Sexual Reproduction in flowering plants

One Mark Questions

1. Name the part of the flower which the tassels of the corn-cob represent.

ANS:- Tassels  of  corn  cob   represent   staminate inflorescence  (cluster  of  male   flowers),  so  they represent androecium  (male reproductive part) of the  flower.

2. Give an example of a plant which came into India as a contaminant and is a cause of pollen allergy.

ANS:- Parthenium or carrot grass is an example of a plant which came to India as a contaminant and is a major contributor to pollen allergy

3. What is pollen-pistil interaction and how is it mediated?

ANS:- Pollen-pistil interaction is the group of events that occur from the time of pollen deposition over the stigma to the time of pollen tube entry  into ovule. The dialogue between pollen grain and the pistil is mediated by chemical components of the pollen interacting with those of the pistil. 

4. What is apomixis? What is its importance?

ANS:- 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.

5. Differentiate between xenogamy and geitonogamy.

ANS:- Differences     between     xenogamy     and geitonogamy are as follows:

6. An anther with malfunctioning tapetum often fails to produce viable male gametophytes. Give any one reason.

ANS:- Tapetum is the microsporangial layer that provides nourishment to the developing microspores. In an anther with malfunctioning tapetum, microspores do not get sufficient nutrition and hence viable male gametophytes are often not produced.

7. Mention the function of coleorhiza.

ANS:-  Coleorhiza is a protective sheath that  covers the young root of the embryo in plants of the grass family.

8. Write the function of tapetum in anthers.

ANS:-  (i) Tapetum  nourishes  the developing microspores.

           (ii)   It produces lipid rich ubisch granules containing sporopollenin, pollenkitt, compatibility recognising proteins etc.

          (iii) It secretes enzyme callase for the separation of microspores.

9. The meiocyte of rice has 24 chromosomes. Write the number of chromosomes in its endosperm.

ANS:-  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.

10. What is pericarp? Mention its function.

ANS:-  Pericarp is the covering of fruit that develops from ovary wall. It protects the fruit and also helps in its dispersal.

11. Give reason why anthers of angiosperm flowers are described as dithecous.

ANS:-   Anthers of most angiosperms(flowering plants) contain two anther lobes, hence are called dithecous.

12. Mention any one application of a pollen bank.

ANS:-   Pollen banks are used to store pollen grains for long time, which can be used in plant breeding programmes. In pollen banks, pollens are stored in liquid nitrogen at a temperature of –196°C.

13. All papaya plants bear flowers but fruits are seen in some. Explain.

ANS:-   Papaya plant is dioecious plant and bears male and female flowers on different plants i.e., male plant and female plant. Fruit is formed by the female part of the   flower after fertilisation. Following double fertilisation, i.e., fusion of one male gamete with egg cell and the other male gamete with secondary nucleus. Ovules turn into seeds and ovary turns into fruits.

14. Write the  characteristic  features  of  anther, pollen and stigma of wind pollinated 

ANS:-   In wind pollinated  flowers, anthers are exserted and versatile. In some cases like Urtica, the anthers burst suddenly to throw the pollen grains (gun powder mechanism).

Pollens of wind pollinated  flowers are light, small, winged, dry, smooth, non-sticky, unwettable and are produced in very large number.

Stigma of wind pollinated  flower is exserted, hairy, feathery or branched to catch the wind borne pollen grains. The large thread-like stigmas and styles of cob of maize hang in air to catch wind borne pollens.

15. A bilobed, dithecous anther has 100 microspore mother cells per microsporangium. How many male gametophytes this anther can produce?

ANS:-   Dithecous anther has four microsporangia or pollen sacs. This anther  will have 400 microspore mother cells. As each microspore mother cell forms 4 pollen grains, 400 microspore mother  cells will form 1600 pollen grains (male gametophytes).

16. Why do the pollen grains of Vallisneria have a mucilaginous covering?

ANS:-   Vallisneria is a water pollinated plant. Pollen grains of Vallisneria have a protective mucilaginous coat that  prevents  the  water from  damaging  the pollen grains.

17. The microscopic pollen grains of the past are obtained as fossils. Mention the characteristic of the pollen grains that makes it happen.

ANS:-   Sporopollenin  is a  major  component  of the hard outer layer called exine of pollen grains. It is chemically very stable and is usually well preserved in soils and sediments. Hence, pollen grains of the part are well preserved as fossils.

18. Name the type of flower which favours cross pollination. 

ANS:-  Unisexual  flowers favour cross pollination.

19. Why is bagging of the  emasculated flowers essential during hybridisation experiments?

ANS:-   Bagging of the emasculated  flowers is essential to prevent the landing of unwanted pollen on the stigma of  flowers during hybridisation experiments.

20.   What is double fertilisation?   

ANS:-  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).  

Two Mark Questions

1. Draw a labelled mature stage of a dicotyledonous embryo.    

ANS:-   Labelled diagram of dicot embryo is as follows:  

2. Angiosperms bearing unisexual flowersare said to be either monoecious or dioecious. Explain with the help of one example of each.

ANS:-  Unisexual   flowers are   flowers bearing  either male or female reproductive structures. Male  flower is  staminate,  i.e., bearing  stamens  while  female  flower is pistillate i.e., bearing pistils. In  some   flowering  plants,  both  male and  female  flowers are present on same plant. Such plants are referred to as monoecious e.g., cucurbits. In some plants,  unisexual  male  and   female   flowers are present on separate plants. Such plants are referred to as dioecious e.g., papaya.

3. Describe the endosperm development in coconut.

ANS:-  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.

4. Gynoecium of a flower may be apocarpous or syncarpous. Explain with the help of an example each.

ANS:-  Gynoecium represents the female reproductive part of a   flower. Gynoecium is called apocarpous if the  carpels are  free, e.g., Michelia. It  is called syncarpous if the  carpels are fused, e.g., Papaver (poppy).

5. Why do hermaphrodite  angiosperms  develop outbreeding  devices? Explain  any  two  such devices with the help of examples.

ANS:-  Hermaphrodite  angiosperms  develop out breeding devices to avoid self pollination and encourage cross pollination. Two outbreeding devices which ensure cross pollination are as follows:

(i)    Dichogamy :  Anthers and  stigmas mature  at different times in a bisexual flower. It is of two types: (a) Protandry : Anthers mature earlier than stigma of the same flower. Their pollen grains become available to stigmas of the older  flowers, e.g., Sunflower, Salvia. (b) Protogyny : Stigmas mature earlier so that they get pollinated before the anthers of the same  flower develop pollen grains, e.g., Mirabilis jalapa, Gloriosa, Plantago.

(ii)   Self sterility  (Self incompatibility)  :   Pollen grains of a  ower do not germinate on the stigma of the same  ower due to presence of similar self sterile gene (S1,S3 in pistil and S1 or S3 in pollen grain) e.g., tobacco, potato, crucifers.

6. Draw a labelled diagram of the L.S. of a monocot seed (maize grain). 

ANS:-  L.S. of a maize grain is as follows:

7. Draw a diagram of a matured  microspore of an angiosperm. Label its cellular components only.

ANS:-  Labelled diagram of mature microspore (or pollen grain) is as follows:

8. (a)  How does cleistogamy ensure autogamy? (b)   State one advantage and one disadvantage of cleistogamy to the plant.

ANS:-  (a) Cleistogamy is the condition  where pollination occurs in closed flowers that do not open at all. In such  flowers, the anthers and stigma lie close to each other. When anthers dehisce in the  ower buds, pollen grains come in contact with the stigma to  effect pollination. Thus, cleistogamous   flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.

(b)   One  advantage  of  cleistogamy is  that  seed setting is assured even in the absence of pollinators as pollen on maturity will always reach the stigma due to their close placement.

One disadvantage of cleistogamy is that it does not allow cross pollination, thereby restricting chances of genetic variability.

9. Geitonogamous  owering plants are genetically autogamous but functionally cross-pollinated. Justify.  

ANS:-  Geitonogamy is the transfer of pollen grains from  the  anther  of one   flower  to  the  stigma  of another   flower of the same plant. It is functionally cross  pollination  involving pollinating  agent  but genetically it is similar to autogamy since the pollen grains come from the same plant.

10.Name all the haploid cells present in an unfertilised mature embryo-sac of a  flowering plant. Write the total number of cells in it.

ANS:-  In  an unfertilised mature  embryo sac of  flowering  plant,  six haploid  cells are present  i.e.,two synergids, one egg, three antipodals. The total number  of cells in an unfertilised mature embryo sac is seven i.e., two haploid synergids, one haploid egg, three haploid antipodals and one central cell (containing two polar nuclei).

11. Differentiate between the two cells enclosed in a mature male gametophyte of an angiosperm.

ANS:-  Two cells enclosed in a mature  male gametophyte of an angiosperm are : a smaller generative cell and a much larger vegetative cell (or  tube  cell). Generative cell is spindle shaped to spherical in outline with thin dense cytoplasm surrounding a prominent nucleus. Vegetative cell has a lobed central nucleus that lies in vacuolated cytoplasm packed with, starch grains, lipids and proteins.

12. Draw a labelled diagram of a mature  pollen grain.

ANS:-  Labelled diagram of mature pollen grain is as follows:

13. Explain the sequence of events in pollen-pistil interaction.

ANS:-  Pollen pistil interaction is the group of events that occur from the time of pollen deposition over the stigma to the time of pollen tube entry  into ovule.

As soon as a pollen grain lands on the stigma, it is hydrated. As a result of hydration,  the exine and intine proteins are released on the stigmatic surface.

 The pollen wall proteins bind to the stigma surface pellicle (receptor site for the pollen wall proteins) within few minutes of the contact.

When pollen is compatible, erosion of the cuticle of the stigma papilla begins beneath the emerging pollen   tube. This is the essential preliminary requirement for the penetration of the pollen tube. In  case  of  incompatible  pollination,  a callosic plug develops between the plasma membrane and pectocellulosic layer of the stigmatic papillae just below the point of contact with the pollen and the growth of pollen tube ceases.

14. Draw labelled diagram of a mature ovule and embryo sac with its contents.

ANS:-  Refer to answer 9 OF LONG QUESTIONS.

15. Draw the labelled structure of mature embryo sac.

ANS:-  Refer to answer 9 OF LONG QUESTIONS.

16. Describe the  process of syngamy and  triple fusion in an angiosperm.   

ANS:-  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.   

17. State one advantage and one disadvantage of cleistogamy

ANS:-  One  advantage  of  cleistogamy is  that  seed setting is assured even in the absence of pollinators as pollen on maturity will always reach the stigma due to their close placement.

18. Where is sporopollenin present in plants? State its signicance with reference to its chemical nature.

ANS:-  Sporopollenin  is  present  in  exine  layer  of pollen grains. Sporopollenin is highly resistant fatty substance  not  degraded  by any enzyme and  not a  affected by high temperature, strong acid or strong alkali therefore pollen grains can be well preserved as microfossils.

19. Explain triple fusion in angiosperm.

ANS:-  In angiosperms, one of the male gametes (n) fuses with  diploid  secondary  nucleus  to  form  a triploid (3n) primary endosperm cell.   is process is referred to as triple fusion.

20. Differentiate between autogamy, geitonogamy and xenogamy.

ANS:-  Difference between autogamy, geitonogamy and xenogamy is as follows:

 Three Mark Questions

1. Explain the post-pollination events leading to seed production in angiosperms.  

ANS:-  Post-pollination events leading to seed formation in angiosperms are as follows :

(i) After pollination, the pistil recognises the pollen whether it is of the right type (compatible) or of the wrong type (incompatible). Compatible pollens are accepted and germinate on the stigma to produce a pollen tube. Pollen tube grows and reaches the ovary and enters the ovule either through micropyle or chalaza or integuments.

(ii) The pollen tube bursts open in one of the two synergids to release the two male gametes. One male gamete fuses with the egg to form a diploid zygote or oospore (syngamy). The second male gamete fuses with the diploid secondary nucleus of the central cell to form a primary triploid endosperm nucleus (triple fusion). The whole process is termed as double fertilisation.

2. Explain the phenomenon of double fertilisation.

ANS:- 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.

3. Make a list of any three outbreeding devices that flowering plants have developed and  explain how they help to encourage cross-pollination.

ANS:- The three outbreeding devices that Flowering plants have developed are as follows:

(i) Dicliny (Unisexuality) : Flowers are unisexual so that self pollination is not possible. The plantsmay be monoecious (bearing both male and female flowers, e.g., maize) or dioecious (bearing male and female flowers on different  plants, e.g., mulberry, papaya).

(ii) Dichogamy : Anthers and stigmas mature at diff­erent times in a bisexual flower so as to prevent self pollination. (a) Protandry : Anthers mature earlier than stigma of the same flower. Their pollen grains become available to stigmas of the older flowers, e.g., sunower, Salvia. (b) Protogyny : Stigmas mature earlier so that they get pollinated before the anthers of the same flower develop pollen grains, e.g., Mirabilis jalapa, Gloriosa, Plantago.

(iii) Heterostyly : There are 2 or 3 types of flowers with different  heights of styles (and stamens), e.g., In diheterostyly (dimorphic heterostyly), there are two types of flowers, pin eyed (long style and short stamens) and thrum eyed (short style and long stamens), e.g., Primula (primrose), jasmine. Pollination occurs between anthers and stigmas of the same height present in different  flowers.

4. Name the two end products of double fertilisation in angiosperms. How are they formed? Write their fate during the development of seed.

ANS:- 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 utilized. 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.

5. Describe the structure of a mature microsporangium of an angiosperm.

ANS:- A microsporangium or pollen sac is a cylindrical sac which appears circular in transverse section. It consists of two parts, outer wall and central homogenous sporogenous tissue. Microsporangial wall has four types of layers - epidermis (common anther covering), endothecium, 1-3 middle layers and tapetum. The outer three perform the function of protection in the young anther and dehiscence of the mature anther. Both endothecium and tapetum consist of larger cells. In a typical anther the endothecial cells develop fibrous thickenings of a-cellulose on the inner and radial walls. In the shallow groove present between the two microsporangia of an anther lobe, the hypodermal cells lying at the level of endothecium remain thin walled. They constitute the stomium or line of dehiscence. The sporogenous tissue gives rise to microspores.

6. Draw a diagram of a mature embryo sac of an angiosperm and label the following parts in it. (a)   Filiform apparatus  (b)  Synergids

(c)   Central cells           (d)  Egg cell

(e)   Polar nuclei            (f)  Antipodals

ANS:-  Refer to answer 9 OF LONG QUESTIONS.

7. Draw  L.S. of  a  pistil  showing  pollen  tube entering the embryo-sac in an angiosperm and label any six parts other than stigma, style and ovary.

ANS:-  Refer to answer 13.

8. Explain the  process of microsporogenesis in angiosperms.

ANS:-  Refer to answer 6 OF LONG QUESTIONS.

9. Explain the  process of megasporogenesis in angiosperms.

ANS:- Megasporogenesis is the formation of megaspore (n) from megaspore mother cell (2n) inside the ovule by the process of meiosis. In the hypodermal region of nucellus towards the micropylar end develops a primary archesporial cell. The primary archesporial cell divides periclinally to form outer parietal cell and inner sporogenous cell. The sporogenous cell functions as megaspore mother cell (MMC), which undergoes reduction division to form four haploid megaspores. Only one megaspore (usually the chalazal one) remains functional and the other three degenerate. Di­fferent stages in the development of linear tetrad of megaspores are shown as follows:

10. Describe the process of megasporogenesis in angiosperms until 8 nucleate stage.

ANS:-  Megasporogenesis is the formation of megaspore (n) from megaspore mother cell (2n) inside the ovule by the process of meiosis. In the hypodermal region of nucellus towards the micropylar end develops a primary archesporial cell. The primary archesporial cell divides periclinally to form outer parietal cell and inner sporogenous cell. The sporogenous cell functions as megaspore mother cell (MMC). The MMC undergoes meiotic division and produce four haploid megaspores. One of the megaspore is functional while other three degenerate. Di­fferent stages in the formation of embryo sac from functional megaspore are represented diagrammatically as follows:

11. Describe the structure of a 3-celled pollen grain of an angiosperm.

ANS:-  Pollen grain is the first cell of male gametophyte. The 3-celled pollen grain of an angiosperm consists of one tube or vegetative cell and two male gametes. The tube cell has a vacuolated cytoplasm which is rich in the food reserves (starch, protein, fat with mostly unsaturated fatty acids) and cell organelles. Its nucleus is large and irregular. Male gametes are formed by division of generative cell. Each male gamete is lenticular to spherical in outline. It has a large nucleus which is surrounded by a thin sheath of cytoplasm.

12. Explain how false, true and parthenocarpic fruits are different from each other. Give one example of each.      

ANS:-  False fruit is the fruit which is derived from the fertilised ovary and accessory oral 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.

13. Draw a longitudinal section of a post-pollinated pistil  showing  entry  of  pollen  tube  into  a mature  embryo-sac. Label filiform apparatus, chalazal end, hilum, antipodals, male gametes and secondary nucleus.   

ANS:-  Longitudinal section of post pollinated pistil showing entry of pollen tube into a mature embryo sac is shown as follows:

14. Describe the development of a 7-celled female gametophyte from a megaspore mother cell in an angiosperm. 

ANS:-  Refer to answer 10.

15. Explain any three out breeding devices in flowering plants.

ANS:-  Refer to answer 3.

16. Describe the stages in embryo development in a dicot plant.     

ANS:-  In a typical dicot flower, after double fertilisation, the zygote elongates and then divides by a transverse wall into two unequal cells. The larger basal cell is called suspensor cell. The other towards the antipodal end is termed as terminal cell or embryo cell. The suspensor cell divides transversely a few times to produce a filamentous suspensor of 6–10 cells. The suspensor helps in pushing the embryo in the endosperm. The first cell of the suspensor towards the micropylar end becomes swollen and functions as a haustorium. The last cell of the suspensor is known as hypophysis. Hypophysis later gives rise to the radicle and root cap. The embryo cell undergoes two vertical divisions and one transverse division to form eight cells arranged in two tiers–epibasal (terminal) and hypobasal (near the suspensor).The epibasal cells eventually form the two cotyledons and the plumule. The hypobasal cells produce the hypocotyl except its tip.

Stages in the development of a dicot embryo are represented diagrammatically as follows:   

17. Draw a diagram  of a vertical section  of an anatropous ovule of an angiosperm. Label the following parts

ANS:-  Sectional view of anatropous ovule is given below :-.

18. State the significance  of pollination. List any four differences between wind-pollinated and animal-pollinated  flowers.

ANS:-  Significance of pollination :

(i) Pollination leads to fertilisation and production of seeds and fruits, which ensures continuity of plant life.

(ii) The pollination, especially cross pollination results in the production of plants with a combination of characters from two plants. So it introduces genetic recombinations and hence variations in the progeny.

Di­fferences between wind pollinated and animal pollinated flowers are as follows:

 19. Differentiate between parthenogenesis and parthenocarpy. 

     ANS:-  Di­fferences between parthenocarpy and parthenogenesis are as follows:

20. Describe the Polygonum type of embryo sac. Why is it generally referred to as monosporic?

     ANS:-  The most common type of embryo sac, found in 80% owering plants is called Polygonum type of embryo sac. It contains 8 nuclei and 7 cells – 3 micropylar, 3 chalazal and one central. It is formed by one meiosis and three mitosis.The three micropylar cells are collectively known as egg apparatus. One middle cell is larger and is called egg or oosphere. The remaining two cells are called synergids or cooperative cells or help cells. The three chalazal cells of the embryo sac are called antipodal cells. The central cell contains two polar nuclei which often fuse to from a single diploid secondary nucleus or definitive nucleus. Polygonum type of embryo sac is developed from a single megaspore (rest three degenerate) and so it is called monosporic type of embryo sac.

 Five Mark Questions

1. (a)   Draw a labelled diagram of the sectional view of microsporangium of an angiosperm.

ANS:-  (a) Sectional view of microsporangium of an angiosperm is as follows:

 (b)   Explain the  development  of  male gametophyte in the microsporangium.

ANS:-  Pollen grain or microspore is the first cell of male gametophyte and represents immature male gametophyte. Development of male gametophyte is precocious, i.e., it begins inside the microsporangiumor pollen sac. Young pollen grain has a centrally placed nucleus embedded in dense cytoplasm covered by plasma membrane. It grows in size with the inflow of nutrients. The protoplast of the pollen grain divides mitotically to form two unequal cells - smaller generative cell and larger tube or vegetative cell. A layer of callose develops around the generative cell which separates the cell from the pollen wall. Later on, callose dissolves and the naked generative cell comes to lie freely in the cytoplasm of the tube cell. The tube cell has vacuolated cytoplasm which is rich in the food reserve and cell organelles. Its nucleus becomes large and irregular.The generative cell is spindle shaped to spherical in outline with thin dense cytoplasm surrounding a prominent nucleus. In some species the generative cell divides into two nonmotile male gametes prior to the dehiscence of anther and release of the pollen grains. therefore, at the time of pollination, the pollen grain is either 2-celled or 3-celled.

2. Explain the development  of the  zygote into an embryo and of the primary endospermic nucleus into an endosperm in a fertilised embryo sac of a dicot plant.  

ANS:-  Endosperm is the food laden tissue formed after double fertilisation. It provides essential nutrients to the growing embryo and also the young seedling at the time of seed germination. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilisation or triple fusion i.e., fusion of a male gamete with secondary nucleus of the central cell. Based on the first and subsequent divisions of primary endosperm nucleus, the development of endosperm takes place in di­fferent ways and accordingly endosperm is of three types – nuclear, cellular and helobial.

• Nuclear type : In the nuclear type of endosperm the first division of primary endosperm nucleus and few subsequent nuclear divisions are not accompanied by wall formation. The nuclei produced are free in the cytoplasm of the embryo sac and they may remain free indefinitely or wall formation takes place later. The multinucleate cytoplasm undergoes cleavage,and gives rise to multicellular tissue, maize, wheat,rice.
• Cellular type : In this case, there is cytokinesis after each nuclear division of endosperm nucleus. The endosperm, thus, has a cellular form, from the very beginning because first and subsequent divisions are all accompanied by wall formation. e.g., Petunia, Datura, Adoxa etc.
• Helobial type: It is an intermediate type between the nuclear and cellular types. The first division is accompanied by cytokinesis but the subsequent ones are free nuclear. The chamber towards micropylar end of embryo sac is usually much larger than the chamber towards chalazal end. A large number of nuclei are formed in the micropylar chamber by free nuclear divisions while the nucleus of the chamber towards chalazal end divides to form a fewer free nuclei or may not divide at all, e.g., Order Helobiales and most monocots. Endosperm provides nourishment to the zygote. So,endosperm development precedes that of zygote.

 3. (a) Draw a longitudinal section of a pistil of an angiosperm showing the growth of the pollen tube up to the micropyle of the ovule.

ANS:-  Longitudinal section of pistil showing growth of pollen tube is shown below.

(b)  Explain the events that occur, upto fertilisation, when the compatible pollen grain lands on the stigma. 

ANS:-  (b) ‑The compatible pollen grain germinates on the stigma; the intine grows out through one of the germ pores as a pollen tube.

– The contents of the pollen grain move into the pollen tube.

– The generative cell divides into two male gametes.

– The pollen tube grows through the stigma and style and reaches the ovary; it enters the ovule through micropyle and then enters the embryo sac guided by the filiform apparatus and discharges the male gametes into one of the synergids.

– One of the male gametes fuses with the egg cell (syngamy) to form the zygote and the other male gamete fuses with the two polar nuclei (triple fusion) to form the primary endosperm nucleus (PEN).    

4. (a)   Draw a diagram of an enlarged view of T.S. of one microsporangium of an angiosperm and label the following parts:

(i)    Tapetum

(ii)   Middle layer

(iii) Endothecium

(iv)  Microspore mother cells

ANS:-  (a) Sectional view of microsporangium of an angiosperm is as follows:

(b)   Mention  the  characteristic features and function of tapetum.

ANS:-  (b) ‑The characteristics of labelled i.e.,tapetum are as follows:

(i) Tapetal cells are filled with protoplasmic contents as well as nutrients.

(ii) They are either multinucleate or their nucleus becomes polyploid due to endoploidy.

(iii) Tapetum is of two types – amoeboid and secretory.

(iv) In amoeboid type, the tapetal cells fuse to form a plasmodium or periplasmodium because it passes in between the sporogenous cells to nourish them.

(v)The cells of secretory tapetum pass out substances over the sporogenous cells for their growth and di­erentiation.

(c)   Explain the following giving reasons:

ANS:-  (i)    Pollen grains are well preserved as fossils.

Sporopollenin is present in exine layer of pollen grains. Sporopollenin is highly resistant fatty substance not degraded by any enzyme and not a­ected by high temperature, strong acid or strong alkali therefore pollen grains can be well preserved as microfossils.

(ii)   Pollen tablets are in use by people these days.  

ANS:-  Pollen tablets are used as food supplement by people to improve health.

5. (a)  Geitonogamy  is  functionally  a  cross pollination but genetically similar to autogamy. Explain.

ANS:-  Geitonogamy is the transfer of pollen grains from the anther of one flower to the stigma of another flower of the same plant. It is functionally cross pollination involving pollinating agent but genetically it is similar to autogamy since the pollen grains come from the same plant.

(b)   Why do  flowering plants need to develop outbreeding  devices. Explain any three such devices developed by  owering plants.

ANS:-  Refer to answer 3 of three mark question.

6. How does the pollen mother cell develop into a mature pollen grain? Illustrate the stages with labelled diagrams.   

ANS:-  In an anther, each cell of the sporogenous tissue is a potential pollen or microspore mother cell (PMC). Each PMC divides by meiosis to form a microspore tetrad. This process is called microsporogenesis. As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. The hard outer layer called the exine is made up of sporopollenin. The inner wall of the pollen grain is called the intine. When the pollen grain is mature it contains two cells, the vegetative cell and the generative cell. Di­fferent stages of microsporogenesis are shown below.

7. Explain  with  the   help  of  a  diagram   the development of a mature  embryo sac from a megaspore mother cell in angiosperm.

ANS:-  Megasporogenesis is the formation of megaspore (n) from megaspore mother cell (2n) inside the ovule by the process of meiosis. In the hypodermal region of nucellus towards the micropylar end develops a primary archesporial cell. The primary archesporial cell divides periclinally to form outer parietal cell and inner sporogenous cell. The sporogenous cell functions as megaspore mother cell (MMC). The MMC undergoes meiotic division and produce four haploid megaspores. One of the megaspore is functional while other three degenerate. Diff­erent stages in the formation of embryo sac from functional megaspore are represented diagrammatically as follows:

The functional megaspore is the first cell of female gametophyte or embryo sac. The nucleus of megaspore divides by mitosis into eight daughter nuclei. Two polar nuclei are present in centre which further fuse to form a secondary nucleus. After fertilisation with a male gamete it produces triploid endosperm. Three nuclei at the base of embryo sac form antipodal cells. The remaining three nuclei at the micropylar end constitute egg apparatus, which consists of two cells known as synergids or help cells and an egg cell or osphere. The egg cell on fusing with one male gamete (fertilisation) gives rise to zygote.

8. How does the megaspore mother cell develop into  7-celled, 8  nucleate  embryo  sac  in  an angiosperm?  Draw  a  labelled diagram  of  a mature embryo sac.   

ANS:-  Refer to answer 7.

9.  (a)   Describe the process of megasporogenesis in angiosperms until 8 nucleate stage.

ANS:-  Refer to answer 7.

(b)   Draw the  labelled structure  of mature embryo sac.  

 ANS:-  (b)

10. (a)   Draw a labelled schematic diagram  of the transverse section of a mature  anther  of an angiosperm plant.

ANS:-  (a) T.S. of mature dehisced anther is as follows :

(b)   Describe the characteristic features of an insect pollinated flower.

ANS:-  (b) Characteristic features of insect pollinated flowers are :

(i) Nectar is produced by nectariferous glands of flower which attracts the pollinators for feeding.

(ii) Some flowers produce edible pollen grains which attract the pollinators to visit the flowers e.g., rose, Papaver, Clematis.

(iii) Flowers are fragrant and emit scent and odour e.g., Jasminum, Cestrum etc.

(iv) Flowers are showy and brightly coloured. They usually have coloured leaves, petals, sepals, stamens or sometimes stigma, e.g., leaves in Euphorbia pulcherrima, bract in Bougainvillea.

(v) Small flowers occur in groups and thus, become more conspicuous.

(vi) Stigma also secretes some exudates which make stigma sticky. This sticky stigma can get pollens easily.

(vii) The pollen grains become sticky or develop spines around their body. This feature help in their attachment with the body of pollinators.

-------------------Compiled by:- AKB Sir-------------

Tags: sexual reproduction in flowering plants, microsporogenesis, megasporogenesis, pollen grains, ovule, tapetum