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Conditions needed for the Seed to Germinate
The dried seed will remain dormant until the outside conditions are right for germination. For one thing, this means that the temperature must be right. Seeds which were dispersed in the autumn will germinate in the following spring when the weather starts to become warmer. Some seeds need a certain number of daylight hours before they germinate. Again, they will start to germinate in springtime when the days begin to get longer (there are more hours of sunlight).
If other conditions are correct there must also be enough water. If there is too much water the seeds will drown and start to rot. If there is too little water the seeds will start to germinate and then dry up and die. When there is the right amount of water the seed will take it in through its micropyle and swell. The food stores in the cotyledons or endosperms are then converted into energy for the development of the embryo.
Development of the Embryo
The radicle grows in length and moves out of the seed through the micropyle. It splits the soft testa as it does so. The radicle always grows downwards towards the earth. It does not matter how the seed is positioned; if the radicle is pointing upwards when it comes out of the seed, it will turn around as it grows so that it always points downwards. Once the radicle has grown out from the seed the plumule starts to develop. In seeds like the bean and the pea, the plumule grows out slightly bent so that the first green leaves are not damaged by anything lying on top of the seed. Once in the air the plumule continues to grow upwards and straightens out so that the first green leaves are facing the light. The plumule always grows towards the light and will turn, as it grows, to make sure that the leaves face the light. The young plant, called a seedling, obtains its energy for the growth of the radicle and plumule from the food stores in the seed. Once the green leaves are in the light, the seedling starts to make its own food by a process called photosynthesis. The radicle develops many side roots and these grow outwards, pushing their way through the soil, taking in water and minerals. What is left of the seed falls away from the seedling.
The seedling grows into a plant using the energy of the sunlight to make its food and obtaining water and minerals from the soil. In the late spring it will produce its own flowers which, after pollination and fertilization, will form fruits and seeds. Peas and beans are annual plants, which means that they survive from germination in the spring until the seeds are dispersed from their pods in the summer. Some plants, such as the carrot, are biennial. These plants do not produce flowers until the second year, after which the plant dies. Many plants, such as trees and bushes, are perennial which means that they live and grow for many years. Some pine trees in the south west United States are thought to have germinated over five thousand years ago.
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© Paul Billiet and Shirley Burchill 2009 |
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