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Plants produce many fruits and therefore many seeds. This is because only a few of these seeds will find suitable places and conditions in which to germinate. The majority of the seeds will be wasted; some wind-dispersed seeds could fall in water or on stones, for example.
When fruits and seeds are dispersed the seeds have very little water in them. The outer covering, the testa, is hard and sometimes rough and creased. Although the seeds seem dry and inactive they are very much alive. They are said to be in a dormant state and some seeds can remain like this for a long period of time. Inside the testa there is just enough water to keep the embryo (the baby plant) living until the external conditions are correct for the seed to germinate.
The Structure of a Haricot Bean Seed
The outer covering of a seed is called the testa. It is usually hard and protects the softer parts of the seed. There is a tiny hole in the testa called the micropyle. When the seed is ready to germinate, water is taken in through the micropyle. The first root, called the radicle, will grow out of the seed through the micropyle. On the surface of the testa there is a scar which shows the place where the seed was attached to the fruit.
A dried seed is difficult to study. If the seed is soaked in water for twelve hours it takes in some of the water and swells. Peas and beans are useful seeds to study after they have been soaked because they are big. The testa can be removed easily and the inside of the seed can be examined under the binocular microscope.
Inside the Seed
Inside both the pea and the bean there are two large food-storing structures called cotyledons. The food contained in the cotyledons gives energy to the embryo when the seed germinates. The embryo is found between the two cotyledons. It is made up of two parts; the radicle, or first root, which points toward the micropyle, and the plumule, or first shoot. Under the binocular microscope the plumule can be seen as two very tiny leaves.
Some types of seeds have only one small cotyledon. Most cereal grains, such as wheat, are like this. The embryo is surrounded by a structure called the endosperm which is also a food storage area. Both the cotyledon and the endosperm will provide energy to the embryo during germination.