SEEDS

SEEDS

Definition of Seeds

  • Embryo: A small embryonic plant enclosed in a covering seed coat, usually with some stored food.

  • The ovule after fertilization develops into a seed, and the formation of the seed completes the reproductive process in seed plants.

Advantages of Seeds for Plants

  • Dormancy: Seeds can maintain dormancy until better environmental conditions arise.

  • Protection: They afford protection to the young plant at a vulnerable developmental stage.

  • Food Supply: Seeds contain an adequate food supply until photosynthesis is possible.

  • Dispersal: Seeds facilitate the dispersal of plants, allowing species to spread.

Structure of a Seed

A seed is made up of three main structures:

  • Seed Coat

  • Embryo

  • Nutritive Storage Tissue (Food store)

1. Seed Coat

  • Definition: The outermost covering of the seed, protective covering that forms around the fertilized ovule.

  • Composition: Made up of sclerenchyma cells with very thick cell walls, normal characteristics include being hard and water-resistant.

  • Layers: Composed of two layers:

    • Testa: The outer seed coat, thick and protective against pathogens.

    • Tegmen: The inner seed coat, thin and protects the inner layer of the seed.

Parts of the Seed Coat

  • Testa:

    • Developed from the outer integument of the ovule.

    • Also known as episperm or the outer coat.

    • Externally marked by a scar indicating the HILUM - the part attached to the funiculus of the placenta.

    • Micropyle: A minute opening above the Hilum.

    • Raphe: A ridge around the seed formed by the funiculus or stalk in many seeds.

  • Tegmen:

    • Inner membrane or coat of the seed which completely adheres to the testa.

    • Also known as endopleura or internal coat.

Functions of the Seed Coat

  • Prevents embryo from dehydration.

  • Protects the embryo from physical or mechanical damage.

  • Shields the embryo from insects, fungi, and bacterial infections.

  • Guards the embryo during seed dispersal (e.g., protection from stomach acids when eaten by animals).

  • Scarification: Artificial methods to break the seed coat in preparation for germination.

2. Embryo

  • Definition: The rudimentary plant developed from the fertilized egg cell in the embryo sac.

  • Components: Made up of:

    • Radicle: Develops into the root.

    • Plumule: Shoot tip with a pair of miniature leaves.

    • Cotyledons: Filled with stored food for the plant's use before photosynthesis begins.

    • Epicotyl: Above the cotyledon, forms part of the stem.

    • Hypocotyl: Below the cotyledon, also contributes to stem formation.

  • Growth Response: The epicotyl and hypocotyl grow upward in response to light.

3. Nutrient Storage Tissue

  • Definition: The term albumen refers to the storage tissue of the seed. It can be designated as either perisperm or endosperm based on its origin.

Endosperm

  • Description: A nutritive tissue useful for the developing embryo.

  • Properties: Compact tissue that lacks intercellular spaces; stores starch, hemicelluloses, proteins, oils, and fats.

  • Formation in Angiosperms: Formed from the primary endosperm nucleus, derived from the fusion of one male gamete with the secondary nucleus of the embryo sac after fertilization (3n - triploid).

  • Formation in Gymnosperms: The female gametophyte, which is formed before fertilization, acts as the endosperm (1n - haploid).

Endospermic vs Non-Endospermic Seeds

  • Endospermic Seeds:

    • The endosperm is present in the mature seed and serves as the food storage organ.

    • Composed of the testa and endosperm as the two covering layers of the embryo.

  • Non-Endospermic Seeds:

    • Cotyledons serve as the sole food storage organs.

    • Example: Pea (Pisum sativum) where cotyledons absorb food reserves from endosperm, which is nearly degraded in mature seeds.

    • The embryo is fully enclosed by the testa in mature seeds.

Comparative Anatomy of Seeds

  • Arabidopsis thaliana (Endospermic Seed):

    • Seed coat (2n)

    • Endosperm (3n)

    • Embryo (2n)

    • Cotyledon (part of the embryo)

    • Shoot of embryo

    • Root of embryo

  • Pisum sativum (Non-Endospermic Seed):

    • Seed coat (2n)

    • Cotyledons

    • Embryonic axis

  • Example in Rice: The endosperm fills most of the seed while the embryo is comparatively small.

  • Example in Peanut: All of the endosperm's stored resources have been transferred to the embryo before maturation.

Monocot vs Dicot Seeds

  • Monocot Seeds:

    • Cotyledons: One.

    • Size and Shape: Usually smaller and less symmetrical.

    • Cotyledon Function: Thin and absorbs food from the endosperm, which stores food for the embryo.

    • Germination: Produces a single leaf that is usually long and narrow upon germination.

  • Dicot Seeds:

    • Cotyledons: Two.

    • Size and Shape: Usually larger and more symmetrical than monocots.

    • Cotyledon Function: Fleshy and store food for the embryo.

    • Germination: Produces two seed leaves that are larger than the true leaves upon germination.