CBSE Class 12 Biology Sexual Reproduction In Flowering Plants Worksheet Set D

 

Question. Differentiate between:
(a) hypocotyl and epicotyl (b) coleoptile and coleorhiza
(c) integument and testa (d) perisperm and pericarp.
Ans. (a) Differences between hypocotyl and epicotyl

S.No.	Hypocotyl	Epicotyl
(i)	The region of the embryonal axis that lies between the radicle and the point of attachment of cotyledons is called hypocotyl.	The region of the embryonal axis that lies between the plumule and cotyledons is called epicotyl.
(ii)	Hypocotyl pushes the seed above the soil in epigeal germination.	Epicotyl pushes the plumule above the soil in hypogeal germination.
(iii)	It is an important component of embryonic root system.	It is an important component of embryonic shoot system.

(b) Differences between coleoptile and coleorhiza

S.No.	Coleoptile	Coleorhiza
(i)	The shoot apex and few leaf primordia are enclosed in a hollow foliar structure in epicotyl region in monocots and is called coleoptile.	The radicle and root cap are situated at the lower end of embryonal axis are enclosed by undifferentiated sheath called coleorhiza.
(ii)	It comes out of the soil.	It remains inside the soil.
(iii)	It emerges from the soil, turns green and carries out photosynthesis.	It remains in the soil and is non-green in colour.

(c) Differences between integument and testa

S. No.	Integument	Testa
(i)	It is the protective covering of the ovule (nucellus).	It is the protective covering of the seed.
(ii)	It is thin and living.	It is thick and dead.
(iii)	It is part of pre-fertilisation.	It is part of post-fertilisation.

(d) Differences between perisperm and pericarp

S.No.	Perisperm	Pericarp
(i)	It represents persistent remains of nucellus (of ovule) in the seed.	It represents the wall of fruit formed by the ovarian wall.
(ii)	It is a part that belongs to seed.	It is a part that belongs to fruit.
(iii)	It is usually dry.	It can be dry or fleshy.

 

Question.Mention the function of coleorhiza.
Answer. Coleorhiza is a protective sheath that covers the young root of the embryo in plants of the grass family.

Question. Why is banana referred to as a parthenocarpic fruit? 
Answer. Banana is a fruit obtained from an unfertilised ovary. It is seedless and produced without pollination and fertilisation, therefore, it is referred to as parthenocarpic fruit.

Question. The meiocyte of rice has 24 chromosomes. Write the number of chromosomes in its endosperm. 
Answer. If the meiocyte (2n) of rice has 24 chromosomes, then the gametes (n) of rice will have 12 chromosomes. Therefore, the number of chromosome in its endosperm will be = 12 × 3 = 36.

Question. Write the function of scutellum.
Answer. Scutellum is the tissue in a monocot seed that lies between the embryonic axis and the endosperm. It is the modified cotyledon of grasses. It is very thin with high surface area and serves to absorb nutrients from the endosperm during germination.

Question. What is pericarp? Mention its function
Answer. Pericarp is the covering of fruit that develops from ovary wall. It protects the fruit and also helps in its dispersal.

Question.Name the mechanism responsible for the formation of seed without fertilisation in angiosperms. Give an example of a species of flowering plants with such seed formation.
Answer. Parthenocarpy is the mechanism which is responsible for the formation of seed without fertilisation in angiosperms. Examples of plants in which parthenocarpic seed formation takes place are banana, pineapple, tomato, papaya etc.

Question. Name the part of flower that contributes to fruit formation in strawberry and guava respectively. 
Answer. Thalamus along with ovary takes part in fruit formation in strawberry and guava.

Question. Mention the exact location or the site in a flowering plant where the following developments take place.
(a) Triple fusion
(b) Release of male gametes. 
Answer. (a) In angiosperms, triple fusion takes place in central part of embryo sac.
(b) In angiosperms, release of male gametes by pollen tube takes place into the cytoplasm of the synergid.

Question. What is double fertilisation?
Answer. Double fertilisation is the fusion of two male gametes brought by a pollen tube to two different cells (egg and secondary nucleus) of central cell of the same female gametophyte in order to produce two different structures (zygote and primary endosperm nucleus).

Question. Write the fate of egg cell and polar nuclei after fertilisation. 
Answer. Egg cell after getting fertilised with one of the two male nuclei forms zygote which further develops into embryo. Polar nuclei fuse with other male nuclei to form primary endosperm nucleus which further develops into endosperm.

Question. In angiosperms, zygote is diploid while primary endosperm cell is triploid. Explain.
Answer. In angiosperms, pollen tube releases the two haploid male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell (haploid) and fuses with its nucleus thus completing the syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus.

Question. Explain triple fusion in angiosperm.
Answer. In angiosperms, one of the male gametes (n) fuses with diploid secondary nucleus to form a triploid (3n) primary endosperm cell. This process is referred to as triple fusion.

Question. Why is fertilisation in an angiosperm referred to as double fertilisation? Mention the ploidy of the cells involved.
Answer. In angiosperms, one of the male gametes fuses with the egg cell to form the zygote (syngamy). The other male gamete fuses with the two polar nuclei to produce a triploid primary endosperm nucleus (triple fusion). Since two types of fusions, syngamy and triple fusion take place in the same embryo sac, the phenomenon is termed as double fertilisation.
The ploidy of the cells involved in double fertilisation are as follows:
(i) Male gamete - Haploid (n) (ii) Egg cell - Haploid (n)
(iii) Central cell - Diploid (2n)

Question. In an angiosperm, the embryo sac is haploid, zygote is diploid and endosperm is triploid. Justify giving reasons for each stage. 
Answer. Embryo sac is female gametophyte formed by haploid megaspores (produced by meiotic division in diploid megaspore mother cells). Therefore, it is haploid in nature.
Zygote is formed by the fusion of one of two male gametes (haploid) with egg (haploid) therefore, it is dipoid (2n).
Endosperm is produced by fusion of one of two male gametes (n) with two polar nuclei (2n) therefore, it is triploid in nature.

Question. List the components of the embryo sac and mention their fate on fertilisation.
Answer. The components of the embryo sac are antipodals, polar nuclei (central cell), egg, synergids and filiform apparatus. Filiform apparatus, antipodals and synergids degenerate after fertilisation, egg after fertilisation with male gamete forms zygote whereas the polar nuclei fuse with other male gamete to form primary endosperm nucleus which gives rise to endosperm.

Question. Why does a pollen grain possess two male gametes? Explain. 
Answer. In angiosperms, there is a unique phenomenon, called double fertilisation, in which there are two fusions involved in the fertilisation of an ovule. A pollen has two male gametes, one of which fuses with the female gamete (egg) to form the zygote, while the other fuses with the polar nuclei (secondary nucleus) to form the primary endosperm nucleus

Question. A flower of tomato plant following the process of sexual reproduction produces 200 viable seeds.
Answer the following questions giving reasons. (a) What would have been the minimum number of ovules present in per pollinated pistil?
(b) How many microspore mother cells would minimally be required to produce requisite number of pollen grains?
(c) How many pollen grains must have minimally pollinated the carpel?
(d) How many male gametes would have used to produce these 200 viable seeds?
(e) How many megaspore mother cells were required in this process? 
Answer.  A flower of tomato plant contains 200 viable seeds.
(a) Minimum number of ovules involved = 200 because following fertilisation one ovule matures into one seed.
Each microspore mother cell undergoes meiosis and gives rise to tetrad of four haploid microspores or pollen grains.
(b) Number of microspore mother cells required to produce 200 pollen grains is 200/4=50
(c) 200 pollen grains must have pollinated 200 carpels for production of 200 viable seeds.
(d) Male gametes involved are 200 × 2 = 400
Each pollen grain releases two male gametes, one fuses with the egg to form zygote and second male gametes fuses with polar nuclei to form endosperm nucleus.
(e) Megaspore mother cell undergoes reduction
division to form four haploid megaspores. Only one megaspore remains functional and other three degenerate. Only the functional megaspore develops into female gametophyte. Hence, number of megaspore mother cell = 200.

Question. A flower of brinjal plant following the process of sexual reproduction produces 360 viable seeds.
Answer the following questions giving reasons: (a) How many ovules are minimally involved?
(b) How many megaspore mother cells are involved?
(c) What is the minimum number of pollen grains that must land on stigma for pollination?
(d) How many male gametes are involved in the above case?
(e) How many microspore mother cells must have undergone reduction division prior to dehiscence of anther in the above case? 
Answer. A flower of brinjal plant contains 360 viable seeds.
(a) Number of minimally involved ovules = 360. This is because following sexual reproduction, one ovule matures into one seed.
(b) Megaspore mother cell undergoes reduction division to forms four haploid megaspores. Only one megaspore remains functional and other three degenerate. Only the functional megaspore develops into female gametophyte. Hence, number of megaspore mother cell = 360.
(c) Compatible pollen grain after landing on stigma forms pollen tube which traverses through style of carpel and enters the ovule to release male gamete. Here, minimum number of pollen grains involved is = 360.
(d) Each pollen grain releases two male gametes in an embryo sac. One male gamete fuses with the egg to form zygote and second male gamete fuses with polar nuclei to form endosperm nucleus. Hence, number of male gametes involved is = 360 × 2 = 720.
(e) Each microspore mother cell undergoes meiosis and gives rise to tetrad of four haploid microspores or pollen grains.
Number of microspore mother cell that have undergone reduction division prior to dehiscence of anther = 360/4 = 90

Question. Give reasons why
(a) most zygotes in angiosperms divide only after certain amount of endosperm is formed?
(b) groundnut seeds are exalbuminous and castor seeds are albuminous ?
Answer. (a) Endosperm is food storing tissue formed during the development of angiospermous seed which provides essential nutrients to the growing embryo and also young seedling at the time of seed germination.
(b) Groundnut seeds are exalbuminous as the seeds usually store food materials in cotyledons and the endosperm is used up by the developing embryo.
Castor seeds are albuminous as they, have copious amounts of endosperm tissue to provide nutrition to the developing embryo.
(c) Micropyle remains as a small pore in the seed coat to facilitate entry of O2 and water into the seed during germination.
(d) Integuments of an ovule harden and the water content is highly reduced, as the seed matures to enable the seed to remain viable for a long time during the period of dormancy. The seed may enter a state of inactivity called dormancy and germinates only when the favourable conditions are available. (e) Apple and cashew are not called true fruits as in these fruits, along with the ovary, thalamus also forms a part of fruit.

Question. Mention advantage of apomictic seeds to farmers. 
Answer. In hybrid apomicts, there is no segregation of characters in hybrid progeny. So, farmers can keep on using the hybrid seeds to raise new crop year after year and they do not have to buy new hybrid seeds every year.

Question. Hybrid seeds have to be produced year after year give reason. 
Answer. Hybrid seeds have to be produced year after year because seeds collected from hybrid plants, if sown subsequently, do not maintain hybrid characters due to segregation of traits

Question. Normally one embryo develops in one seed but when an orange seed is squeezed many embryos of different shapes and sizes are seen. Mention how it has happened.
Answer. In orange seed, embryos originate by adventive embryony from diploid cells of nucellus or integuments and thus, if orange seed is squeezed many embryos of different sizes can be observed.

Question. What is apomixis? What is its importance?
Answer. Apomixis is a mode of reproduction which does not involve formation of zygote through gametic fusion. It is a form of asexual reproduction that mimics sexual reproduction, in which seeds are produced without fertilisation. It is common in grasses and species of Family Asteraceae.
Apomicts have advantages in horticulture and agriculture, particularly hybrid seed industry.
(i) The hybrid varieties are more productive but production of these seeds is very costly and time consuming and such seeds do not maintain hybrid vigour in progeny due to segregation of characters / genes during meiosis. If the hybrids are made into apomicts, there will be no segregation of genes due to no meiosis. Thus, apomictic hybrid seeds can be used year after year which will reduce the cost on purchasing hybrid seeds every year.
(ii) Adventive embryos are better clones than cuttings.
(iii) Apomictic embryos are generally free from infections.

Question. Mention the scientific term used for modified form of reproduction in which seeds are formed without fusion of gametes.
Answer. Apomixis is a mode of reproduction that produces seeds without fertilisation

Question. Suggest the advantage to a farmer for using apomictic seeds of hybrid varieties?
Answer. Advantages of apomictic seeds of hybrid varieties to farmer are as follows:
(i) It will reduce the cost on purchasing hybrid seeds every year.
(ii) Apomixis is genetically controlled so genes of apomixis can be introduced in hybrid varieties.

Question. Explain the different ways apomictic seed can develop. Give an example of each.
Answer. There are several methods of apomictic development in seeds. The two common ones are recurrent agamospermy and adventive embryony.
(i) Recurrent agamospermy : Agamospermy is the formation of seed that has an embryo formed without meiosis and syngamy. It is of two types, nonrecurrent and recurrent. In nonrecurrent agamospermy, the embryo is haploid. Therefore, the seed having it is non-viable. In recurrent agamospermy all the cells of embryo sac are diploid as it is formed directly either from a nucellar cell (apospory) or diploid megaspore mother cell (diplospory), e.g., Rubus, apple.
(ii) Adventive embryony : An embryo develops directly from a diploid cell other than egg like that of nucellus and integument, e.g., Citrus, Opuntia

Question. (a) Why are seeds of some grasses called apomictic? Explain.
(b) State two reasons to convince a farmer to use an apomictic crop. 
Answer. (a) Seeds of some grasses are called apomictic because they are produced without fertilisation and do not involve formation of zygote through gametic fusion.
(b) Two reasons to convince a farmer to use an apomictic crop are as follows:
(i) Production of infection free embryo for development of plant.
(ii) Production of better clones as adventive embryos are better clones than cuttings.

Question. Why are some seeds of Citrus referred to as polyembryonic? How are they formed?
Answer. Polyembryony is the occurrence of more than one embryo in a seed. In Citrus, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. So, their seed contains many embryos and are referred as polyembryonic seed.

Question. Fertilisation is essential for production of seed, but in some angiosperms, seeds develop without fertilisation
Answer. a) In the members of Family Asteraceae, seeds develop without fertilisation. This process is called apomixis.
(b) Two ways by which seeds develop without fertilisation are as follows:
(i) In some species, the diploid (2n) egg cell is formed without reduction division and develops into embryo without fertilisation.
(ii) In many varieties of Citrus and mango some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos.

Question. (a) Describe the endosperm development in coconut.
(b) Why is tender coconut considered healthy source of nutrition?
Answer. (a) In the most common type of endosperm development of coconut the PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage of endosperm development is called free-nuclear endosperm. Subsequently cell wall formation occurs and the endosperm becomes cellular. The number of free nuclei formed before cellularisation varies greatly. The coconut water from tender coconut is free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.
(b) Tender coconut is rich in minerals, vitamins, proteins and is isotonic with our body fluids therefore it is considered healthy source of nutrition.
(c) Pea seeds are exalbuminous seeds i.e., the endosperm is completely eaten up by growing embyro and food for later development of embryo is stored in cotyledon which becomes massive whereas castor seeds are albuminous seeds i.e., the endosperm persists in the seed as food storage tissue.

Question. Write the fate of the products of triple fusion in the mature fruit of coconut.
Answer. In coconut, the product of triple fusion is primary endosperm cell which develops into endosperm. Coconut has cellular endosperm (called coconut meal) in the outer part and free nuclear endosperm (called coconut milk) in the centre. This endosperm persists in the seed hence the seed is known as albuminous seed.

Question. Differentiate between albuminous and non- albuminous seeds, giving one example of each.
Answer. Albuminous seeds are those seeds in which endosperm is present and serves as the reserve food material for the developing embryo. Most monocot and some dicot seeds are albuminous seeds, e.g., cereal, coconut, castor etc.
Non-albuminous seeds are those seeds in which endosperm is consumed during seed development and food is stored in cotyledons. Most dicot and some monocot seeds are non-albuminous seeds, e.g., pea, gram, bean, orchids etc.

Question. Describe the development of endosperm in coconut.
Answer. In Cocos nucifera (coconut), the coconut water represents free-nuclear endosperm and the surrounding kernel represents the cellular endosperm. The primary endosperm nucleus (PEN) first undergoes a number of free nuclear divisions without wall formation to form a large number of free nuclei (free nuclear endosperm). When the fruit is about 50 mm long, the embryo sac gets filled with a clear fluid in which float numerous nuclei of various sizes. At a later stage (about 100 mm long fruit), the suspension shows, in addition to free nuclei, several cells each enclosing a variable number of nuclei. Gradually these cells and free nuclei start settling at the periphery of the cavity, and layers of cellular endosperm start appearing. This forms the coconut meal. The quantity of the cellular endosperm increases further by divisions of the cells.

Question. Banana is a parthenocarpic fruit whereas oranges show polyembryony. How are they different from each other with respect to seeds?
Answer. Banana shows parthenocarpy. Its fruit is formed without fertilisation and is therefore seedless. Polyembryony refers to presence more than one embryo in a seed e.g., orange. Some of the nucellar cells surrounding the embryo sac start dividing and protrude into the embryo sac and develop into the embryos. Thus each ovule contains many embryos. With respect to seeds, bananas and oranges differ as bananas are seedless whereas oranges contain many seeds.

Question. Double fertilisation is reported in plants of both castor and groundnut. However, the mature seeds of groundnut are non-albuminous and castor are albuminous. Explain the post fertilisation events that are responsible for it.
Answer. In angiosperms double fertilisation produces two structures - a diploid zygote and a triploid primary endosperm cell. The latter gives rise to tissue called endosperm. Zygote forms the embryo and endosperm provides nourishment to the growing embryo. With the growth of embryo the central part of the endosperm is utilised. In some seeds, the endosperm persists in the seed as food storage tissue. Such seeds are called endospermic or albuminous seeds, e.g., castor, cereals, etc. In others the endosperm is completely eaten up by growing embryo. The food for later development of embryo is then stored in cotyledons which become massive. Such seeds are called nonendospermic or exalbuminous, seeds e.g., groundnut, pea etc.

Question. List the post-fertilisation events in angiosperms.
Answer. Soon after the act of double fertilisation, the flower begins to lose its shine. The petals, stamens and style either fall or wither away. The calyx, however, may persist in some cases (e.g., tomato, brinjal). The major events include –
(i) Development of endosperm from triploid primary endosperm nucleus in the central cell of embryo sac.
(ii) Development of embryo from diploid zygote. (iii) Development of seed from ovule.
(iv) Development of fruit from ovary.

Question. Explain any three advantages the seeds offer to angiosperms.
Answer. The three advantages that seeds offer to angiosperms are as follows:
(i) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
(ii) They have sufficient food reserves, young seedlings are nourished until they are capable to photosynthesise on their own and hard seed coat provides protection to the young embryo.
(iii) Seeds are product of sexual reproduction, they generate new genetic combinations leading to variations.

Question. Explain how false, true and parthenocarpic fruits are different from each other. Give one example of each.
Answer. False fruit is the fruit which is derived from the fertilised ovary and accessory floral parts e.g., thalamus also contribute to the fruit formation. Examples : apple, strawberry, guava etc. True fruit is the fruit which is derived only from the ovary of a flower and is not associated with any non-carpellary part , for example : mango, tomato, etc. Parthenocarpic fruit is the fruit that is formed without fertilisation and does not bear seeds, e.g., pineappale, banana, etc.

Question. Strawberry is sweet and eaten raw just like any other fruit. Why do botanists call it a false fruit?
Answer. True fruits are the fruits in which only ovary participates in the formation of fruit whereas false fruits are the fruits in which along with ovary some other parts also participate in the formation of fruit. In strawberry, along with ovary thalamus also contributes to fruit formation, therefore, botanists call it a false fruit even though it is sweet and eaten raw just like any other fruit.

Question. Banana crop is cultivated by farmers without sowing of seeds. Explain how the plant is propagated.
Answer. Banana crop is cultivated by vegetative propagation without sowing seeds. It has subaerial stem structures called suckers which are slender branches that develop from base of aerial shoot, grow for some distance and form new aerial shoots or crowns. Breaking of suckers forms new plant. Rhizomes of banana also forms new plant.

Question. Write the changes a fertilised ovule undergoes within the ovary in an angiosperm plant.
Answer. Double fertilisation in angiosperms triggers the transformation of ovule into a seed. Following changes usually occur in the ovule during the development of seed:
(i) The zygote develops into an embryo.
(ii) The triploid primary endosperm nucleus gives rise to a nutritive tissue called endosperm. The endosperm may persist or gets completely digested during embryogenesis.
(iii) The nucellus is generally used up during the development of embryo but in some cases it remains outside the endosperm in the form of a thin layer, called perisperm.
(iv) The outer integument becomes hard and forms leathery testa or outer seed coat which ensures survival of seeds.
(v) The inner integument, if persists, forms the tegmen.
(vi) The micropyle remains in the form of a fine pore on the surface of seed. Funicle is transformed into stalk of the seed. The hilum marks the point of attachment to the stalk.

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