Let us learn about Seeds. After reading this article we will learn about: 1. Definition of Seed 2. Structure of Seed.
Definition of Seed:
A true seed is defined as a fertilized mature ovule that possesses embryonic plant, stored material, and a protective coat or coats. Seed is the reproductive structure characteristic of all phanerogams. The structure of seeds may be studied in such common types of pea, gram, bean almond or sunflower.
They are all built on the same plan although there may be differences’ in the shape or size of the seed the relative proportion of various parts.
There are hundreds of variations in the seed size, shape, colour and surface. The seeds range in size from tiny dust particles, as found in some orchids, to large double-coconuts. The seed surface may be smooth, wrinkled, striate, ribbed, furrowed, reticulate, tuberculate, alveolate, hairy, and pulpy or having patterns like finger prints.
In the seed, life activities are temporarily suspended in order to enable the plant to successfully pass through unfavourable and injurious climatic conditions. On the approach of favourable conditions, the seed resumes active life and grows into full plant. In the form of seeds, a plant can be carried to long distances without special precautions.
Structure of Seed:
The various parts of a seed may be easily studied after it has been soaked in water for a day or so varying according to the nature of the seeds. A mature seed contains an embryonic plant (with a radicle and plumule), and is provided with reserve food materials and protective seed coats. A mature pod of pea (Pisum sativum) has a number of seeds arranged in two rows.
The seeds are attached to the fruit wall by a small stalk, the funiculus. At maturity, on one side of the seed coat a narrow, elongated scar representing the point of attachment of seed to its stalk is distinctly seen, this is the hilum. Close to the hilum situated at one end of it there is a minute pore, micropyle. During seed germination, water is absorbed mainly through this pore, and the radicle comes out through it.
Continuous with the hilum there is sort of ridge in the seed coat, the raphe. The seed is covered by two distinct seed coats; the outer whitish one is the testa, while the other inner thin, hyaline and membranous covering is the tegmen. The seed coats give necessary protection to the embryo which lies within.
The whitish fleshy body, as seen after removing the seed-coats is the embryo. It consists of two fleshy cotyledons and a short axis to which the cotyledons remain attached. The position of the axis lying outside the cotyledons, bent inward and directed towards the micropyle is the radicle and the other portion of the axis lying in between the two cotyledons is the plumule.
The plumule is crowned by some minute young leaves. The radicle gives rise to the root, the plumule to the shoot and the cotyledons store up food material. Since the reserve food material is stored in the massive cotyledons and the seed lacks a special nutritive tissue, the endosperm.
The seeds which lack endosperm at maturity are called non-endospermous or exalbuminous. On the other hand in several other plants such as castor bean (Ricinus communis), coconut (Cocos nucifera) and cereals, food is stored in the endosperm. Such seeds where endosperm persists and nourishes the seedling during the initial stages are called endospermous or albuminous.
On the basis of the number of cotyledons in the embryo the angiosperms have been divided into two large groups:
1. Dicotyledons, having embryos with two cotyledons, and
2. Monocotyledons, with only one cotyledon.
A maize grain is a single-seeded fruit in which the seed coat and the fruit wall are un-separable. On one side of the grain a small, opaque, whitish, deltoid area is seen to be distinctly marked out from the region. The embryo lies embedded in this area. There is only a thin layer surrounding the whole grain.
This layer is made up of the seed-coat and the wall of the fruit fused together. The grain remains divided into two unequal portions by a definite layer known as the epithelium. The bigger portion is the endosperm, and the smaller portion, the embryo. The endosperm, is the food storage tissue. The embryo consists of one shield shaped cotyledon, known as the scutellum and axis.
The upper portion of the axis, with minute leaves arching over it, is the plumule, and the lower portion provided with the root cap the radicle. The plumule is surrounded by a leaf-sheath or coleoptile and the radicle is surrounded by a root sheath or coleorhiza. These are the protective sheaths of the plumule and the radicle respectively.
Besides the basic structures (endosperm, embryo and seed-coat) certain special structures may arise during seed development. In castor bean a fleshy whitish tissue, the caruncle, develops at one end of the seed. It is derived from the integument.
The juicy edible part of the litchi fruit (aril) is an outgrowth of the funiculus that develops after fertilization. The cotton fibres are the elongated epidermal cells of the seed-coat. These fibres are single-celled and thin walled. They attain a length of upto 45 mm and have characteristic twists.