Double Fertilisation, Embryo, Seed and Fruit: Quick Revision

A brief note on Double Fertilisation, Embryo, Seed and Fruit. Read it before a test. Try each question at the end before you open its answer.

Double fertilisation and the endosperm

• The pollen tube releases two male gametes into a synergid.
• Syngamy: male gamete + egg gives the zygote, $2n$.
• Triple fusion: male gamete + two polar nuclei gives the primary endosperm nucleus (PEN), $3n$.
• Double fertilisation is unique to flowering plants.
• The endosperm starts before the embryo. It is free-nuclear first, as in tender coconut water, and cellular later, as in the white kernel.
• The endosperm is used up in pea, groundnut and beans. It stays in castor, coconut, wheat and maize.

Embryo and seed

• Stages: zygote, proembryo, globular, heart-shaped, mature embryo.
• Dicot embryo: two cotyledons. Epicotyl (with plumule) above, hypocotyl (with radicle and root cap) below.
• Monocot embryo of a grass: scutellum, coleoptile around the shoot apex, coleorhiza around the radicle.
• Ovule to seed: integuments to seed coat, micropyle to a pore, zygote to embryo, PEN to endosperm.
• Non-albuminous: pea, groundnut. Albuminous: wheat, maize, barley, castor, sunflower. Perisperm: black pepper, beet.
• A mature seed has 10 to 15 per cent water and may be dormant.

Fruit, apomixis and polyembryony

• The ovary becomes the fruit, and the ovary wall becomes the pericarp. A false fruit includes the thalamus (apple, strawberry).
• Parthenocarpy: fruit without fertilisation (banana), seedless.
• Apomixis: seeds without fertilisation, from a diploid egg or from nucellar cells. The offspring are like the mother.
• Polyembryony: more than one embryo in a seed (Citrus, mango).

A worked example

A plant has 14 chromosomes in its root-tip cells. How many chromosomes are in its zygote, and how many in an endosperm cell?

1. The zygote is diploid, with 14 chromosomes.
2. The endosperm is triploid. One haploid set has 7 chromosomes, so the endosperm cell has $3 \times 7 = 21$.

Practice with new numbers

A program to try

The program counts the chromosomes in the parts of a seed.

Mistakes to avoid

• Calling the endosperm diploid. It is triploid.
• Mixing up the coconut parts. The water is free-nuclear, and the kernel is cellular.
• Mixing up the sheaths of a grass embryo. The coleoptile covers the shoot, and the coleorhiza covers the root.
• Calling an apple a true fruit. It is a false fruit.

Check yourself

1. What does the primary endosperm nucleus become?

2. Which part of the ovule becomes the seed coat?

3. What is a parthenocarpic fruit?

4. Name a plant with polyembryony.

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 1.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.