Sexual Reproduction in Flowering Plants — NCERT Solutions

Triple fusion is the fusion of one male gamete with the two polar nuclei in the central cell of the embryo sac.

It takes place inside the embryo sac after the pollen tube enters one of the synergids and releases the male gametes. One male gamete fuses with the egg cell, while the other fuses with the two polar nuclei.

The nuclei involved in triple fusion are:

• one male gamete
• two polar nuclei

This forms a triploid primary endosperm nucleus (PEN).

The zygote remains dormant for some time so that endosperm develops first. The chapter says that most zygotes divide only after some amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.

(a) Hypocotyl and epicotyl

• Hypocotyl: the cylindrical portion of the embryonal axis below the cotyledons; it ends in the radicle.
• Epicotyl: the portion of the embryonal axis above the cotyledons; it ends in the plumule.

(b) Coleoptile and coleorrhiza

• Coleoptile: a hollow foliar sheath that encloses the shoot apex and leaf primordia in monocot embryos.
• Coleorrhiza: an undifferentiated sheath that encloses the radicle and root cap in monocot embryos.

(c) Integument and testa

• Integument: the protective covering(s) of the ovule.
• Testa: the hard seed coat formed when the integuments harden after fertilisation.

(d) Perisperm and pericarp

• Perisperm: persistent nucellus remaining in some seeds, for example black pepper and beet.
• Pericarp: the fruit wall, formed from the ovary wall after fertilisation.

Apple is called a false fruit because the fruit develops not only from the ovary but also from the thalamus.

So, in apple, the thalamus contributes to fruit formation along with the ovary. Hence it is a false fruit.

Emasculation means the removal of anthers from a flower bud before the anthers dehisce.

A plant breeder employs this technique in artificial hybridisation when the female parent has bisexual flowers. It is done before anther dehiscence so that self-pollination cannot occur. This ensures that the stigma receives only the desired pollen from the male parent.

If growth substances can induce parthenocarpy, fruits such as banana would be good candidates. The chapter specifically mentions banana as a parthenocarpic fruit that develops without fertilisation and is therefore seedless. Such fruits are useful because seedless fruits are often preferred for consumption.

The tapetum is the innermost wall layer of the microsporangium. It nourishes the developing pollen grains.

In relation to pollen-grain wall formation, the tapetum supplies materials needed for the development of the exine. The exine is made of sporopollenin, and the tapetum helps in providing/supporting its formation while the pollen develops.

Apomixis is the formation of seeds without fertilisation. It is a form of asexual reproduction that mimics sexual reproduction.

Its importance is that it can help in the production of hybrid seeds without segregation of characters in the progeny. Farmers could then use the same hybrid seeds repeatedly, reducing the need to buy new hybrid seeds every year. It is therefore very useful in agriculture and horticulture.