Seeds are produced by plants following the fertilization of the flower, as a means of reproducing the plant. Each seed is a plant embryo, which consists of a minute shoot and root and a store of food. The food reserve enables the embryo to grow before its root is developed to absorb nutrients from the soil and before the leaves emerge above the ground and make sugars by photosynthesis, a complex process. In some seeds, such as those of sunflowers or
peas, the food reserve is starch but in others it may be oils or fats. The food reserve occupies the bulk of the volume of a seed. The seed is enclosed in a protective coat called the testa, which frequently has a small hole through which water can enter before germination.
Many seeds undergo a dormant period for some time before germination. This dormant period is useful in that it prevents the seed from germinating in the mild autumn only to be killed off when the frosts arrive. In many seeds the dormant period occurs because germination is delayed by reason of the very hard testa which has to be cracked open by the expansion and shrinkage that occurs during the cold weather. In other types of seeds growth-inhibiting chemicals have to be washed out by the rain before germination will occur. These growth-inhibiting chemicals can prevent the germination of the seeds of other species of plants. The rain washes them into the soil and germination of neighboring seeds is prevented. Some seeds such as those of lettuce and mistletoe, require light before they will germinate, others not only will germinate in the dark but actually are prevented from germination by light.
When the seed is ready to germinate, water enters and the food reserve provides energy for the growth of root.
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