Introduction of Seed
- The seed is a ripened ovule developed after fertilization.
- It consists of an embryo and reserve food surrounded by seed coat.
- The food may be stored in the cotyledons of embryo in non-endospermic or ex-albuminous seeds (e.g.pea, grams, bean) or in a special tissue of seed, called endosperm in endospermic or albuminous seed (e.g. castor, maize).
Structure of Dicotyledonous Seed (Gram)
- The seed is covered by two distinct layers of seed coats, testa and tegmen.
- Testa is the outer seed coat which is smooth, thick, impermeable and may be coloured.
- Tegmen is the inner seed coat which is a thin membranous and hyaline.
- The seed coats provide necessary protection to the embryo.
- The seed coat contains a narrow elongated scar representing the point of attachment of seed to its stalk.
- It is called hilum.
- At one end of hilum is found a minute pore, known as micropyle.
- The opposite end of hilum is continued into a narrow ridge which is called raphe.
- Inner to the seed coats is embryo.
- A typical dicot embryo consists of a short axis with two cotyledons.
- The cotyledons are attached laterally to the embryonal axis.
- The portion of the axis lying outside the cotyledons, bent inward and directed towards the micropyle is the radicle.
- The other portion of the axis lying in between the two cotyledons is the plumule.
- On germination of the radicle gives rise to the root and the plumule to the shoot.
- The cotyledons store up food materials.
- The region of embryo axis between the radicle and its point of attachment to the cotyledon is called hypocotyl.
- Similarly the region of embryo axis between the plumule and its point of attachment is called the epicotyl.
Structure of Monocotyledonous Seed (Maize)
- A maize grain is a single seeded fruit called caryopsis.
- The seed coat and the wall of the fruit are fused together to form a thin layer around the whole grain.
- The seed coat surrounds the endosperm and the embryo.
- In longitudinal section, the grain shows two unequal portions separated by a definite epithelium.
- Endosperm is the bigger portion occupying the major portion of the seed which stores starch, protein and oil.
- The endosperm is externally surrounded by a portion sheath called aleurone.
- The smaller portion is the embryo which lies on one side of the grain in a distinctly marked area.
- The embryo consists of one shield shaped cotyledon, known as scutellum and a short axis.
- The upper portion of the axis with minute leaves is called plumule and the lower end is called the radicle.
- The plumule is covered by a protective sheath, the coleoptile whereas the radicle has two protective sheaths
- The inner sheath is root cap and the outer sheath is known as coleorhiza.
- On germination the radicle gives rise to the root and the plumule to the shoot.
Image source: meritnation
Seed and its structure