Seeds: Structure and Germination — ICSE Class 9 Biology Important Questions

Answer: (b) Radicle.

The radicle emerges first, grows downward (positive geotropism) and forms the primary root; the plumule emerges later to form the shoot.

Answer: (b) Endosperm.

Maize is a monocot endospermic seed; its single cotyledon (scutellum) absorbs food from the persistent endosperm during germination.

Answer: (b) Epigeal germination.

Rapid elongation of the hypocotyl arches upward and lifts the cotyledons above the soil, as in the bean and castor seed.

Answer: (c) Both water and oxygen.

X lacks water so it fails; Z has water but the boiled, oil-sealed water excludes oxygen so it fails; Y has both water and oxygen and germinates. Hence both water and oxygen are essential.

Answer: (a) In the pea the epicotyl grows rapidly so the plumule is pushed up while the cotyledons stay underground, which is exactly the definition of hypogeal germination. R correctly explains A.

Germination is the process by which the dormant embryo of a mature seed resumes active growth and develops into a young seedling when supplied with water, oxygen and a suitable temperature.

The two main types based on the fate of the cotyledons are:

1. Epigeal germination — cotyledons are pushed above the soil (e.g. bean).
2. Hypogeal germination — cotyledons remain below the soil (e.g. maize, pea).

(i) Radicle: it is the embryonic root; on germination it emerges first, grows downward and develops into the primary root system that fixes the plant and absorbs water and minerals.

(ii) Micropyle: it is a small pore in the seed coat through which water is chiefly absorbed (imbibition) at the start of germination; it also allows entry of oxygen for respiration of the embryo.

The bean seed is a dicotyledonous, non-endospermic seed. Its main parts are:

• Seed coat — the outer protective covering made of an outer testa and an inner tegmen; it bears a scar called the hilum and a pore called the micropyle.
• Two cotyledons — fleshy seed leaves that store food (starch and protein) used by the growing embryo.
• Embryo — consisting of the radicle (embryonic root) and the plumule (embryonic shoot), joined by the tigellum (embryonic axis).

Because food is stored in the cotyledons and the endosperm is absent, the bean is called non-endospermic.

In addition, the bean shows a distinct hilum and micropyle, while maize shows a whitish scutellum and a coleoptile and coleorhiza that protect the plumule and radicle.

(i) A boiled seed does not germinate because boiling kills the living embryo and denatures the enzymes and stored food; the dead embryo can no longer resume growth.

(ii) Seeds sown too deep fail to germinate because deep in the soil there is very little oxygen for respiration, and the small stored food reserve is exhausted before the plumule can reach the surface and begin photosynthesis.

(iii) Germinating seeds feel warm because they respire rapidly, and respiration is an exothermic process that releases heat energy as the stored food is oxidised.

Germination needs both external and internal conditions.

External conditions

1. Water (moisture): Water is absorbed through the micropyle causing the seed to swell (imbibition). It softens the seed coat, activates enzymes, dissolves stored food and helps transport it to the growing embryo. Without water the enzymes remain inactive.

2. Oxygen: The germinating embryo respires aerobically to release energy for growth. Oxygen from soil air is needed for this respiration; waterlogged or deeply buried seeds get too little oxygen and may die or respire anaerobically.

3. Suitable temperature: Enzymes controlling germination work best at an optimum warm temperature (about 25 to 35 degrees Celsius for most seeds). Too low a temperature slows enzyme action; too high a temperature denatures the enzymes.

4. Light: Most seeds germinate in darkness, but a few (e.g. lettuce, tobacco) need light, while some are inhibited by it.

Internal conditions

1. A viable and mature embryo capable of growth.
2. Adequate stored food in the cotyledons or endosperm.
3. End of dormancy — the seed must have completed its resting period and the germination-inhibiting substances must be removed or the seed coat made permeable.

Epigeal germination (bean, castor, sunflower)

• The hypocotyl (region below the cotyledons) elongates rapidly and forms an arch that pushes the cotyledons above the soil.
• The cotyledons turn green, may carry out photosynthesis for a short time, and then shrivel and fall as the true leaves develop.

Hypogeal germination (maize, pea, gram)

• The epicotyl (region above the cotyledons) elongates while the cotyledon(s) remain below the soil.
• In maize the plumule is protected by a sheath called the coleoptile and the radicle by the coleorhiza as they grow through the soil; the cotyledon stays underground and its food is absorbed.

Thus the essential difference lies in whether the hypocotyl or the epicotyl elongates, which decides whether the cotyledons are lifted out of the soil.

(i) Comparing A and B tests the need for water (moisture) — both are at the same temperature and open to air; only A has moisture, and only A germinates, showing water is essential.

(ii) The seeds in Tube C failed to germinate because the low refrigerator temperature (about 4 degrees Celsius) is far below the optimum; at this temperature the germination enzymes are almost inactive, so growth of the embryo cannot proceed.

(iii) The layer of oil in Tube D seals the surface and prevents oxygen from the air from dissolving into the water. Since the water was also boiled to drive out dissolved air, the seeds are deprived of oxygen for respiration and therefore do not germinate — this shows that oxygen is essential.

(iv) The process is imbibition, the absorption of water by the dry hydrophilic substances of the seed causing it to swell.