Exploration Class 9 Science Chapter 11: Reproduction: How Life Continues — NCERT Solutions

Explanation: The female reproductive system is built to produce eggs, allow fertilisation and nourish the embryo till birth, so each organ labelled in Fig. 11.19 matches one of these jobs. The two ovaries are the primary sex organs; they produce the ova (egg cells) by meiosis and also secrete the female sex hormones. An ovum released by an ovary is picked up by the fallopian tube (oviduct), and it is inside this tube that a sperm fuses with the egg, so fertilisation in humans is internal. The fertilised egg, or zygote, then moves down into the uterus, a muscular pear-shaped organ whose thick lining allows the embryo to implant and grow into a baby. The cervix is the narrow lower neck of the uterus that opens into the vagina, and the vagina receives the sperms during mating and later forms the birth canal through which the baby is delivered.

Explanation: A pea flower is bisexual and its petals form a closed keel that encloses the stamens and the pistil, so pollen falls on the stigma of the same flower while the flower is still a bud; this is self-pollination. A muslin bag only keeps out insects and foreign pollen and cannot stop this self-pollination, so a bud or a flower bagged with its stamens intact still forms a fruit. Removing the stamens from a bud takes away the only source of pollen before pollination can happen, so the stigma receives no pollen, there is no fertilisation, and no fruit is formed. Removing the stamens from an already opened flower is too late, because pollination has already taken place inside the bud, so that flower still develops into a fruit. The experiment therefore shows that pollination followed by fertilisation is necessary before the ovary can grow into a fruit.

Explanation: Grafting is a method of artificial vegetative propagation in which the stem of one plant is joined to the rooted stem of another so that the two grow as a single plant. The lower rooted part is the stock and it supplies water and minerals absorbed from the soil, while the upper cut twig, the scion, carries the buds and decides the characters of the shoot, flowers and fruit. The cuts on the stock and the scion must match and be pressed together so that their cambium layers, the actively dividing tissue, are in contact; only then can new cells form and fuse the two parts into one continuous plant. Tying the joint holds the parts firm and prevents drying and infection, and watering with shade until the graft takes ensures the scion survives on the water supplied by the stock. Since the scion is a part of the parent plant and no gametes are involved, the new shoot is genetically identical to the parent, which is how a desired variety of fruit is reproduced exactly.

Explanation: A cutting is a piece of stem bearing nodes, and the node is the important part because it carries a bud and meristematic (actively dividing) tissue from which new roots and shoots can arise. The buried nodes, kept moist and in contact with the soil, give out adventitious roots, while the buds at the nodes above the soil grow into new shoots with leaves, so the same cutting builds both the organ systems it needs. The cutting is planted slantingly at about 45 degrees so that a greater length of stem stays in contact with moist soil, which increases the surface available for root formation and helps the cutting stand firm. Compost supplies nutrients and regular watering prevents the cutting from drying out before its roots form, since at first it has no root system of its own. As no gametes and no fertilisation are involved, this is vegetative (asexual) propagation and the new plant is a clone, genetically identical to the parent, so it keeps the parent's flower colour and other desirable characters.

Explanation: The male reproductive system has to produce sperms, nourish them and deliver them into the female body, and each part labelled in Fig. 11.18 performs one of these jobs. The testes are the primary sex organs; they produce sperms by meiosis and secrete the male hormone testosterone, and they lie in the scrotum, a sac outside the abdomen that keeps them a few degrees cooler than body temperature, a condition sperm formation requires. Sperms made in a testis travel through the vas deferens, which opens into the urethra, a common tube that carries both sperms and urine and runs through the penis, the organ that transfers sperms into the female body. The seminal vesicles and the prostate gland are the accessory glands: they add their secretions to the sperms, providing nutrition and a fluid medium in which the sperms can swim. This mixture of sperms and glandular secretions is called semen.

Explanation: A flower is the reproductive part of a plant and is made of four whorls borne on the thalamus, which is why the observation is made from the outermost whorl inwards. The outermost calyx is made of green sepals that enclose and protect the flower while it is still a bud, and next comes the corolla of brightly coloured, often scented petals that attracts insects and birds for pollination. The third whorl, the androecium, is the male part made of stamens, each having an anther that produces pollen grains carrying the male gametes on a stalk called the filament; the innermost gynoecium or pistil is the female part with a sticky stigma that receives the pollen, a style, and a swollen ovary containing the ovules. A transverse section of the ovary shows the ovules attached to the placenta inside chambers called locules and a longitudinal section shows the same ovules in side view, which matters because after fertilisation each ovule becomes a seed and the whole ovary becomes the fruit. In Table 11.1 flower A has sepal, petal and pistil but no stamen, so it is a unisexual female flower such as a papaya or pumpkin female flower, whereas flowers having both stamens and pistil, like hibiscus or mustard, are bisexual.

Explanation: During meiosis the two chromosomes of a pair separate and only one of them enters each gamete, and every pair segregates independently of the other pairs. So each pair offers 2 possible choices, and with 3 pairs the total number of combinations is 2 x 2 x 2 = 2³ = 8, which is why one bead of each colour is picked to build each gamete in Fig. 11.9. Applying the same rule to the 23 pairs of chromosomes in a human gives 2²³ = 8,388,608 genetically different gametes from a single person. Since a zygote is formed by the chance fusion of one such sperm with one such egg, the possible combinations rise to about 2²³ x 2²³, roughly 7 x 10¹³, and crossing over between the chromosomes of a pair makes the number effectively unlimited. This independent assortment during meiosis is the main source of variation and explains why children of the same parents differ from one another and from their parents.

Explanation: Layering is a method of artificial vegetative propagation in which a branch is made to produce roots while it is still attached to the parent plant. The buried node is the key part, because a node carries buds and meristematic tissue from which adventitious roots arise when it is kept in moist, dark soil. The advantage over a cutting is that the twig is not separated at first, so it keeps receiving water, minerals and food from the parent through its own vascular tissue and does not dry out or starve while its roots are forming, which makes layering more reliable. Only after the buried part has grown a good root system is the twig cut away from the parent, and it then survives on its own as a new plant, as shown for the lemon plant in Fig. 11.4. Since only a body part of a single parent is involved and no gametes or fertilisation take place, the new plant is a clone, genetically identical to the parent, and it retains all the parent's desirable characters.

Explanation: In asexual reproduction only one parent is involved and no gametes fuse, so the offspring are clones, genetically identical to the parent. This is exactly what a farmer wants when a variety already has good qualities such as sweet fruit, high yield or a particular flower colour, because those characters are passed on unchanged; it is also faster than growing from seed, gives a uniform crop that ripens together, and is the only choice for plants such as banana, sugarcane and seedless grapes that do not form viable seeds. Sexual reproduction, on the other hand, involves meiosis and the fusion of gametes from two parents, so the seeds give rise to plants showing variation. A farmer prefers it when variation is useful, that is to develop new and improved varieties and to breed hybrids that resist disease, pests or drought, and also because seeds are small and easy to store, transport and sow over large areas. Variation also protects the crop, since a genetically varied field is far less likely to be wiped out completely by a single disease or by a change in the weather.

Explanation: A gamete is haploid, which means it receives only one chromosome from each pair, and during meiosis each pair separates independently of every other pair. Because each pair offers 2 choices and the choices are independent, the total number of combinations is obtained by multiplying 2 by itself once for every pair, that is 2ⁿ for n pairs of chromosomes. With the three pairs of beads used in the activity this gives 2³ = 8 different gametes, and with the 23 pairs of chromosomes in a human it gives 2²³ = 8,388,608 genetically different gametes. Crossing over, in which the chromosomes of a pair exchange segments, shuffles the characters still further, so the number of possible gametes in practice is even larger than this. This is why every gamete is unique and why sexual reproduction produces so much variation.

Explanation: Pollination is the transfer of pollen grains from the anther to the stigma and it is the necessary first step of sexual reproduction in a flowering plant. Only after pollen lands on the stigma does it germinate and grow a pollen tube down the style to the ovule, carrying the male gamete to the female gamete for fertilisation. If pollination never happens, no pollen tube grows, the male and female gametes never fuse and no zygote is formed. As a result the ovules cannot develop into seeds and the ovary cannot ripen into a fruit, so the unfertilised flower dries up and falls off. The plant would then have no seeds to give rise to the next generation, and unless it can multiply by vegetative propagation its kind would not continue.

Explanation: Humans have 23 pairs of chromosomes, of which one pair is the sex chromosomes: XX in a female and XY in a male. During meiosis a gamete receives only one chromosome of each pair, so a mother (XX) can make only one kind of egg as far as sex is concerned, one that carries an X. A father (XY), however, makes two kinds of sperms in equal numbers, half carrying an X and half carrying a Y. If an X-bearing sperm fertilises the egg the zygote is XX and the child is a girl, and if a Y-bearing sperm fertilises it the zygote is XY and the child is a boy. Since the mother's contribution is always X and only the father's contribution varies, it is the father who determines the sex of the child. As X-bearing and Y-bearing sperms are produced in equal numbers the chance of a boy or a girl is about 50 percent each, so it is scientifically wrong and unjust to blame the mother for the sex of a child.

Explanation: Vegetative propagation involves a single parent with no gametes and no fertilisation, so every new plant is produced from body cells by mitosis and is a clone, genetically identical to the parent and to all the other plants raised from it. Genes decide resistance to disease, so if the parent's genes make it susceptible to a particular fungus or virus, then every plant in that field carries exactly the same weakness. A pathogen that can overcome one plant can therefore overcome all of them, and the disease sweeps through the whole crop. In sexual reproduction, meiosis and the fusion of gametes from two parents reshuffle the genes, so plants grown from seed show variation and are not all alike. Some of these varied plants happen to carry genes for resistance; they survive the attack and continue the population, which is why variation acts as a natural insurance for a species.

Explanation: Yeast and hydra have a very simple body design with little division of labour, so their cells are not highly specialised and almost any part of the body can grow back into a complete organism, which makes budding or fission a quick and successful way to reproduce. In complex animals and flowering plants the body is organised into specialised tissues, organs and organ systems, and a specialised body cell such as a nerve cell or a leaf cell cannot by itself build a whole new individual. Such organisms therefore maintain a separate set of reproductive organs that produce gametes by meiosis, and two gametes fuse to form a zygote that develops into the new individual. Sexual reproduction brings a second and deeper advantage: the shuffling of genes during meiosis and the fusion of gametes from two parents create variation among the offspring. Complex organisms are long-lived, produce fewer offspring and must survive changing surroundings, so this variation is essential, since it lets some individuals adapt and survive and it supplies the raw material for evolution.

Explanation: Removing the anthers (emasculation) prevents self-pollination. Introducing pollen from another plant of the same species is the definition of cross-pollination.