NCERT Solutions for Class 9th Science Chapter 11 Chapter opener — Think It Over

Book page 208 Updated on2026-09-08

Q1.

When does a farmer prefer asexual or sexual methods of reproduction for crops production?

Answer

A farmer uses an asexual (vegetative) method when the plant must be an exact copy of a chosen parent, and a sexual (seed) method when he wants seed that stores well, travels cheaply and carries new combinations of characters.

Farmer wants…	Method chosen	Crops	Reason
Every plant identical to one elite mother plant	Vegetative — cutting, grafting, layering, tissue culture	Sugarcane setts, potato tubers, banana plantlets, grafted mango and rose, ginger	Only mitosis is involved, so all offspring are clones: same sweetness, same fruit size, same ripening date
A crop that sets no usable seed	Vegetative only	Banana, seedless grape, sugarcane	There is no other route — the plant cannot be raised from seed at all
A quick harvest	Vegetative	Sugarcane, potato	A cutting or tuber already has nodes and stored food, so it skips seed germination and seedling stages
Cheap, storable, transportable planting material and new varieties	Sexual — seed	Wheat, rice, maize, mustard, pulses	Seeds are small and dry; every seed carries a fresh combination of characters, so better plants can be selected

Why it happens: the two routes differ in one step. In vegetative propagation the new plant is built by mitosis from cells of the parent, so the chromosome sets are copied without any reshuffling — the daughter plant is a clone. In seed production the parent first makes gametes by meiosis (chromosome pairs separate independently) and two gametes then fuse. That reshuffling is exactly what the farmer wants when he is breeding, and exactly what he does not want when he is multiplying a variety he has already perfected.

Tip: the convenience of clones has a price. A field of clones has no genetic variation, so a disease that can attack one plant can attack every plant in it. That is why the chapter recommends virus-free tissue-culture banana plantlets, and why a wise farmer keeps more than one variety.

Q2.

Why do you think most complex animals and flowering plants use sexual reproduction, while many simple organisms, like yeast and hydra mainly reproduce asexually?

Answer

Sexual reproduction buys variation; asexual reproduction buys speed. Complex organisms need variation more, simple organisms need speed more.

Why complex organisms go sexual. A mango tree or an elephant lives for years, produces relatively few offspring and meets a changing world — new pests, new diseases, changing rainfall. Sexual reproduction gives every offspring a new combination of characters, so some individuals in a family will happen to cope with the new condition.

number of gamete types from n chromosome pairs = 2ⁿ
in humans, n = 23 → 2²³ = 8,388,608 kinds of gamete
zygote combinations = 2²³ × 2²³ = 2⁴⁶ ≈ 7.0 × 10¹³

Seven lakh crore possibilities — and that is before counting the extra mixing the chapter hints at. No two children of the same parents are alike, and that is the point.

Why simple organisms go asexual. A yeast cell in a warm sugar solution or a hydra in a pond is small, short-lived and sitting in food. What matters there is filling the space before the food runs out. Budding needs no partner, no gamete, no meeting of two cells — a single cell simply divides by mitosis. A yeast cell can put out a bud within a couple of hours, so the population climbs steeply while conditions last.

Why it happens: finding a mate and making gametes costs time and energy, and only half of a parent's genes reach any one offspring. Asexual reproduction avoids all of that. The cost is that all offspring are identical, so one unfavourable change hits the whole population. A long-lived, slow-breeding organism cannot afford that risk; a fast-breeding one can, because it turns over so many generations that it will meet the next good patch anyway.

Did you know? Yeast and hydra are not only asexual. When food runs short or the water cools, both can switch to sexual reproduction — exactly when variation becomes worth its cost.

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