NCERT Solutions for Class 12 Science Biology Chapter 2 - Sexual Reproduction In Flowering Plants

Question 1:

Among the terms listed below, those that of are not technically correct names for a floral whorl are:

i. Androecium
ii. Carpel
iii. Corolla
iv. Sepal

(a) i and iv,
(b) iii and iv
(c) ii and iv
(d) i and ii.

Answer:

The four whorls of a flower are calyx, corolla, androecium, and gynoecium. The sepal is the individual unit of the outermost whorl of the flower, calyx and carpel is the individual unit of the innermost whorl of the flower, gynoecium.

Hence, the correct answer is option (c).

Question 2:

Embryo sac is to ovule as _______ is to an anther.

a. Stamen
b. Filament
c. Pollen grain
d. Androecium

Answer:

Embryo sac is the female gametophyte and contains the female gamete, egg. It is present in the nucellus of the ovule which is also called megasporangium. Similarly, the pollen grain is the male gametophyte and contains the male gametes. It is produced in the anther which is referred to as microsporangium.

Hence, the correct answer is option c.

Question 3:

In a typical complete, bisexual and hypogynous flower the arrangement of floral whorls on the thalamus from the outermost to the innermost is:

a. Calyx, corolla, androecium and gynoecium
b. Calyx, corolla, gynoecium and androecium
c. Gynoecium, androecium, corolla and calyx
d. Androecium, gynoecium, corolla and calyx

Answer:

A typical complete, bisexual and hypogynous flower has all the four whorls (accessory and reproductive whorls) and the ovary is superior. The arrangement of floral whorls in such a flower on the thalamus from outermost to the innermost is calyx followed by corolla, androecium and gynoecium.

Hence, the correct answer is option a.

Question 4:

A dicotyledonous plant bears flowers but never produces fruits and seeds. The most probable cause for the above situation is:

a. Plant is dioecious and bears only pistillate flowers
b. Plant is dioecious and bears both pistillate and staminate flowers
c. Plant is monoecious
d. Plant is dioecious and bears only staminate flowers.

Answer:

After fertilisation, the ovary develops into a fruit and ovules develop into seeds. So, a dicotyledonous plant will produce fruits and seeds if it is monoecious and bears bisexual flowers or both pistillate and staminate flowers or it is dioecious and bears pistillate flowers only.
Hence, the correct answer is option d.

Question 5:

The outermost and innermost wall layers of microsporangium in an anther are respectively:

a. Endothecium and tapetum
b. Epidermis and endodermis
c. Epidermis and middle layer
d. Epidermis and tapetum

Answer:

A typical microsporangium in an anther is generally surrounded by four wall layers – the epidermis, endothecium, middle layers and the tapetum, of which epidermis is the outermost layer, endothecium present inner to epidermis followed by middle layers and the tapetum is the innermost layer.

Hence, the correct answer is option d.

Question 6:

During microsporogenesis, meiosis occurs in:

a. Endothecium
b. Microspore mother cells
c. Microspore tetrads
d. Pollen grains.

Answer:

Microsporogenesis is the process of formation of microspores (haploid) from a pollen mother cell (diploid), also called microspore mother cell (MMC) through meiosis.

Hence, the correct answer is option b.

Question 7:

From among the sets of terms given below, identify those that are associated with the gynoecium.

a. Stigma, ovule, embryo sac, placenta
b. Thalamus, pistil, style, ovule
c. Ovule, ovary, embryo sac, tapetum
d. Ovule, stamen, ovary, embryo sac

Answer:

The female reproductive part of the flower, gynoecium consists of the pistil(s). Each pistil has three parts- stigma, style, and ovary. The ovary contains ovules that are borne on the placenta in the ovarian cavity. An ovule contains the embryo sac in its nucellus.

Hence, the correct answer is option a.

Question 8:

Starting from the innermost part, the correct sequence of parts in an ovule are,

a. egg, nucellus, embryo sac, integument
b. egg, embryo sac, nucellus, integument
c. embryo sac, nucellus, integument, egg
d. egg, integument, embryo sac, nucellus.

Answer:

Each ovule has one or two protective envelopes called integuments. These integuments encircle a mass of cells called nucellus where an embryo sac or female gametophyte containing the female gamete, the egg is present. So, the correct sequence of parts of an ovule from innermost to outermost is egg, embryo sac, nucellus, and integuments.

Hence, the correct answer is option b.

Question 9:

From the statements given below choose the option that is true for a typical female gametophyte of a flowering plant:

i. It is 8-nucleate and 7-celled at maturity
ii. It is free-nuclear during the development
iii. It is situated inside the integument but outside the nucellus
iv. It has an egg apparatus situated at the chalazal end

(a) i and iv,
(b) ii and iii
(c) i and ii
(d) ii and iv

Answer:

In angiosperms, a typical female gametophyte or embryo sac consists of an egg apparatus and is situated at the micropylar end, two polar nuclei at the center, and three antipodal cells at the chalazal end of the ovule. The megaspore mother cell undergoes meiotic division to produce four megaspores. Out of them, three are degenerate and only one is functional. This megaspore undergoes three mitotic divisions to form 8 nuclei, hence, a free nuclear division. Out of the 8 nuclei, three move to the chalazal end and form the antipodal cells. Three form the egg apparatus at the micropylar end, while two nuclei occupy the central region, forming a single-celled binucleate polar cell on maturity.

Hence, the correct answer is option c.

Question 10:

Autogamy can occur in a chasmogamous flower if:

a. Pollen matures before maturity of ovule
b. Ovules mature before maturity of pollen
c. Both pollen and ovules mature simultaneously
d. Both anther and stigma are of equal lengths.

Answer:

Autogamy is the transfer of the pollen grains from the anther to the stigma of the same flower. Chasmogamous flowers are open flowers in which both stigma and anthers are exposed. Thus, autogamy in chasmogamous flowers occurs only when

both stigma and anther lie close to each other and
pollen grains are released at the time when stigma becomes receptive.

Hence, the correct answer is option b.

Question 11:

Choose the correct statement from the following:

a. Cleistogamous flowers always exhibit autogamy
b. Chasmogamous flowers always exhibit geitonogamy
c. Cleistogamous flowers exhibit both autogamy and geitonogamy
d. Chasmogamous flowers never exhibit autogamy

Answer:

Cleistogamous flowers are closed flowers so they never expose their anthers and stigma. Thus, there is no chance of landing pollen grains from other plants. In such flowers, their stigma lie close to the anthers so, when anthers dehisce in the flower buds, pollen grains come in contact with the stigma bringing about pollination. Thus, cleistogamous flowers always exhibit autogamy. In contrast, chasmogamous flowers are open flowers with their exposed stigma and anthers. The stigma of such flowers can receive pollen grains from the same flower or same or different plant. Thus, chasmogamous flowers can exhibit autogamy, geitonogamy, or xenogamy.

Hence, the correct answer is option a.

Question 12:

A particular species of the plant produces light, non-sticky pollen in large numbers and its stigmas are long and feathery. These modifications
facilitate pollination by:

a. Insects
b. Water
c. Wind
d. Animals.

Answer:

Light and non-sticky pollen grains are suitable for their transport through wind currents. They are produced in large numbers to compensate for the loss associated with their transport by wind. The long and feathery stigma of wind-pollinated flowers is adapted to easily trap the pollen grains carried by air.

Hence, the correct answer is option c.

Question 13:

From among the situations given below, choose the one that prevents both autogamy and geitonogamy.

a. Monoecious plant bearing unisexual flowers
b. Dioecious plant bearing only male or female flowers
c. Monoecious plant with bisexual flowers
d. Dioecious plant with bisexual flowers

Answer:

Autogamy is the transfer of pollen grains from the anther to the stigma of the same flower. Thus, plants exhibiting autogamy must be monoecious with bisexual flowers. Geitonogamy is the transfer of pollen grains from the anther of a flower to the stigma of another flower on the same plant. Thus, plants exhibiting geitonogamy must be monoecious bearing both staminate and pistillate flowers. Dioecious plants with male and female flowers present on different plants will exhibit only xenogamy which is the transfer of pollen grains from the anther of a flower to the stigma of a flower present on a different plant.

Hence, the correct answer is option b.

Question 14:

In a fertilised embryo sac, the haploid, diploid and triploid structures are:

a. Synergid, zygote and primary endosperm nucleus
b. Synergid, antipodal and polar nuclei
c. Antipodal, synergid and primary endosperm nucleus
d. Synergid, polar nuclei and zygote.

Answer:

A mature embryo sac has seven haploid cells which are two synergids, one egg, three antipodal cells, and two polar nuclei. During fertilization, one of the male gametes fuses with the egg to form a diploid zygote and the other male gamete fuses with the diploid nucleus to form the triploid primary endosperm nucleus. So, a fertilized embryo sac has haploid synergid, diploid zygote, and triploid primary endosperm nucleus.

Hence, the correct answer is option a.

Question 15:

In an embryo sac, the cells that degenerate after fertilisation are:

a. Synergids and primary endosperm cell
b. Synergids and antipodals
c. Antipodals and primary endosperm cell
d. Egg and antipodals.

Answer:

After fertilization, synergids and antipodal cells degenerate, the primary endosperm nucleus develops into endosperm and the zygote develops into an embryo.

Hence, the correct answer is option b.

Question 16:

While planning for an artificial hybridization programme involving dioecious plants, which of the following steps would not be relevant:

a. Bagging of female flower
b. Dusting of pollen on stigma
c. Emasculation
d. Collection of pollen

Answer:

In artificial hybridization, the stigma of the female plant is pollinated with the desired pollen grains so, it has to be protected from contamination from unwanted pollen grains. To prevent this, anthers from the flower bud are removed before their dehiscence using a pair of forceps, that is, emasculation is carried out in female plants with bisexual flowers. Since dioecious plants have unisexual flowers, emasculation is not necessary.

Hence, the correct answer is option c.

Question 17:

In the embryos of a typical dicot and a grass, true homologous structures are:

a. Coleorhiza and coleoptile
b. Coleoptile and scutellum
c. Cotyledons and scutellum
d. Hypocotyl and radicle.

Answer:

In the embryo of grasses, the cotyledon is called the scutellum. So, cotyledons of embryos of a typical dicot and scutellum of the embryo of grass are homologous structures.

Hence, the correct answer is option c.

Question 18:

The phenomenon observed in some plants wherein parts of the sexual apparatus is used for forming embryos without fertilisation is called:

a. Parthenocarpy
b. Apomixis
c. Vegetative propagation
d. Sexual reproduction.

Answer:

Apomixis is a special phenomenon that evolved in a few flowering plants such as some species of Asteraceae and grasses. In this phenomenon, embryos develop either from an unreduced egg cell or any nucellar cell without fertilisation.
Hence, the correct answer is option b.

Question 19:

In a flower, if the megaspore mother cell forms megaspores without undergoing meiosis and if one of the megaspores develops into an embryo sac, its nuclei would be:

a. Haploid
b. Diploid
c. A few haploid and a few diploid
d. With varying ploidy.

Answer:

Normally, a megaspore mother cell undergoes meiosis to form four haploid megaspores, out of which one haploid megaspore undergoes three mitotic divisions to form a mature 7-celled and 8-nucleate embryo sac. All the cells of the typical embryo sac are haploid. If the megaspore mother cell does not undergo meiosis, four diploid megaspores will be formed. Out of these four diploid megaspores, if one megaspore undergoes three mitotic divisions to develop into an embryo sac, the nuclei of the so formed embryo sac will also be diploid.

Hence, the correct answer is option b.

Question 20:

The phenomenon wherein, the ovary develops into a fruit without fertilisation is called:

a. Parthenocarpy
b. Apomixis
c. Asexual reproduction
d. Sexual reproduction

Answer:

In many plants, the ovary develops into a fruit after fertilization. But in some plant species, the ovary develops into a fruit without fertilization. This phenomenon is called parthenocarpy and such fruits are called parthenocarpic.

Hence, the correct answer is option a.

Question 1:

Name the component cells of the ‘egg apparatus’ in an embryo sac.

Answer:

The egg apparatus of an embryo sac has three component cells - two synergids and one egg cell.

Question 2:

Name the part of gynoecium that determines the compatible nature of pollen grain.

Answer:

The stigma of the pistil determines the compatible or incompatible nature of the pollen grain.

Question 3:

Name the common function that cotyledons and nucellus perform.

Answer:

Cotyledons of the embryo and nucellus of the ovule store food reserves that provide nourishment to the embryo and embryo sac, respectively.

Question 4:

Complete the following flow chart

Answer:

Question 5:

Indicate the stages where meiosis and mitosis occur (1, 2 or 3) in the flow chart.
Megaspore mother cell $\overset{1}{\to}$ Megaspores $\overset{2}{\to}$ Embryo sac $\overset{3}{\to}$ Egg

Answer:

1 – Meiosis, 2 – Mitosis, 3 – Mitosis

Question 6:

In the diagram given below, show the path of a pollen tube from the pollen on the stigma into the embryo sac. Name the components of egg apparatus.

Answer:

The diagram showing the path of a pollen tube from the pollen on the stigma into the embryo sac is given below.

The components of egg apparatus are three cells- two synergids with filiform apparatus and one egg cell.

Question 7:

Name the parts of pistil which develop into fruit and seeds.

Answer:

The ovary of the pistil develops into fruit and ovules develop into seeds after fertilisation.

Question 8:

In case of polyembryony, if an embryo develops from the synergid and another from the nucellus which is haploid and which is diploid?

Answer:

Synergid is haploid so the embryo that develops from it will also be haploid. Nucellus is diploid so the embryo developing from this will be diploid.

Question 9:

Can an unfertilised, apomictic embryo sac give rise to a diploid embryo? If yes, then how?

Answer:

Yes, an unfertilised embryo sac can give rise to a diploid embryo if it develops from a diploid megaspore which is produced by a megaspore mother cell without meiotic division. All the cells of such an embryo sac will be diploid. The diploid egg cell will develop into a diploid embryo without fertilisation.

Question 10:

Which are the three cells found in a pollen grain when it is shed at the three celled stage?

Answer:

The pollen grain shed at the three-celled stage has one vegetative cell and two male gametes.

Question 11:

What is self-incompatibility?

Answer:

Self-incompatibility is a genetic mechanism in which pollen grains from the same flower or other flowers of the same plant are failed to fertilise the ovules of the same plant by inhibiting pollen germination or pollen tube growth in the pistil.

Question 12:

Name the type of pollination in self-incompatible plants.

Answer:

Self-incompatible plants exhibit xenogamy in which pollen grains from anther are transferred to the stigma of a different plant.

Question 13:

Draw the diagram of a mature embryo sac and show its 8-nucleate, 7-celled nature. Show the following parts: antipodals, synergids, egg, central cell, polar nuclei.

Answer:

The diagram showing 8-nucleate and 7 celled mature embryo sac along with the labels mentioned in the question is given below.

Question 14:

Which is the triploid tissue in a fertilised ovule? How is the triploid condition achieved?

Answer:

Endosperm is the triploid tissue in a fertilised ovule. It develops from the repeated divisions of triploid primary endosperm cell, which in turn, results from the triple fusion, that is, fusion of a male gamete with two polar nuclei.

Question 15:

Are pollination and fertilisation necessary in apomixis? Give reasons.

Answer:

No, pollination and fertilisation are not necessary in apomixis. In apomixis, diploid embryos are formed from diploid egg cell or nucellar cell without fertilisation.

Question 16:

Identify the type of carpel with the help of diagrams given below:

Answer:

In the given figure, (a) shows the multicarpellary syncarpous pistil, that is, gynoecium has more than one pistil which are fused and (b) shows multicarpellary syncarpous gynoecium, that is, gynoecium has many pistils which are free.

Question 17:

How is pollination carried out in water plants?

Answer:

Pollination in aquatic plants occurs in three ways:

In the majority of water plants, flowers emerge above the water level and insects or wind bring about pollination.
In aquatic plants like Vallisneria, the female flowers reach the surface of the water by the long stalk and are pollinated by the male flowers or pollen grains released on to the surface of the water.
In another group of water pollinated plants such as seagrasses, female flowers remain submerged in water and are pollinated by the pollen grains released inside the water.

Question 18:

What is the function of the two male gametes produced by each pollen grain in angiosperms?

Answer:

Out of two male gametes released by the pollen tube after entering the embryo sac, one male gamete fuses with the egg cell resulting in the formation of a diploid zygote. The other male gamete fuses with two polar nuclei and as a result a triploid primary endosperm nucleus forms.

Question 1:

List three strategies that a bisexual chasmogamous flower can evolve to prevent self pollination (autogamy).

Answer:

The three strategies that a bisexual chasmogamous flower can evolve to prevent self-pollination (autogamy) are:

Non-synchrony between pollen release and stigma receptivity- This implies either the pollen grains are released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen grains.
Differential position of anther and stigma - In some other species, the anther and stigma are placed at different positions so that the pollen grain cannot come in contact with the stigma of the same flower.
Self-incompatibility - This is a genetic mechanism wherein pollen grain from the same flower or other flowers of the same plant is prevented from fertilising the ovules of the same plant by inhibiting pollen germination or pollen tube growth in the pistil.

Question 2:

Given below are the events that are observed in an artificial hybridization program. Arrange them in the correct sequential order in which they are followed in the hybridization program.

(a) Re-bagging
(b) Selection of parents
(c) Bagging
(d) Dusting the pollen on the stigma
(e) Emasculation
(f) Collection of pollen from the male parent.

Answer:

The correct sequential order of the different events that are followed in an artificial hybridisation programme is:

(b) Selection of parents
(e) Emasculation
(c) Bagging
(f) Collection of pollen from male parent
(d) Dusting the pollen on stigma
(a) Re-bagging

Question 3:

Vivipary automatically limits the number of offspring in a litter. How?

Answer:

Vivipary is a condition in which an organism gives birth to young ones. It automatically limits the number of offspring in a litter because

limited number of eggs are produced and fertilised.
limited resources like space and nutrients are available inside female's body.
higher chances of survival of young ones as an embryo gets proper care and protection during its development inside the female body.

Question 4:

Does self incompatibility impose any restrictions on autogamy? Give reasons and suggest the method of pollination in such plants.

Answer:

ans

Question 5:

In the given diagram, write the names of parts shown with lines.

Answer:

The given diagram is of an embryo of grass in longitudinal section. The names of the parts shown with lines in the given diagram are as follows:

Question 6:

What is polyembryony and how can it be commercially exploited?

Answer:

The occurrence of more than one embryo in a seed is called polyembryony. This process can be commercially exploited in the following ways such as:

Polyembryony helps in plant breeding experiments.
This also helps in horticulture.

Question 7:

Are parthenocarpy and apomixis different phenomena? Discuss their benefits.

Hint: Yes, they are different. Parthenocarpy leads to development of seedless fruits. Apomixis leads to embryo development.

Answer:

Yes, both apomixis and polyembryony are different processes as in parthenocarpy the fruits are developed without fertilization. While apomixis is a condition in which seeds are developed without fertilization. Fertilization is absent in both processes. But seeds are produced only in apomixis. Parthenocarpic fruits are seedless fruits. Genetically identified mother cells are produced by apomixis whereas genetically identified offspring are produced by parthenocarpy.

Benefits of Parthenocarpy:
It is easier to consume seedless fruits.
With the help of parthenocarpy, those fruits are produced seedless that contains too many seeds such as papaya, watermelon, etc.,

Benefits of Apomixis: The process of apomixis is used for the production of apomitic hybrid seeds and this helps the farmers economically.
Seeds produced by apomixis are clones of their parent plant, so that a plant having desired characteristics can be grown apomictically than, through sexual reproduction.

Question 8:

Why does the zygote begin to divide only after the division of the Primary endosperm cell (PEC)?

Answer:

The zygote requires nourishment during its development. During maturation, the fertilized embryo sac provides very little nourishment to the zygote. So, the PEC (Primary endosperm cell) divides and generates the endosperm tissue that provides nourishment to the zygote. Due to this, the zygote always divides after the division of PEC.

Question 9:

The generative cell of a two-celled pollen divides in the pollen tube but not in a three-celled pollen. Give reasons.

Answer:

The generative cell in pollen grain divides mitotically to give rise to the two male gametes before pollen grains are shed. This is a 3-celled stage i.e., one vegetative cell and two male gametes in over 40 percent species of angiosperms. While in 60 percent of angiosperms, the pollen grains in the 2-celled stage have 2-cells- a vegetative cell and a generative cell. In such species, the division of the generative cell did not occur, so the generative cell of two-celled pollen divides in the pollen tube.

Question 10:

In the figure given below label the following parts: male gametes, egg cell, polar nuclei, synergid and pollen tube

Answer:

The labelled figure is given below.

Question 1:

Starting with the zygote, draw the diagrams of the different stages of embryo development in a dicot.

Answer:

Different stages of embryo development starting with the zygote in a dicot are shown below.

Question 2:

What are the possible types of pollinations in chasmogamous flowers? Give reasons.

Answer:

Chasmogamous flowers are similar to flowers of other species with exposed anthers and stigmas. In such flowers, cross-pollination is possible-

1. Geitonogamy –

This process involves the transfer of pollen grains from the anther to the stigma of another flower of the same plant. Geitonogamy is functionally cross-pollination that involves a pollinating agent but genetically it is similar to autogamy because the pollen grains come from the same plant.

2. Xenogamy –

This type of cross-pollination involves the transfer of pollen grains from the anther to the stigma of a different plant. This is the only type of pollination that during pollination brings genetically different types of pollen grains to the stigma.

Question 3:

With a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.

Answer:

A mature embryo sac of angiosperm is a 7-celled and 8 nucleate structure.

The seven cells of embryo sac are one egg, two synergids, one central cell and three antipodal cells.
The egg cell along with two synergids, present at the micropylar end, constitute the egg apparatus. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus.
The central cell is the largest and contains two polar nuclei which fuse to form a secondary nucleus.
Three antipodal cells are present at the chalazal end of the embryo sac.

Role of synergids: One of the synergids provides the seat for the entry of the pollen tube into the embryo sac. The filiform apparatus present at the micropylar end of the synergids guides the pollen tube towards the embryo sac.

Question 4:

Draw the diagram of a microsporangium and label its wall layers. Write briefly on the role of the endothecium.

Answer:

The microsporangium is surrounded by four wall layers- the epidermis, endothecium, middle layers, and the tapetum. The outer three wall layers perform the function of protection and help in the dehiscence of anther to release the pollen.

Question 5:

Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition.

Answer:

The given statement is true that the embryo sac of some apomictic species appears normal but contains diploid cells. The embryo sac possesses diploid cells due to the failure of meiotic division at the megaspore mother cell stage. As the megaspore mother cell contains a diploid nucleus, only if it undergoes mitosis instead of meiosis all the resulting nuclei and cells will be diploid in nature.

NCERT Solutions for Class 12 Science Biology Chapter 2 - Sexual Reproduction In Flowering Plants

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