Seed and its structure

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

References:

i) https://ib.bioninja.com.au/higher-level/topic-9-plant-biology/untitled-3/seed-structure.html

ii) https://www.britannica.com/science/seed-plant-reproductive-part

Seed and its structure