THE SEED - STRUCTURE & GERMINATION
THE SEED - STRUCTURE & GERMINATION
What is a seed?
A seed is a fertilized, matured ovule of a flowering plant, containing an embryo.
This tiny embryo enclosed within the seed develops to form a new plant by the process of germination. Plant embryos in seeds have structures called cotyledons. A cotyledon is the central part of a seed embryo to which the plumule – the immature shoot, and the radicle – the immature roots, are attached.
Seeds are classified according to the number of cotyledons present in the embryo. If the embryo has one cotyledon (monocotyledon) it is a monocot plant, and if there are two cotyledons (dicotyledon), it is a dicot plant.
Example:
Monocot seeds – Maize, Wheat, etc.
Dicot seeds – Pea, Gram, kidney Bean, etc.
The structure of Dicot Seed:
A bean (dicot) seed is normally kidney-shaped, flat, and has a notch on one side. Dicots have two seed leaves inside the seed coat. They are usually rounded and fat because they contain the endosperm to feed the embryo plant.
Dicot seed consists of the following parts:
§ Hilum:
The hilum is the point where the seed is attached to the inner margin of the fruit.
§ Micropyle:
This is the opening through which the pollen tube enters the ovule. In the seed, the Micropyle imbibes water and softens the seed coats, and allows the embryo to emerge. It allows the diffusion of gases for the respiration of the embryo.
§ Seed coats:
In dicots, the seed coat is further divided into an outer coat, known as the testa, and the inner coat, known as the tegmen. The inner coat is thin, membranous, and generally united with the testa. The seed coat is tough and waterproof. It covers and protects the embryo and the food supply.
§ Embryo:
The embryo is the baby plant, which emerges from the seed and germinates into a mature plant. The embryo consists of Cotyledons or seed leaves or nurse leaves. They are fleshy, green, leaf-like structures and provide nourishment to the growing embryo. It also consists of The radicle or future root of the plant. It is the first to emerge out of the Micropyle during germination and develops into a taproot in the bean plant. Plumule or the embryonic shoot system. It has a growing point covered by undeveloped, delicate tiny leaves. It is attached to the radicle by the hypocotyl.
The structure of Monocot Seed:
The maize grain (monocot) is roughly oval in shape and normally flattened. The lower narrower portion shows a white triangular region, which is the embryo. The broader and more yellowish region is the endosperm. Monocots have only one seed leaf inside the seed coat. It is often only a thin leaf because the endosperm to feed the new plant is not inside the seed leaf.
Parts of the maize seed:
§ Endosperm:
Most of the grain is occupied by the endosperm and is filled with reserved food. A thin epithelial layer separates it from the embryo. The outermost layer of the endosperm which is continuous with the fused seed coat and fruit wall is rich in protein and is called the aleurone layer. The inner starchy endosperm is made of starch cells and a few lipid cells.
§ Embryo:
It is seen below the endosperm. It is made of a single cotyledon called scutellum and embryo axis with plumule and radicle. It has special cells which secrete digestive enzymes to digest and absorb the nutrients in the endosperm to provide nourishment for the developing embryo. The root cap protects the tip of the radicle. The radicle is surrounded by a protective sheath called coleorhiza. Plumule is also protected by a covered sheath known as coleoptile.
Germination :
Germination is the development of a plant from a seed or spore under specific environmental conditions. In other words, Seed germination is a process in which the dormant embryos of seed resumes metabolic activities and grow to produce a seedling.
There are 2 types of Germination, namely:
1. Epigeal Germination.
Epigeal Germination:
In epigeal germination (or epigeous germination), the hypocotyl elongates and forms a hook, pulling rather than pushing the cotyledons and apical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air.
Example: Kidney Beans, tamarind, and papaya are examples of plants that germinate this way.
Hypogeal Germination:
In hypogeal germination, the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose.
Example: Maize, peas, mango. Here the cotyledon remains below the soil. They dry up and become shriveled when the food reserves are exhausted. The epicotyl elongates and pushes the plumule above the ground.
In both types of germination 3 processes are common:
1. The seed soaks up water and the seed coat may burst.
Viviparous Germination:
This is a special type of germination found in mangrove plants. Mangrove plants are found in marshy areas and on the sea coast. In viviparous germination, the seeds germinate while still attached to the parent plant. The embryo grows out of the seed and then out of the fruit and projects from it in the form of a green seedling displaying the root and the hypocotyl.
Seeds are of immense biological and economic importance. They contain high protein, starch, and oil reserves that help in the early stages of growth and development in a plant. These reserves are what make many kinds of cereal and legumes major food sources for a large proportion of the world's inhabitants.
References:
Concise Biology I.C.S.E. Selina Publishers PVT. LTD.
Link for structure of a BEAN seed (Dicotyledonous seed)
Link for structure of a MAIZE grain (Monocotyledonous seed)
Link for EPIGEAL and HYPOGEAL GERMINATION
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