Parts of Seed: The seed consists of several important parts that contribute to plant growth and reproduction. A seed is composed of the seed coat, embryo, endosperm, and sometimes the perisperm. The seed coat provides protection and aids in seed dispersal. The embryo contains tissues that give rise to the root, stem, and leaves of a new plant. The endosperm serves as a nutrient source for the developing embryo, while the perisperm, if present, acts as a nutrient storage tissue.
What is a Seed?
Seed is fertilized ovule. After fertilization, ovules are converted into seeds, and the integument of the ovule forms a seed coat. The seed is the reproductive unit of plants that contains all the necessary components to initiate the growth of a new plant.
Parts of a Seed
A seed is made up of four main parts: the seed coat, the embryo, the endosperm, and the perisperm. The seed coat protects the seed and helps it spread to new places. The embryo contains important tissues that grow into roots, stems, and leaves. The endosperm acts as a source of nutrition for the developing plant. The perisperm, if present, stores additional nutrients for the seed's growth. Together, these parts make up the reproductive unit of a plant, ensuring its survival and propagation. Let's understand these parts in detail.
Seed Coat
The seed coat provides protection and helps in the dispersal of seeds. The seed coat is made up of two layers: the outer testa and the inner tegmen. The testa is tough and protective and it prevents mechanical damage and the entry of pathogens. The tegmen is thin and delicate and it provides a protective covering for the embryo.
Embryo
The embryo contains the important tissues that give rise to the root, stem, and leaves of the new plant. The embryo consists of the epicotyl, the hypocotyl, the plumule, the radicle, and the cotyledons. The plumule is the embryonic shoot, and the radicle is the embryonic root. The epicotyl gives rise to the shoot system above the ground, including the leaves and stem. The hypocotyl connects the epicotyl and the radicle. Dicot seeds have two cotyledons. In monocot seeds, only one cotyledon is present towards the lateral side of the embryonal axis which is known as 'scutellum' in the grass family of monocots.
Endosperm
The endosperm serves as a nutrient reservoir and provides nourishment to the developing embryo. It is rich in carbohydrates, proteins, and lipids. It is triploid(3n). Monocot seeds retain the endosperm even after germination. Also, the aleurone layer, made up of protein, covers the endosperm in monocots. The endosperm is primarily starch in the case of rice. This aleurone layer contains an enzyme known as Amylase that digests starch.
While in most dicot seeds endosperm is consumed completely during germination. The cotyledons are responsible for storing nutrients and providing nourishment to the developing embryo.
Perisperm
The persistent nucellus is known as the perisperm. It is not present in all seeds. Nucellus normally gets used up in the development of the embryo sac. If remained, then known as perisperm. Perisperm is present in seeds of sugar beet, black pepper, etc.
Types of Seed
There are two types of seeds based on the number of cotyledons:
- Monocotyledonous Seeds: They contain a single cotyledon. Example: Rice
- Dicotyledonous Seeds: They contain two cotyledons Example: Tomato
There are two types of seed based on the presence or absence of endosperm:
- Non-Endospermous Seeds: Non-Endospermous Seeds, also known as Ex-Albuminous Seeds, are those seeds in which cotyledon develops properly and endosperm gets consumed completely. The growing embryo is nourished by cotyledon. These types of seeds are generally found in dicot plants except for the orchid family which belongs to monocot plants but still lack endosperm. Examples include Peas, Beans, Groundnuts, Mustard, Orchid family, etc.
- Endospermous Seeds: Endospermous Seeds, also known as Albuminous Seeds, are those seeds in which the endosperm does not get consumed completely and also provide nourishment to growing embryos along with cotyledon. These types of seeds are generally found in monocot plants except for the castor, sunflower, and Solanaceae family which belong to dicot plants but still contain endosperm. Examples include Maize, Wheat, Rice, Coconut, etc.
Advantages of Seed
- The seed has a hard seed coat that protects the embryo.
- Seed is better adapted for dispersal.
- Out of all the stages of the plants like pollination, embryo development, etc. Only seed germination depends completely on water. Thus, seed helps in searching for water for plants.
- Seed is a product of sexual reproduction and thus helps in producing variation.
- Due to dormancy and dehydration, seeds can be transported. Also, farmers can store seeds for the next season.
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