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Germination
A seed enters a period of inactivity, or dormancy, that is necessary before development can continue. During dormancy, chemical changes occur that prepare the embryo for further change. When a seed reaches a suitable environment, the embryo within the seed germinates. Germination occurs if conditions such as amounts of oxygen and moisture and a suitable temperature are right. Each type of seed has unique growth requirements. Oxygen is needed for respiration to provide the rapidly growing embryo with energy.
Seed plant embryos have regions that develop into definite structures during germination. The radicle is the first part of the embryo to emerge from the seed. It becomes the primary root. The radicle is the tip of the hypocotyl. In some seeds, such as a bean, the hypocotyl forms an arch, pushes above the soil, and becomes a small part of the base of the stem. In other seeds, such as corn and peas, the hypocotyl remains in the soil and becomes the upper part of the primary root. The epicotyl is a part of the embryo that gives rise to the shoot (stems ans leaves).
The embryo is attached to one or two cotyledons, or seed leaves. Plants such as beans are dicots and have two cotyledons per seed. Corn, a monocot, has one per seed. In monocot seeds, food is stored mainly endosperm tissue. In many dicot seeds, the cotyledons absorb food from the endosperm as the seed matures. In either case, food stored in seeds provides a source of enegry during early development of the sporophyte.
แหล่งข้อมูล : Biology living systems. Page 426.
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