Chapter 6 · ICSE Class 9 Biology (Selina Concise)

Seeds: Structure and Germination

Inside a bean and a maize grain, and what a seed needs to wake up and grow.

🫘 Chapter 6

Dicot seed and monocot grain Bean seed with seed coat, two cotyledons, plumule, radicle, hilum and micropyle; maize grain with pericarp, endosperm, scutellum, plumule and radicle. Dicot seed (bean) Monocot grain (maize) Seed coat Cotyledon (2) Plumule Radicle Hilum Micropyle Endosperm Scutellum Plumule Radicle Pericarp Bean: 2 cotyledons, no endosperm in the mature seed. Maize: 1 cotyledon, food stored in the endosperm.

↔ Swipe the diagram sideways to see all of it

6.1

Structure of seeds

A seed has a protective seed coat (testa and tegmen), a small pore (micropyle), a scar (hilum) and an embryo: radicle (future root), plumule (future shoot) and cotyledons (seed leaves). A dicot seed such as the bean has two cotyledons which store food. A monocot grain such as maize has one cotyledon (the scutellum), a large endosperm that stores food, and a fruit wall fused to the seed coat.

1

Differentiate a dicot seed from a monocot seed.

Dicot: two cotyledons, usually no endosperm in the mature seed, e.g. bean. Monocot: one cotyledon, endosperm present, e.g. maize.

2

What are the radicle and the plumule?

The radicle is the embryonic root; the plumule is the embryonic shoot.

3

What is the function of the micropyle?

It lets water enter the seed and the radicle emerge.

6.2

Germination and conditions needed

Germination is the growth of the embryo into a seedling. Needs: water (softens the coat, activates enzymes), oxygen (for respiration) and a suitable warm temperature. Light is not needed for the germination of most seeds. Epigeal germination: cotyledons are pushed above ground (bean, castor). Hypogeal germination: cotyledons stay underground (pea, gram, maize).

1

Four sets of bean seeds are kept: (A) moist cotton, warm, (B) dry cotton, warm, (C) under a layer of oil on boiled water, (D) moist cotton in a refrigerator. Which germinate?

Only A. B has no water; C has no oxygen; D is too cold.

2

Out of 250 seeds sown, 215 germinate. Find the germination percentage.

215 ÷ 250 × 100 = 86%.

3

Seeds of mass 5.0 g weigh 7.5 g after soaking. Find the percentage gain in mass.

(7.5 − 5.0) ÷ 5.0 × 100 = 50%.

6.3

Epigeal and hypogeal germination

In epigeal germination the hypocotyl (below the cotyledons) lengthens and arches, lifting the cotyledons and plumule out of the soil. In hypogeal germination the epicotyl (above the cotyledons) lengthens, so the cotyledons stay in the soil.

1

Which part elongates in (a) epigeal, (b) hypogeal germination?

(a) Hypocotyl. (b) Epicotyl.

2

Is maize germination epigeal or hypogeal?

Hypogeal; the single cotyledon stays underground.

Practice

Exercises

1. Name the parts of an embryo.

2. What does the radicle grow into? The plumule?

3. What is the hilum?

4. Why is the seed coat important?

5. Name three conditions necessary for germination.

6. Why do seeds need water to germinate?

7. Name one epigeal and one hypogeal seed.

8. In a test, 40 of 50 seeds germinate. Find the germination percentage.

9. Does a seed need light to germinate? Explain.

10. What is dormancy?

Next chapter — Respiration in Plants →