeds:
Dispersal by Wind:
Either the whole fruit or the individual seeds may be suited to dispersal by wind. Seeds that are thus disseminated are characteristically light. The means of adaptation to wind dispersal may be grouped under the headings of minute seeds, flattened fruits or seeds, wing like outgrowths, feathery appendages and the so-called censor mechanisms.
The seeds of orchids are very small and besides, have a light, inflated outer covering. These dust-like seeds can be blown by the wind for great distance.
In many cases, seeds and in others, whole fruits are flattened or have wing-like outgrowths or they may be both flattened and winged. This type of structure results in the scattering of the seed by the wind (e.g., the fruits of Terminalia, ash, maple, Macrozanonia, Pterocymbidium, Dipterocarpus, linden, etc.).
Seeds or fruits may have feathery appendages, (e.g., Asclepias, Vernonia, etc.) which greatly increase their buoyancy, so that they are frequently carried by the wind to considerable altitudes. These feathery appendages are very characteristic of the seeds of Asclepias and of the achenes or many Compositae. Commercial cotton consists of trichomes which grow from the epidermal cells of the seed of the cotton plant.
These trichomes form a flossy mass round the seed.
The capsules of many plants open in such a way that the seeds can escape only when the capsules are violently shaken as by a strong wind, (e.g., Aristolocliia, Poppy, etc.). This has a tendency to scatter the seeds. The seeds may in addition have a flat shape or winged outgrowths (as in Aristolocliia)-, and as they are likely to escape when there is a strong wind, they may be blown for considerable distances.
Dispersal by Water:
Adaptations for dispersal by water occur in many seashore and aquatic plants. Either the whole fruit or the seed may be adapted for floating. The pericarp of a fruit may be composed of light tissue, (e.g., in Terminalia catappa) or the fruit may be inflated, (e.g., in Heritiera littoralis).
The coconut is an excellent example of a fruit with a light pericarp. Floating seeds may likewise contain either a mass of light tissue of large air spaces (e.g., Entada scmdens). In the lotus fruit the torus is a greatly enlarged mass of loose, air-filled tissue which floats readily, while the individual fruits are also adapted for floating.
Many seeds that are not especially fitted for floating may occasionally float for short distances, or seeds that by themselves would not float may be carried in floating debris.
Dispersal by Animals:
Seeds that are adapted for dispersal by animals are disseminated in two general ways: in the case of fleshy fruits, a portion of the fruit is eaten by the animals, while any dry fruits adhere to animals.
Fleshy fruits are generally adapted to being eaten by animals. Such fruits are usually constructed so that the fleshy part may be eaten without injury to the seed. In many cases the seed coat is very hard.
While in drupes the seed is protected by the stony endocarp. Owing to this protection a seed is protected by the stony endocarp. Owing to this protection a seed may pass without injury through the digestive tract of an animal.
Birds are particularly prominent in disseminating the seeds of fleshy fruits. Sometimes they eat the fleshy portion of a fruit and throw the seeds away. Dry fruits are often carried off for food by seed-eating animals which lose them in one way or another and leave them to grow.
Many dry fruits have hook-like appendages (e.g., Cosmos, Bidens, Mimosa pudica, Triumfetta, etc.). which are particularly fitted for grasping the hair of animals. Animals to which the fruits adhere carry them about and thus distribute the seeds. In a similar way fruit may adhere to clothing and thus be disseminated by man.
Some seeds and fruits have a sticky covering which will adhere to the hair of an animal. Fruits of Chinese forget-me-not adhere by sticky hook-like appendages. Feathery appendages are usually capable of adhering to fur as well as of flying on the wind.
Many plants have minute seeds which are disseminated by being caught in mud that adhere to the feet or other parts of birds or other animals.
Dispersal by Explosive Mechanism:
Many fruits are so constructed that they explode when ripe and scatter the seeds. This method of dispersal is frequently conspicuous in members of the bean family, where the explosive forces are due to stresses arising from the drying of the valves of the pod.
The balsam has somewhat fleshy capsules which are very turgid. When these are disturbed by contact the segments of the pericarps roll up with considerable force and in such a way that they scatter the seeds. An unusual explosive mechanism is found in the squirting cucumber (Ecaballium elaterium).
Dehiscence of Fruits:
The dehiscent fruits rupture to liberate their seeds in several ways.
They are as follows:
The fruits of Portulaca and Celosia liberate their seeds through transverse rupture.
In poppy (Papaver) and Luffa the seeds are liberated through pores in fruits.
In pea (Pisum) and bean the seeds are liberated through sutural valves.
The fruits of cotton (Gossypium) and lady’s finger (Abelmoschus) the seeds are liberated through locules.
In linseed (Linum) and mustard (Brassica) the seeds are liberated through septa or partition wall.
In Datura and other such plants, the fruits rupture loculicidally throwing valves away from fruit and leaving seeds attached to the central axis.