The Seed

A seed is the final product of sexual reproduction in angiosperms. It is a fertilised, mature ovule that contains an embryo, stored food material (endosperm), and a protective coat (seed coat).

After fertilisation, the following simultaneous transformations occur in the ovule:

A typical seed has three primary components:

Seed Coat: Protective outer layer derived from integuments.

• Testa - Outer hard, thick layer
• Tegmen - Inner thin, papery layer

Endosperm: Triploid (3n) food-storing tissue formed by triple fusion during double fertilisation.

Embryo: The miniature plant. Consists of:

• Radicle - Develops into the root
• Plumule - Develops into the shoot
• Embryonal axis - Connects radicle and plumule
• Cotyledon(s) - Seed leaf/leaves (1 in monocots, 2 in dicots)

• The seed remains attached to the placenta via a stalk-like structure called the funicle.
• When detached, the point of attachment leaves a scar called the hilum.
• In some seeds, a bright-coloured outgrowth develops on the surface called the aril.

Seeds show remarkable variation in size, shape, colour and surface. Some plants can even be identified on the basis of seeds alone.

A state of metabolic inactivity in mature seeds where germination does not occur even under seemingly favourable conditions.

A. External Structure

B. Internal Structure

Seed Dormancy: A physiological state in which a viable seed fails to germinate even under favourable conditions of water, temperature, and oxygen. It is an adaptive mechanism for survival.

Seed Viability: The ability of a seed to germinate under appropriate conditions. It indicates whether a seed is alive or metabolically active.

Conditions for Germination:

Seeds require the following to break dormancy and germinate:

• Adequate water (seeds typically contain 10–15% water in the dormant state)
• Suitable temperature
• Adequate oxygen supply
• Sometimes: exposure to light (photoblastic seeds)

Seed Viability Records:

As ovules mature into seeds, the ovary simultaneously develops into a fruit. The ovary wall becomes the pericarp (fruit wall = epicarp + mesocarp + endocarp).

• A seed is a fertilised, mature ovule with an embryo, stored food (endosperm), and a protective seed coat (outer testa, inner tegmen).
• The embryo consists of the radicle (→ root), plumule (→ shoot), embryonal axis, and cotyledons (one in monocots, two in dicots); endosperm is triploid (3n).
• Dicot seeds have two cotyledons and store food in them (pea, bean); monocot seeds have one cotyledon (scutellum), a bulky endosperm, and a coleoptile and coleorhiza (maize, wheat).
• Albuminous seeds retain the endosperm (wheat, castor); ex-albuminous seeds store food in cotyledons (pea, groundnut).
• The seed attaches by a funicle, leaving a hilum, with a micropyle above it for water entry.
• Seed dormancy is the failure of a viable seed to germinate even in favourable conditions; germination needs water, suitable temperature, and oxygen.
• True fruits form only from the ovary (mango); false fruits include other floral parts (apple); parthenocarpic fruits are seedless and form without fertilisation (banana).