Seed

SEED 

 

  • Ovules, post-fertilization, transform into seeds.
  • A seed comprises a seed coat and an embryo.
  • Composition of a Seed:

Seed Coat:

- The outer protective layer of the seed.

- Offers physical protection to the embryo.

- Helps prevent desiccation and damage.

 

Embryo:

The central part of the seed, containing the developing plant.

Composed of distinct structures, including the radicle, embryonal axis, and cotyledons.

 

Components of the Embryo-

 

Radicle:

The embryonic root.

The first part to emerge during germination.

Develops into the primary root of the mature plant.

 

Embryonal Axis:

The portion of the embryo between the radicle and the cotyledons.

Includes the hypocotyl (stem-like region) and the plumule (future shoot).

 

Cotyledons:

Structures that store and provide nutrients for the developing embryo.

The number of cotyledons varies among plant species.

Monocots (e.g., wheat, maize) have one cotyledon, while dicots (e.g., gram, pea) have two.

 

Structure of dicotyledonous seed-

 

  • The seed coat is the outermost layer of a seed, serving as a protective covering.
  • The seed coat comprises two layers: the outer testa and the inner tegmen.
  • Seed Coat Features:

Testa: The outer layer of the seed coat.

Tegmen: The inner layer of the seed coat.

Hilum: A scar on the seed coat where the seed was attached to the fruit during development.

Micropyle: A small pore above the hilum that allows for water and gases to enter during germination.

 



  • Embryo Structure:

Within the seed coat resides the embryo.

The embryo consists of the embryonal axis and two cotyledons.

Cotyledons: These structures are often fleshy and packed with reserve food materials.

Embryonal Axis: This axis includes the radicle (embryonic root) and the plumule (future shoot).

 

 

  • Types of Seeds:

Endospermic Seeds: In some seeds like castor, double fertilisation results in endosperm formation, which serves as a food storage tissue.

Non-Endospermous Seeds: Seeds of plants like bean, gram, and pea lack endosperm in their mature state. 

Structure of Monocotyledonous Seed-

 

  • Monocotyledonous seeds can be endospermic or non-endospermic, with orchids being a notable exception.
  • In cereals like maize, the seed coat can be membranous and fused with the fruit wall.
  • Monocotyledonous Seed Structure:

Endosperm: Typically bulky and stores food.

Aleurone Layer: A proteinous layer that separates the embryo from the outer endosperm.

Embryo: Small and located in a groove within the endosperm.

Scutellum: A large, shield-shaped cotyledon that stores nutrients.

Embryonic Axis: Contains the plumule (future shoot) and radicle (embryonic root).

 

  • Protective Sheaths for Embryo:

Coleoptile: A sheath that covers and protects the plumule.

Coleorhiza: A sheath that encloses and safeguards the radicle.

 

  • Seed Coat and Endosperm Interaction:

In some cases, the seed coat is membranous and fuses with the fruit wall.

The endosperm is rich in nutrients and plays a vital role in nourishing the developing embryo.

 

  • Variation in Monocotyledonous Seeds:

Orchids are an exception among monocots, as their seeds can be non-endospermic.