A seed is the product of sexual reproduction in angiosperm plants. It contains a globular embryo or endosperm, which is a nutritive tissue, and a tough seed coat. It is also called an ovule. The term is generally used to describe a fertilised ovule, which is the product of a male sperm and a female gamete. In angiosperms, the seeds are formed inside the fruit. During the germination process, the embryo develops from the micropyle, a small pore within the seed coat, and begins to use the food-laden endosperm to obtain the necessary nutrients for growth. If the germination occurs in an appropriate environment, the embryo may germinate and begin to grow into a plant. However, if the conditions are not optimal, the embryo may enter a state of dormancy.
The embryo development process is the same in dicotyledonous and monocotyledonous plants. Regardless of the type of structure, it requires an oxygen supply to initiate germination. Moreover, the embryo needs sufficient moisture for germination. Typically, a seed consists of a seed coat and a cotyledon. The cotyledon is a simple structure that is either slender or swollen to accommodate the storage of food reserves. An embryo cotyledon is typically larger than the seed.
There are two main types of seed: albuminous and non-albuminous. Albuminous seeds are those that contain large amounts of endosperm. These include the seeds of corn, wheat, maize, sunflower, and other cereals. When an embryo grows, it uses the endosperm to get the necessary nutrients, and to store some of the food reserves that it will need for future growth. Non-albuminous seeds are those that lack endosperm. Examples of non-albuminous seeds are the pea, groundnut, almond, grountnut, and other legumes.
Seeds are usually composed of a seed coat, a cotyledon, and an embryo axis. Cotyledons are food-storage tissues, and are normally slender and whitish. Some seeds, such as black pepper, contain remnants of the nucellus, which is the endosperm of a developing seed. Although the nucellus is present in both ends of the seed, the persistent nucellus (perisperm) is not present in mature seeds. This is because the endosperm in a mature seed does not contain a perisperm, or the persistent nucellus.
There are three types of endosperm: nuclear, helobial, and cellular. Each form of endosperm is characterized by its chemical composition. Free nuclear, helobial, and cellular endosperm are all formed by the fusion of the male gametes with the secondary nucleus of the central cell. Embryos that are formed through double fertilization are referred to as globular embryos, while those that are produced through single fertilization are called perisperm.
During the germination process, the endosperm of an embryo carries oil, starch, and other foods. Endosperm is usually yellowish or orange in color, and is made of protein. The seed’s outer layer, the kernel, is a protective structure that helps it survive and reduces water content. After germination, the seed is relatively dry, with about 10 to 15 percent moisture by mass. Depending on the condition of the seed, it may begin to rot if too much moisture is not present.