Seed and its structure

  • 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 exalbuminous 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.

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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.

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Seed and its structure

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