What is Seed?

What is a seed?

Before discussing seed germination, it's helpful to review the meanings of the terms fruit, seed, and grain. A fruit is an enlarged, ripened ovary with its wall forming the fruit wall, which encloses the seed. The fruit protects the seed and aids in its dispersal.

Examples include mango and pea pod.

Seed

A seed is a mature ovule. It contains an embryo that can grow into a new plant. The seed coat protects the embryo from physical harm. Examples include bean seeds and peas.

Grain

In plants like maize and wheat, the grain is a fusion of the fruit wall and the seed coat, forming a single protective layer.

Classification and Structure of Seeds

Types of Seeds

Seeds are broadly classified into two types: monocotyledonous and dicotyledonous.

• Monocotyledonous seeds have one cotyledon (seed leaf), such as in maize and grasses.
• Dicotyledonous seeds have two cotyledons, like in peas, grams, and beans.

Seed Size and Variety

Seeds come in various sizes:

• Some are tiny and barely visible to the naked eye, like poppy and orchid seeds.
• Some are large, such as in watermelons, pumpkins, or even mangoes (the stony part).
• The largest seeds are from coconuts and double coconuts.

While the size, shape, and structure of seeds differ among plant species, most seeds share a similar basic structure.

Based on the presence of endosperm, seeds can be classified as follows:

(i) Albuminous (endospermic): In these seeds, the cotyledons are thin and membranous, and the endosperm remains present. Examples include:

• Dicot albuminous seeds: poppy, custard apple.
• Monocot albuminous seeds: cereals, millets, palm.

(ii) Exalbuminous (non-endospermic): In these seeds, the cotyledons store food, becoming thick and fleshy. Examples include:

• Dicot exalbuminous seeds: gram, pea, mango, mustard.
• Monocot exalbuminous seeds: Vallisneria, orchids, amorphophallus.

The Bean Seed

There are various kinds of beans, such as broad bean, lima bean, and French bean, but their seeds generally share the same structure. Most are kidney-shaped with both a convex and a concave side.

Structure of Bean Seed

A&B – External Structure, C – Testa Removed, D – Seed Cut Open to Show Various Parts

• Seed Coat (Testa): The outermost, hard, brownish layer that protects the inner parts of the seed from damage and attacks by bacteria, fungi, and insects.

• Tegmen: A thin inner layer next to the testa, providing additional protection.

• Hilum: A distinct, white, oval scar on the concave side of the seed, indicating where the ovule was attached to the ovary wall.

• Micropyle: A tiny pore near the hilum, marking the entry point of the pollen tube. It serves two functions:

  1. Absorbs water for the embryo during germination.
  2. Allows the diffusion of respiratory gases for the growing embryo.

• Cotyledons: Two thick structures beneath the seed coat containing food for the embryo and providing protection. Separating them reveals the tiny embryo.

• Embryo: Consists of:

  • Radicle: Later forms the root.
  • Plumule: Later forms the shoot, including a short stem with tiny leaves and a growing point.

(Note: Do not confuse "radicle" with "radical," a chemistry term, to avoid losing marks in exams.)

Maize Grain

• Maize Grain: A fruit where the fruit wall and seed coat are fused into a protective layer, making it a grain.

• Embryo Location: Indicated by a small, light-colored oval area on one side of the grain.

• Endosperm: The major part of the grain, rich in starch, separated from the embryo by a thin epithelial layer. The outermost layer of the endosperm is rich in protein and called the aleurone layer.

• Embryo: Contains a single cotyledon (scutellum), a radicle, and a plumule.

  • Radicle: Located towards the pointed end, enclosed in a protective sheath called the coleorhiza.
  • Plumule: Located towards the upper broader side, enclosed in a protective sheath called the coleoptile.

• Monocotyledonous and Endospermic: Maize grain is a type of grain with one cotyledon and an endosperm. Examples include rice, wheat, and oats.

Major Differences between Bean Seed and Maize Grain