One of the major differences between animals and plants is that animals can move whereas plants cannot. They remain static at one place even though they grow vertically horizontally depending on their growth pattern. But how do these plants spread their seeds to grow at different places? It is interesting to note that nature helps them in this activity and this process is termed as dispersion of seeds.
There are mainly four agents responsible for the dispersion of seeds, viz. air, water, birds and animals, and the plants themselves.
When the dispersion is carried out by the plants themselves the process is called dehiscence. In dehiscence (or dispersion by explosion), a seed-containing structure such as a pod, bursts shooting the seeds into the air. These seeds land up at a distance depending upon the weight of the seed and the force with which they are thrown. Some of the plants that disperse their seeds in this manner include plants of the pea, mustard, and balsam family.
Air is also an effective dispersing agent for the seeds. Lightweight seeds fly from one place to another along with the wind. Some grass seeds have been detected at heights of more than a thousand metres. Lightweight orchid seeds are carried away by air to great distances from their parent plants. Seeds of some plants, such as dandelion and cattail, have puffy hair that helps them float through the air and land up elsewhere. The seeds of maple, ash and box elder have wing like structures to move like small helicopters in the wind and thus get dispersed.
For dispersion by water, the seeds must be waterproof and able to float. Mangroves and coconuts have floating seeds with tough coats.
Seeds are dispersed by birds and animals in a variety of ways. Some seeds have ‘burrs’ or hooks which get attached to the fur or the tails of the animals. They can be carried to great distances travelled by the animals before falling off. Some seeds, such as mistletoe, are enclosed in sticky berries. When a bird or animal eats these berries, the seeds get stuck to its feet, mouth or body. Some seeds are enclosed in juicy, tasty fruits which are consumed by birds, animals, and human beings. These seeds remain unaffected by the digestive system and passed out of the body as such in the faeces. The seeds of grape, mulberry, pear etc. are dispersed this way. The clover and cereal crop seeds are often dispersed by grazing animals such as cattle and sheep. Human beings often disperse seeds by throwing away apple cores, peach pits and other inedible seeds from juicy fruits People carry seeds to great distances when they carry them unknowingly in fruits and other food items in long journeys.
Apart from these main agents of dispersion of seeds, there are other irregular agents that help in this process.
