CBSE Class 12 Biology Sexual Reproduction in Flowering Plants Set B

Very Short Answer Type Questions

Question. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?
Answer: Only fleshy fruits like orange, watermelon, lemon, etc., should be selected as parthenocarpic fruits. Here seeds of fruits are irritant during consumption so seeds are removed so as to make the fruits even more valuable. It is easy to make fruit juices, jams, etc. with seedless fruits.

Question. What is meant by monosporic development of a female gametophyte?
Answer: Out of the four megaspores, three degenerate and only one remains functional which develops into a female gametophyte or embryo sac. This is called monosporic development, i.e., when embryo sac develops from one single megaspore it is called monosporic embryo sac.

Question. How many microsporangia are present in a typical anther of an angiosperm? 
Answer: Four

Question. Pea flowers produce assured seed sets. Give a reason.
Answer: Pea flowers are cleistogamous, i.e., anther and stigma lie close to each other in closed flowers. So when anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, assured seeds are produced in pea.

Question. Name the common function that cotyledons and nucellus perform. 
Answer: Cotyledons and nucellus provide nourishment.

Question. Which are the three cells found in a pollen grain when it is shed at the three-celled stage?
Answer: One vegetative cell and two male gametes.

Question. What is self-incompatibility? Why does self-pollination not lead to seed formation in selfincompatible species?
Answer: Self-incompatibility or self sterility is the inability of an intersexual or bisexual plant to produce viable seeds on self-pollination, in spite of producing functional male and female gametes. Since,fertilisation does not take place, no seeds are produced. It is a genetic mechanism that prevents self pollen from fertilizing ovules by inhibiting pollen tube growth in pistil.

Question. Write the function of scutellum. 
Answer: It provides nourishment and protection to the developing embryo.

Question. Name the part of gynoecium that determines the compatible nature of pollen grain.
Answer: Stigma

Question. How do flowers of Vallisneria get pollinated? 
Answer: In Vallisneria, the female flower stalk is long to reach the water surface to receive the pollen grains carried by water currents and then it gets coiled after pollination.

Question. What features of flowers facilitate pollination by birds?
Answer: Presence of a large quantity of nectar, bright colours of petal, fragrance and large flowers attract birds from long distances.

Question. How is it possible in Oxalis and Viola plants to produce assured seed-sets even in the absence of pollinators? 
Answer: By presence of cleistogamous flowers. 

Question. Explain the role of tapetum in the formation of pollen grain wall.
Answer: Tapetum is the innermost wall layer of a microsporangium. It nourishes the developing pollen grains and also help in the formation of wall of pollen grains. The cells of tapetum secrete Ubisch granules that provide sporopollenin and other materials for exine formation.

Short Answer Type Questions

Question. Explain the function of each of the following:
(a) Coleorhiza
(b) Germ pores 
Answer: (a) Coleorhiza protects the radical of (monocot) embryo.
(b) Germ pores allow germination of pollen grain and formation of pollen tubes.

Question. (a) Plan an experiment and prepare a flow chart of the steps that you would follow to ensure that the seeds are formed only from the desired sets of pollen grains. Name the type of experiment that you carried out.
(b) Write the importance of such experiments. 
Answer: (a) Selection of flowers from desired plants emasculation bagging dusting of the pollens on the stigma of the flowers that were bagged (pollination) rebagging of flower fruit formed The name of the experiment is Artificial hybridisation.
(b) (i) Production of superior or improved varieties of pliai)n tIsm. p( roves crop yield.

Question. (a) Why is the process of fertilisation in angiosperms termed as double fertilisation? Explain.
(b) Draw a diagram of an angiospermic embryo sac where fertilisation is just completed.
Label the following parts:
(i) Micropylar end of the embryo sac.
(ii) The part that develops into an embryo.
(iii) The part that develops into an endosperm.
(iv) The degenerating cells at the chalazal end.
(c) Draw a labelled diagram of globular embryonic stage of an angiosperm.
Answer:  (a) In angiosperms, fusion of haploid egg cell with one haploid male gamete to form diploid zygote is called syngamy. Also, fusion of two (diploid) polar nuclei with the other haploid male gamete to form triploid primary endosperm nucleus is called triple fusion. Hence, the fertilisation is referred to as double fertilisation.

Question. Draw a sectional view of an apple and label the different parts of an ovary in it. Fruits develop from an ovary. Then why is apple referred to as a false fruit? 
Answer: Refer to Fig. 2.16(a).
In apple, the thalamus also contributes to fruit formation. Therefore, it is called a false fruit.

Question. What is pericarp? Mention its functions.
Answer: The wall of the ovary that develops into wall of the fruit is called pericarp.
Functions: (i) Protects the seed till its maturity.
(ii) Helps in seed dispersal.

Question. Mention the reasons for difference in ploidy of zygote and primary endosperm nucleus in an angiosperm. 
                                                                             OR
In angiosperms, zygote is diploid while primary endosperm cell is triploid. Explain.
Answer: A zygote is formed by the fusion of haploid male gamete with the haploid egg to form a diploid cell; whereas, primary endosperm nucleus (PEN) is formed by the fusion of haploid male gamete with two haploid polar nuclei, forming a triploid nucleus.

Question. Some angiosperm seeds are said to be ‘albuminous’, whereas few others are said to have a perisperm. Explain each with the help of an example.
Answer: Albuminous seeds are those which retain a part of endosperm as it is not completely used up during embryo development. For example, in wheat and maize. In some seeds remnants of nucellus are also persistent. This residual, persistent nucleus is the perisperm. For example, in black pepper and beet.

Question. (a) Mature seeds of legumes are non-albuminous. Then, can it be assumed that double fertilisation does not occur in legumes? Explain your answer.
(b) List the differences between the embryos of dicot (pea) and monocot (grass family).
Answer: (a) No it cannot be assumed so because fertilisation does takes place but the endosperm is consumed during embryo development
(b) Dicot embryo
(i) It has two cotyledons
(ii) Radicle and plumule are not covered with sheath.
Monocot embryo
(i) It has one cotyledon
(ii) Radicle is covered with coleorhiza and plumule is covered by coleoptile.

Question. A non-biology person is quite shocked to know that apple is a false fruit, mango is a true fruit and banana is a seedless fruit. As a biology student how would you satisfy this person?
Answer: In apple only the thalamus (along with ovary) portion contributes to fruit formation. Therefore, it is a false fruit. Mango develops only from the ovary, therefore it is a true fruit.
Banana develops from ovary but without fertilisation. The method is known as parthenocarpy.
Since there is no fertilisation, no seeds are formed in banana.

Question. What is agamospermy? How is agamospermy different from parthenogenesis and parthenocarpy?
Answer: The phenomenon of asexual reproduction that mimics sexual reproduction as it forms seed without fertilisation is called agamospermy. Parthenogenesis refers to the development of unfertilised egg into an adult individual. On the other hand, parthenocarpy is the phenomenon of formation of fruits without fertilisation of an ovary.

Question. During an excavation assignment, scientists collected pollen grains of a plant preserved in deeper layers of soil. Analyse the properties of pollen grains which help in the fossilization.
Answer: Pollen has an outer layer called exine which is made of sporopollenin. It is the most resistant organic material known. It can withstand high temperature, strong acids and alkali as well. No enzyme that degrades sporopollenin is so far known.

Question. List the two steps that are essential for carrying out artificial hybridisation in crop plants and why. 
Answer: (a) Selection of parents: Only those plants should be selected which have desired traits.
Emasculation: Removal of anthers from flower before they are mature and dehisce.
(b) Crossing over: Pollen grains from selected male plant is collected and transferred to the female plant after which it is bagged.

Question. Explain the steps that ensure cross pollination in an autogamous flower. 
Answer: A bisexual flower is emasculated at unopened stage to prevent self-pollination in the flower and it is bagged after emasculation to prevent contact of unwanted pollen grain with the stigma of the flower. Artificial pollination is then performed when the stigma is ready and the flower is rebagged.

Question. (a) How are parthenocarpic fruits produced by some plants and apomictic seeds by some others? Explain.
(b) When do farmers prefer using apomictic seeds? 
Answer: (a) Parthenocarpic fruits are formed when ovary develops into fruit without fertilisation.
Apomictic seeds are formed when formation of seeds take place without fertilisation.
(b) To maintain hybrid characters (year after year in a desired plant) and to avoid buying hybrid seeds every year (expensive seeds) farmers prefer using apomictic seeds. 

Question. (a) Pistil of a flower does not accept pollen from any plant other than from its own kind. How does it happen? Explain.
Answer: (a) The pistil has the ability to recognise pollen, whether it is of right type (compatible) or of the wrong type (incompatible). It is mediated by chemical components of the pollen interacting with those of the pistil.

Long Answer Type Questions

Question. Explain the events upto fertilisation that occur in a flower after the pollen grain has landed on its compatible stigma.
Answer: When pollen grain lands over the stigma, it starts germinating and produces a pollen tube through a germ pore. Pollen tube passes through style and reaches the ovule. The generative cell divides and forms two male gametes. Finally the pollen tube enters the embryo sac through micropyle. Now the pollen tube enters the egg apparatus through one of the synergids with the help of filiform apparatus. The vegetative nucleus degenerates while pollen tube leaves two male gametes in embryo sac. Now one of the male gamete fuses with the egg cell to form diploid zygote known as syngamy. The other male gamete fuses with the two already fused polar nuclei (called secondary nucleus) and forms triploid primary endosperm nucleus (PEN) which later gives rise to endosperm. This is called triple fusion. Hence syngamy and triple fusion together are known as double fertilisation.

Question. (a) Write the changes a fertilised ovule undergoes within the ovary in an angiosperm plant.
Answer: (a) In a fertilised ovule, following changes occur:
(i) Zygote changes into embryo
(ii) Integument develops into the seed coat
(iii) Synergids and antipodals degenerate
(iv) Ovule changes to form the seed.

Question. (a) Draw a schematic labelled diagram of a fertilised embryo sac of an angiosperm.
(b) Describe the stages in embryo development in a dicot plant.
Answer: (a) Refer to Fig. a2)..13
(b) Embryogeny in Dicots
• The zygote undergoes transverse division forming a large basal cell and a small apical or terminal cell.
• The large basal cell enlarges and undergoes transverse division to from a group of 6–10 cells called suspensor.
• The first cell of the suspensor towards the micropylar end is called haustorium, whereas the last cell of the suspensor towards the chalazal end is called hypophysis that later develops into radicle.
• The smaller terminal or apical cell undergoes one vertical division. The two cells formed from terminal cell divide by a transverse division thus forming four embryonal cell (quadrant stage).

Question. (a) As a senior biology student you have been asked to demonstrate to the students of secondary level in your school, the procedure(s) that shall ensure cross-pollination in a  hermaphrodite flower. List the different steps that you would suggest and provide reasons for each one of them.
(b) Draw a diagram of a section of a megasporangium of an angiosperm and label funiculus, micropyle, embryo sac and nucellus.
Answer: (a) The following steps would be followed:
(i) Emasculation or removal of anthers from the flower bud, before the anther dehisce, to avoid self pollination.
(ii) Bagging, to prevent contamination of its stigma with unwanted pollen grains.
(iii) Rebagging, the stigma of the mature ovary are dusted with desired pollen grains and rebagged to allow the fruit to develop.

Question. (a) Why does endosperm development precede embryo development in angiosperm seeds?
State the role of endosperm in mature albuminous seeds.
(b) Describe with the help of three labelled diagrams the different embryonic stages that include mature embryo of dicot plants. 
Answer: (a) Endosperm development precedes embryo development because endosperm provides nutrition for the developing embryo. It is an adaptation to provide assured nutrition to the developing embryo.
The endosperm provides nutrition during seed germination.
(b) The zygote (in the embryo sac) divides to give rise to pro embryo and subsequently to the globular, heart shaped and mature embryo as shown in the diagram.

Question. Give reasons why:
(i) most zygotes in angiosperms divide only after certain amount of endosperm is formed.
(ii) groundnut seeds are exalbuminous and castor seeds are albuminous.
(iii) micropyle remains as a small pore in the seed coat of a seed.
(iv) integuments of an ovule harden and the water content is highly reduced, as the seed matures.
(v) apple and cashew are not called true fruits. 
Answer: (i) To obtain nutrition from the endosperm for the developing embryo, zygotes, divide after its formation. 
(ii) The groundnut seeds are exalbuminous because the endosperm is completely consumed during embryo development. Whereas, castor seeds are albuminous because the endosperm persists and is used up during seed germination.
(iii) Micropyle remain as a small pore in the seed coat of a seed for the entry of water and oxygen required for germination.
(iv) To protect the embryo and keep the seed viable, until favourable conditions return for germination.
(v) In apple and cashew, apart from ovary, thalamus also contributes to fruit formation so they are not true fruits.

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 another in the above case?
Answer: (a) 360 ovules are involved. One ovule after fertilisation forms one seed.
(b) 360 MMC are involved. Each MMC forms four megaspores out of which only one remains functional.
(c) 360 pollen grains. One pollen grains participates in fertilisation of one ovule.
(d) 720 male gametes are involved. Each pollen grain carries two male gametes (which participate in double fertilisation) (360 × 2 = 720).
(e) 90 MMC undergo reduction division. Each microspore mother cell meiotically divides to form four pollen grains. (360/4 = 90).

Question. (a) A capsicum flower has 240 ovules in its ovary. But, it produces a fruit with only 180 viable seeds.Explain giving a reason that could be responsible for such a result.
(b) Describe the development of an endosperm in a viable seed. Why does endosperm development precede embryo development?
(c) Give an example of an angiosperm seed that has a perisperm. Name the part the perisperm develops from.
Answer: (a) (i) 240 ovules giving rise to only 180 viable seeds, can be possible only if less number of pollen grains or male gametes were available.
(ii) All pollen grains did not germinate or did not form pollen tubes.
(iii) Many pollen were not compatible
(b) For the development of an Endosperm,
♦ Endosperm develops first, followed by an embryo.
♦ Endosperm develops from PEN.
♦ PEN undergoes successive nuclear divisions to give rise to free nuclei and this stage of endosperm development is called free nuclear endosperm.
♦ Subsequently, cell wall is formed on the periphery and endosperm becomes cellular. It is now called cellular endosperm.
♦ Cells of endosperm are filled with reserve food materials that are used for nutrition of developing embryo. Thus endosperm needs to develop before embryo.
(c) Black pepper and beet have a perisperm. The perisperm develop from the nucellus.

Question. (a) Explain the different ways apomictic seeds can develop. Give an example of each.
(b) Mention one advantage of apomictic seeds to farmers.
(c) Draw a labelled mature stage of a dicotyledonous embryo. 
Answer: (a) (i) Diploid egg cell is formed without reduction division and develops into embryo without fertilisation, e.g., Asteraceae/grasses.
(ii) In citrus/mango, some of the diploid nucellar cells surrounding the embryo sac start dividing, protrude into embryo sac and develop into a embryo.
(b) No segregation of character in hybrid seeds, economically beneficial and desired varieties are cultivated.
(c)

Question. Explain double fertilisation and trace the post-fertilisation events in sequential order leading to seed formation in a typical dicotyledonous plant.
Answer: Double fertilisation:
• On reaching synergid, pollen tube releases the two male gametes into cytoplasm of synergid.
• One of the male gamete fuses with egg nucleus to form a diploid cell called zygote. This event is called syngamy.
• Other male gamete fuses with polar nuclei at the centre to produce a triploid primary endosperm nucleus (PEN). This is termed as triple fusion.
• As syngamy and triple fusion take place simultaneously in the embryo sac, it is termed as double fertilisation.
• The central cell after triple fusion forms primary endosperm cell (PEC) which later develops into endosperm.
• The zygote later develops into an embryo.
Following are the post-fertilisation events:
(i) Development of embryo: Embryo develops in fertilised ovule, from the zygote. The early stages of embryo development from a zygote is known as embryogeny. The formation of embryo starts only after certain amount of endosperm formation has taken place to assure the nutrition supply, for development and growth of embryo. As a result of double fertilisation number of changes takes place in an ovule due to which ovule is converted into seeds.

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

(b) Differences between coleoptile and coleorhiza

(c) Differences between integument and testa

(d) Differences between perisperm and pericarp