Seeds: Structure and Germination
ICSE Class 9 · Biology · Chapter 6
What this chapter covers
- The seed: a plant waiting to grow
- Structure of the bean seed
- Structure of the maize grain
- Differences between monocot and dicot seeds
- Germination: from seed to seedling
- Germination of the bean seed
- Germination of the maize grain
- Hypogeal and epigeal germination
- Conditions for seed germination
- Experiments on the conditions for germination
Key points
- A seed is a mature, fertilised ovule with an embryo, stored food and a seed coat made of an outer testa and an inner tegmen.
- The embryo has a radicle (future root), a plumule (future shoot) and one or two cotyledons; the hypocotyl lies below the cotyledons and the epicotyl above them.
- Seeds are dicotyledonous or monocotyledonous by cotyledon number, and endospermic or non-endospermic by whether the endosperm persists.
- The bean seed is a dicot, non-endospermic seed: kidney-shaped, with hilum and micropyle, and food stored in two fleshy cotyledons.
- The maize grain is a caryopsis, a one-seeded fruit with the fruit wall fused to the seed coat; it is a monocot, endospermic seed.
- In maize the endosperm (with its protein-rich aleurone layer) stores food, the scutellum absorbs it, the coleoptile protects the plumule and the coleorhiza protects the radicle.
- Germination is the resumption of growth by the embryo to form a seedling; it begins with imbibition and enzyme activity, and the radicle always emerges first.
- In epigeal germination (bean, castor, onion) the hypocotyl elongates and lifts the cotyledons above the soil; in hypogeal germination (pea, gram, maize) the epicotyl elongates and the cotyledons stay below.
- The type of germination does not follow the monocot and dicot division.
- Water, a suitable temperature and oxygen are the three essential external conditions for germination; light is not needed by most seeds.
- Each condition is proved by an experiment that withdraws only that condition and compares the result with a control in which all conditions are supplied.
Questions answered
Define a seed, and name its three basic parts.
A seed is a mature, fertilised ovule that contains an embryo, a store of food and a protective seed coat. Its three basic parts are the seed coat, the embryo and the stored food (in the cotyledons or the endosperm).
Distinguish between the hilum and the micropyle of a bean seed.
The hilum is an oval scar on the concave side of the seed, marking where the seed was attached to its stalk in the pod. The micropyle is a tiny pore near one end of the hilum through which water enters the seed; the radicle points towards it.
Name the parts of the embryo of a bean seed, and state what each grows into.
The two cotyledons store food and are used up during germination; the radicle grows into the primary root; the plumule grows into the shoot. The hypocotyl and epicotyl are the parts of the axis below and above the cotyledons.
Why is a maize grain called a fruit and not a seed?
In a maize grain the fruit wall (pericarp) is fused with the seed coat, so the grain is a one-seeded fruit called a caryopsis, not a seed alone.
Give the functions of the scutellum, the coleoptile and the coleorhiza.
The scutellum, the single cotyledon, absorbs digested food from the endosperm and passes it to the embryo axis. The coleoptile is a sheath that protects the plumule as it grows through the soil. The coleorhiza is a sheath that protects the radicle as it emerges.
Give four differences between a bean seed and a maize grain.
The bean has two cotyledons, the maize grain one. The bean stores food in its cotyledons, the maize grain in its endosperm. The bean has no coleoptile or coleorhiza, while the maize grain has both. The seed coat of the bean is free, with a visible hilum and micropyle, while in the maize grain the seed coat is fused with the fruit wall and neither is visible.
Castor has two cotyledons and a large endosperm. Classify its seed in two ways.
By the number of cotyledons it is a dicotyledonous seed; by its food store it is an endospermic (albuminous) seed. Castor shows that not every dicot seed is non-endospermic.
What is imbibition? Why does a germinating seed need water?
Imbibition is the absorption of water by dry seeds and other dry plant materials, which makes them swell. Water softens and bursts the seed coat, activates the enzymes that digest stored food, dissolves the digested food so it can travel to the growing parts, and is needed by every growing cell.