Very Short Answer Type Questions
Question. Give an example of a plant that came to India as a contaminant and is a cause of pollen allergy.
Answer : Parthenium or carrot grass.
Question. State the function of filiform apparatus found in mature embryo sac of an angiosperm.
Answer : Filiform apparatus helps to guide the path of pollen tubes into synergid.
Question. How many pollen grains and ovules are likely to be formed in the anther and the ovary of an angiosperm bearing 25 microspore mother cells and 25 megaspore mother cells respectively?
Answer : Microspore mother cells 25 × 4=100 pollen grains and from megaspore mother cells 25 ovules respectively.
Question. An anther with a malfunctioning tapetum often fails to produce viable male gametophytes. Give any one reason.
Answer : Because the tapetum provides nourishment for the development of pollen grains.
Question. How many microsporangia are present in a typical anther of angiosperm.
Answer : Usually Four.
Short Answer Type Questions – l
Question. How many cells are present in the grains at the time of their release from anther ? Name the cells.
Answer : Pollen grain may be released at 2-celled stage. One vegetative and one generative cell.
Question. A pollen grain in angiosperm at the time of dehiscence from an anther could be 2-celled or 3-celled. Explain. How are the cells placed within the pollen grain when shed at a 2-celled stage ?
Answer : In 2-celled stage, the mature pollen grain contains a generative and vegetative cell, whereas in 3-celled stage, one vegetative cell and two male gametes are present.
The generative cell floats in the cytoplasm of vegetative cell.
Question. Write the function of tapetum in anthers.
Answer : The tapetum is the inner most wall layer of the microsporangia (pollen sacs) in anthers. These cells nourish the developing microspore mother cells and pollen grains. Besides this it forms the exine, secrete pollenkitt and special proteins for pollen grains so as to recognize compatible stigmas.
Question. Name the organic materials exine and intine of an angiosperm pollen grain are made up of. Explain the role of exine.
Answer : The exine is made up of sporopollenin, which is one of the most resistant organic material. The intine layer is made up of cellulose and pectin materials. The exine is hard and hence protects the pollen grains during adverse conditions.
Question. Differentiate between two cells enclosed in a mature male gametophyte of an angiosperm.
Answer : There are three cells enclosed in the male gametophyte of angiosperms out of which two are male gametes and one is tube cell or vegetative
cell. The two male gametes are small, round and surrounded by a little cytoplasm.
They are situated towards the proximal part of the pollen tube, whereas the tube cell or vegetative cells are irregular in outline and is present in the distal part of the pollen tube. Two male gametes are functional and take part in double fertilization whereas the tube cell/vegetative degenerates after the growth of pollen tube.
Question. ”Pollen grains in wheat are shed at 3-celled stage while in peas they are shed at 2-celled stage.” Explain. Where are germ pores present in a pollen grain ?
Answer : At the time of shedding wheat pollen consist of one vegetative and two male gametes (3 celled), While pea pollen consists of one vegetative and one generative cell (2 celled)
Germ pores are present on the exine (Where sporopollenin is absent)
Question. Where is sporopollenin present in plants ? State its significance with reference to its chemical nature.
Answer : Present in exine of pollen / pollen grain Sporopollenin is the most resistant organic materials. It can withstand high temperature and strong acids and alkali. It cannot be degraded by enzymes.
Question. In a flowering plant, a microspore mother cell produce four male gametophytes while a megaspore mother cell form only one female gametophyte. Explain.
Answer : A microspore mother cell/PMC on meiosis forms 4 functional pollen grains/male gametophyte A megaspore mother cell/MMC on meiosis also forms four megaspores but out of it only one is functional and other three degenerate
Question. Mention the ploidy of the different types of cells present in the female gametophyte of an angiosperm.
Answer : Synergids = n/haploid, egg = n/haploid, polar
nuclei = n/haploid, antipodals = n/haploid = × 4 // all types of cell of female gemetophyte are haploid / n = 2
Short Answer Type Questions – ll
Question. (i) Do all pollen grains remain viable for the same length of time ? Support your answer with two suitable examples.
(ii) How are pollen grains stored in pollen banks ? State the purpose of storing pollen grains in banks.
Answer : (i) No 1
Examples :
(a) Cereals / rice / wheat – pollen grains / loose viability within thirty minutes of their release.
(b) In some members of Rosaceae / leguminosae maintain viability for months.
(ii) Using cryopreservation techniques / in liquid nitrogen (– 196º C)
Maintaining viability / preserving threatened species / preserving commercially important plants / to be used for crop breeding programmes
Question. Why are angiosperm anther called dithecous ?
Describe the structure of its microsporangium.
Answer : A typical angiosperm anther is bilobed with each lobe having two pollen sacs. Hence, angiosperm anther are called dithecous.
Structure of Microsporangium : It is circular and is generally surrounded by wall layers namely, an endothecium, 2 or 3 middle layers and a tapetum.
(i) The endothecium performs the function of protection and helps in dehiscence of anther to release the pollen.
(ii) The middle layers and the innermost layer, tapetum nourishes the developing pollen grains. The cells of the tapetum possess dense cytoplasm and more than one nuclei.
(iii) When the anther is young, a group of compactly arranged homogenous cells called sporogenous tissues occupies the centre of each microsporangium which produce microspores or pollen grains.
Question. The embryo sac in female gametophyte is seven celled and eight nucleated structure. Justify the statement with the help of a labelled diagram.
Answer : The typical female gametophyte or embryo sac has three cells that are grouped together at the micropylar end and constitute the egg apparatus.
The egg apparatus, in turn, consists of two synergids and one egg cell.
Three cells are at the chalazal end and are called the antipodals.
The large central cell has two polar nuclei.
Thus, a typical angiosperm embryo sac, at maturity is 8-nucleate is 7-celled.
For diagram: Refer to Topic 1/ Revision Notes/
Important Diagrams/ Fig 2.7 1
Question. Draw a diagram of a male gametophyte of an angiosperm. Label any four parts. Why is sporopollenin considered the most resistant material.
Answer : Diagram of male gametophyte : Refer to Topic 1/ Revision Notes/ Important Diagrams/ Fig 2.3 2 Sporopollenin is considered to be the most resistant organic material because it is chemically very stable and can withstand high temperature, acidic and alkaline conditions and enzymes.
Question. (i) Describe in sequence the process of microsporogenesis in angiosperms.
(ii) Draw a labelled diagram of two celled final structure formed.
Answer : (i) The process of formation of microspores or pollen grains from microspore or pollen mother cell (MMC or PMC) by meiosis is called microsporogenesis.
It takes place in pollen sacs or microsporangia of each anther lobe.
The cells of sporogenous tissue of microsporangium functions as potential MMC/PMC in the anther. They undergo meiosis and as a result form four microspores or pollen grains arranged in tetrads. The pollen grains separate from the tetrads and give rise to two celled male gametophytes while still in situ. In the majority of angiosperms, the pollen is released from the anther at 2 celled stage while in some at 3-celled stage as the generative cell divides to form 2 male gametes.
(ii) Refer to Revision Notes/ Important Diagrams/ Fig 2.3
Question. (i) Name the organic material exine of the pollen grain is made up of. How is this material advantageous to pollen grain
(ii) Still it is observed that it does not form a continuous layer around the pollen grain. Give reason.
(iii) How are ‘pollen banks’ useful ?
Answer : (i) Sporopollenin.
Most resistant to high temperature / strong acids / alkali / no enzymes can degrade it.
(ii) (Germs pores) to allow pollen tube to emerge out / pollen germination.
(iii) Helps in storing pollen grains for years / for crop breeding programmes
Question. Draw a labelled diagram of the sectional view of a mature pollen grain of angiosperms. Explain the function of any two of its parts.
OR
Draw a labelled diagram of a mature pollen grain.
Answer : Refer to Topic 1/ Revision Notes/ Important Diagrams/ Fig 2.3
Exine : It can withstand high temperature/strong acids/alkali. It protects pollens.
Intine : It is a thin and continuous layer made up of cellulose and pectin. The intine produces pollen tube.
Vegetative cell : It is bigger and has abundant food reserve.
Generative cell : It divides mitotically to give rise to two male gametes.
Question. Explain giving reasons why pollen grains can be best preserved as fossils.
Answer : Pollen grains are best preserved as fossils because of the following reasons :
(i) The sporopollenin of exine is highly resistant to the action of strong acids and alkali and can withstand a high temperature.
(ii) It is not easily degraded by any of the enzyme known so far.
Long Answer Type Questions
Question. (i) Describe the process of megasporogenesis of angiosperms until the 8-nucleate stage.
(ii) Draw the labelled structure of a mature embryo sac.
OR
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.
Answer : (i) The process of formation of the megaspore from the megaspore mother cell is called megasporogenesis. Ovules generally differentiate a
single megaspore mother cell in the micropylar region of the nucellus. This mother cell undergoes meiosis and as a result forms a linear tetrad of 4 megaspores.
Usually one of the four megaspores towards the micropylar end is functional, while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac).
The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles, forming 2-nucleate embryo sac.
Two more sequential mitotic nuclear divisions result in the formation of the 4 nuclei and the later 8-nucleate stage of the embryo sac.
(ii) Refer to Topic 1/ Revision Notes/ Important Diagrams/ Fig 2.
Question. Explain the process of microsporogenesis in angiosperms.
Answer : Process of microsporogenesis:
(i) When the anther develops, each cell of sporogenous tissue functions as microspore mother cell (MMC) or pollen mother cell (PMC) and undergoes meiotic divisions to form microspore tetrads.
(ii) The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.
(iii) The microspores get arranged in a cluster of four cells and hence are microspore tetrad.
(iv) As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
(v) In each microsporangium, thousands of pollen grains are formed and released with the dehiscence of anther.
Question. (i) Describe the structure of a 3-celled pollen grain of an angiosperm. U
(ii) Trace its development from sporogeneous tissue in the anther.
OR
Describe in sequence the events that lead to the development of a 3-celled pollen grain from a microspore mother cell in angiosperms.
OR
Describe the development of male gametophyte in angiosperms.
Answer : (i) Structure of 3-celled pollen grain :
(a) Pollen grains are normally spherical in shape.
(b) Each pollen grain has a prominent two-layered wall.
(c) The outer layer is called exine and the inner layer is called intine.
(d) Exine is hard and made up of sporopollenin.
(e) Intine is thin, and it is made up of cellulose and pectin.
(f) At certain places, exine is either absent or very thin and such places are called germ pores.
(g) A mature pollen grain has two cells – a vegetative cell and a generative cell.
(h) The vegetative cell is larger, has abundant reserve food and a large irregular-shaped nucleus.
(i) The generative cell is small and spindle shaped and it floats in the cytoplasm of vegetative cell.
It is a cell with in the cell.
(j) The generative cell in some cases divides to form two male gametes, thus making the pollen 3-celled.
(ii) Development of 3 celled pollen grain :
(a) Every cell of the sporogeneous tissue has a potential pollen mother cell (PMC) and can give rise to microspore tetrad or pollen grains.
(b) Each microspore mother cell undergoes meiosis to form a cluster of four haploid cells, called microspore tetrad.
(c) As the anther matures, the microspores dissociate from the tetrad and develop into pollen grains.
(d) The nucleus of the microspore undergoes mitosis to form a large vegetative cell or tube cells and a small spindle-shaped generative cell that floats in the cytoplasm of the vegetative cell.
(e) They develop a two-layered wall, the outer exine made of sporopollenin and the inner intine made of cellulose and pectin.
(f) Usually the pollen grains are liberated at this two-celled stage. In certain species the generative cell divides mitotically to form two male-gametes and the pollen grains are three celled during liberation.
For diagram: Refer to Topic 1/ Revision Notes/Important
Question. Describe the structure of microsporangium.
OR
Draw a labelled diagram of an anther lobe at microscopic-mother cell stage. Mention the roles of different wall layers of anther.
Answer : Structure of microsporangium of Pollen or Pollen sac :
(i) It is circular and is generally surrounded by wall layers namely, Epidermis, Endothecium, or 3 Middle layers and Tapetum.
(ii) The first two layers perform the function of protection and help in dehiscence of anther to release the pollens.
(iii) The middle layers and the innermost layer, (tapetum) nourishes the developing pollen grains.
(iv) The cells of the tapetum possess dense cytoplasm and more than one nuclei.
(v) When the anther is young, a group of compactly arranged homogenous cells called sporogenous tissues occupies the centre of each microsporangium.
For figure, Refer to Topic 1/ Revision Notes/ Important Diagrams/ Fig 2.2
Question. (i) Describe the development of a 7-celled female gametophyte from a megaspore mother cell in an angiosperm.
(ii) What is the role of endothecium and tapetum in an anther ?
Answer : (i) Development of female gametophyte :
The female reproductive part of a flower is gynoecium, which consists of three parts— stigma, style and ovary. The ovules are formed in the ovary and attach to it through placenta. The ovule is surrounded by one to two protective layers called integuments, leaving a small opening at one end termed as a micropyle. The stalk of the ovule is called funiculus. The ovule is composed of multi celled cellular tissue called the nucellus. A hypodermal cell of nucellus at the micropylar end enlarges and becomes a megaspore mother cell that undergoes meiosis to form a linear tetrad of four megaspores. Out of four, only one remains functional and three megaspores degenerate. The functional megaspore undergoes three successive mitotic divisions to form eight nuclei, which arrange themselves into three groups. Three nuclei migrate towards the micro-pylar end and form the egg apparatus. Other three nuclei form antipodal cells at chalazal end. The remaining two nuclei come together as polar nuclei which fuse to form secondary nucleus in the centre of the embryo sac.
(ii) Role of endothecium : Endothecium performs the function of protection and helps in dehiscence of anther to release the pollen.
Role of tapetum : It nourishes the developing pollen grains.
TOPIC-2
Pollination and Fertilization
Very Short Answer Type Questions
Question. If the stamens are well exposed, usually which mode of pollination the plant is expected to follow ?
Answer : If the stamens are well exposed, the plant is expected to follow wind pollination.
Question. These pictures show the gynoecium of
(i) Papaver and (ii) Michellia flowers. Write the difference in the structure of their ovaries.
Answer : (i) Multicarpellary ovary showing fused syncarpous pistil.
(ii) Multicarpellary ovary showing free apocarpous pistil.
Question. Why do cleistogamous flowers assure seed sets ?
Answer : Cleistogamous flowers are invariably autogamous as there is no chance of cross pollination.
Question. How many pollen grains and ovules are likely to be formed in the anther and ovary of an angiosperm bearing 25 microspore mother cells and 25 megaspore mother cells respectively.
Answer : 100 pollen grains, 25 ovules.
Question. How do the pollen grains of Vallisneria protect themselves ?
OR
Why do the pollen grains of Vallisneria have mucilage covering ?
Answer : The pollen grains of most of the water pollinated species have a mucilaginous covering to protect from wetting.
Question. Explain the process of pollination in Vallisneria.
Answer : The female flower reaches the surface of water by long stalk, male flower releases the pollen grains on surface of water, pollen grains are carried by water currents, some of them reach the stigma and achieve pollination.
Question. Write one advantage and one disadvantage of cleistogamy to flowering plants.
Answer : Advantage – Assured seed set / maintain purelines
Disadvantage – No variation / only parental characters are preserved / it can lead to inbreeding depression
Question. What is pollen–pistil interaction and how is it mediated ?
Answer : It is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
The interaction takes place through the chemical components produced by them.
Question. A bilobed anther has 100 microspore mother cells per microsporangium. How many male gametophytes can this anther produce.
Answer : 1600.
Short Answer Type Questions – l
Question. Gynoecium of a flower may be apocarpous or syncarpous. Explain with the help of an example each.
Answer : Carpels are free (apocarpous), e.g. : Michelia. Carpels are fused (syncarpous), e.g. : Papaver.
(Any other suitable correct e.g.)
Question. Write the differences between wind–pollinated and insect–pollinated flowers. Give an example of each type.
Answer :
Question. State the similarity and differences between geitonogamy and xenogamy.?
OR
Differentiate between geitonogamy and xenogamy in plants. Which one between the two will lead to inbreeding depression and why ?
Answer :
Geitonogamy will lead to inbreeding depression because genetically it is similar to autogamy.
Question. Geitonogamous flowering plants are genetically autogamous but functionally cross-pollinated. Justify.
Answer : Geiotonogamy is cross-pollination involving a pollinating agent, it is genetically similar to autogamy since the pollen grains come from the same plant.
Question. (i) How does cleistogamy ensure autogamy ?
(ii) State one advantage and one disadvantage of cleistogamy to the plant.
Answer : (i) The cleistogamous flowers remain closed and never open. They are bisexual. Therefore, this condition of flowers i.e., cleistogamy ensures autogamy i.e., they ensure the transfer of pollen from the anther to the stigma of the same and single bisexual flower e.g., Commelina and Viola.
(ii) The main advantage of cleistogamy is guaranteed or assured pollination and therefore, fertilization and seed setting. The main disadvantage of cleistogamy is that variants cannot be produced due to autogamy.
It results in poor crop yield and poor resistance to environmental stresses.
Question. Comment upon the mode of pollination in Vallisneria and Eichhornia which have emergent flowers.
Answer : In Eichhornia the flowers emerge above the level of water and are pollinated by insects or wind.
In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried passively by water currents and some of them eventually reach the female flowers and the stigma.
Question. How does the study of different parts of a flower help in identifying wind as its pollinating agent ?
Answer : Wind pollinating flowers has the following characteristics :
(i) Pollen grains are light and non sticky.
(ii) The flowers often possess well-exposed stamen and large feathery stigma.
(iii) Wind-pollinated flowers have a single ovule in each ovary.
(iv) These flowers have small calyx and corolla.
Question. Why is fertilization in angiosperms referred to as double fertilization ? Explain.
Answer : Fertilization in angiosperms is referred as double fertilization, because two male gametes of a single male gametophyte fuse differently with two different cells/nuclei of the same embryo sac to produce two different structures. One male gamete fuses with the egg to form diploid zygote which gives rise to diploid embryo and another male gamete fuses with two haploid polar nuclei or diploid secondary nucleus to form triploid primary endosperm nucleus (3n) which gives rise to triploid endosperm. Hence, the phenomenon of two fusions (syngamy and triple fusion occur) in an embryo sac is known as double fertilization.
Question. Identify the type of flower shown in A and B. Which out of the two will produce an assured seed set ?
Answer : The flower A is chasmogamous flower having exposed anthers and stigma whereas B is cleistogamous flower which do not open at all. Cleistogamous flowers produce an assured seed set.
Short Answer Type Questions – l
Question. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. Explain three such devices.
Answer : (i) Pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.
(ii) Anther and stigma are placed at different positions in such a way that pollen cannot come in contact of stigma of the same flower.
(iii) Self incompatibility which inhibits the pollen germination/pollen tube growth in the pistil of same flower or another flower of same plant.
(iv) Production of unisexual flowers. This condition prevents both autogamy and geitonogamy.
Question. Write the mode of pollination in Vallisneria and water lily. Explain the mechanism of pollination in Vallisneria.
Answer : Mode of Pollination—Water (hydrophily) Pollination in Vallisneria :
The plant is dioecious. On maturity, the male flowers get detached from the parent plant and float up and come to the surface of water. At the same time, the female flowers also rises up to the surface of water by straightening of the coiled stalk. The detached male flowers cluster around the floating female flower and dehisce, thereby performing pollination. The long stalks of the female flower begin to coil down to the bud level where the fruit ripens.
Question. Make a list of any three out breeding devices that flowering plants have developed and explain how they help to encourage cross-pollination.
Answer : Outbreeding devices that help to encourage cross pollination are as follows:
(i) Avoiding synchronization :
In some species, pollen release and stigma receptivity are not synchronized.
Either the pollen is released before the stigma becomes receptive or stigma becomes receptive before the release of pollen. It prevents autogamy.
(ii) Arrangement of anther and stigma at different positions :
In some species, the arrangement of anther and stigma at different positions prevents autogamy.
(iii) Self-incompatibility : It is a genetic mechanism which prevent pollen of one flower to germinate on the stigma of same flower.
(iv) Production of unisexual flowers :
In monoecious plants such as castor & maize, the male and the female flowers are present on the same plant prevents autogamy but not geitonogamy. On the other hand, in dioecious plants like papaya, the male and female flowers are present on different plants prevents both autogamy and geitonogamy.
Question. Explain the steps that ensures cross-pollination in an autogamous flower.
Answer : (i) Pollen release and stigma receptivity are not synchronized, i.e., either the anther matures earlier or stigma.
(ii) The anther and the stigma are placed at different positions so that pollens cannot come in contact with the stigma of the same flower.
(iii) Certain plants produce unisexual flowers, i.e male and female flowers are present on different plants.
Question. (i) What are the benefits of choosing a dioecious plant species for plant breeding experiments ?
(ii) How would you proceed to cross-pollinate a monoecious flower ?
Answer : (i) (Unisexual) self pollination avoided, emasculation not required
(ii) (a) Emasculation
(b) Bagging
(c) Pollination by spraying desired pollen
(d) Rebagging
Question. Enumerate any six adaptive floral characteristics of a wind – pollinated flower.
Answer : (i) Large production of pollen grains.
(ii) Anther is well exposed.
(iii) Flowers are not attractive and scent emitting.
(iv) Feathery and sticky stigma.
(v) The pollen grains are light and non-sticky so that they can be transported in wind currents.
(vi) Flowers do not possess nectar.
Question. How is it possible in Oxalis and Viola to produce assured seed-sets even in the absence of its pollinators ?
Answer : Oxalis and Viola bear cleistogamous flowers which never open. They have bisexual flowers with anther and stigma lying very close to each other.
When anthers dehisce in the closed flowers the pollen grains fall down on stigma, thus effecting pollination followed by fertilization which leads to assured seed set.
Question. Explain three outbreeding devices.
Answer : (i) Pollen release and stigma receptivity is not synchronised.
(ii) Anther and stigma are placed at different position.
(iii) Self incompatibility.
(iv) Production of unisexual flowers.
Question. What does an interaction between pollen grains and its compatible stigma result in after pollination
? List two steps in sequence that follow after the process.
Answer : There is continuous interaction between pollen grain and pistil which is mediated by the chemical components of pollen.
Two steps :
Pollen grain germinates on the stigma to produce pollen tube and the contents of the generative cell move into the pollen tube. Pollen tube grows through the tissue of stigma and style by secreting enzyme.
Question. Explain the process of emasculation and bagging of flowers. State their importance in breeding experiments.
Answer : Emasculation: If the female parent bears bisexual flowers, removal of anthers from the flower’s bud before the anther dehiscence, using a pair of forceps is referred to as emasculation.
Bagging: Emasculated flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This process is called bagging.
Importance: When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parents are dusted on the stigma, the flowers are rebagged and the fruits are allowed to develop.
If the female parent produces unisexual female flowers, there is no need of emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower is rebagged.
Question. If a chromosome number of a plant species is 16, what would be the chromosome number and ploidy of the (a) microspore mother cell and the (b) endosperm cells ?
Answer : (a) Microspore mother cell → 16, which is a diploid (2n) number.
(b) Endosperm → 24—Because it is a triploid (3n) structure, formed as a result of triple fusion i.e, the fusion of one male gamete with two haploid
(n) polar nuclei or with a diploid secondary nucleus.
Long Answer Type Questions
Question. (i) 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.
(ii) Draw a diagram of a section of a megasporangium of an angiosperm and label funiculus, micropyle, embryo sac and nucellus.
Answer : (i) Emasculation, removal of anthers from the flower bud before the anther dehisce to avoid self pollination.
Bagging, to prevent contamination of its stigma with unwanted pollen grains. Rebagging, the stigma of the mature ovary are dusted with desired pollen grains and rebagged to allow the fruit to develop.
(ii) Refer to Topic 1/ Revision Notes/ Important Diagrams/ Fig 2.6
Question. (i) Explain the post-pollination events leading to seed production in angiosperms.
(ii) List the different types of pollination depending upon the source of pollen grain.
Answer : (i) Pollen pistil interaction, germination of pollen tube that carries two male gametes, double fertilization/syngamy and triple fusion, development of endosperm, development of embryo, maturation of ovule into seed.
(ii) Autogamy / self pollination / Geitonogamy 1 Xenogamy / cross pollination.
Question. (i) State one difference and one similarity between geitonogamy and xenogamy.
(ii) Explain any three devices developed in flowering plants to discourage self pollination and encourage cross pollination.
Answer : (i) Difference : In geitonogamy, pollen grains from one flower are transferred to the stigma of another flower on the same plant whereas in xenogamy the pollen grains are transferred to the stigma of a flower on another plant (of the same species) which is genetically different. Similarity : In both types of pollination, pollen grains from the anther are transferred to the stigma of another flower of the same species
(ii) (a) Pollen release & stigma receptivity not synchronised / hence the maturity of stigma and pollen are different / Protandry / Protogyny
(b) Anther and Stigma are placed at different positions so that pollen cannot come in contact with stigma of the same flower.
(c) Self incompatibility / Self sterility.
(d) Production of unisexual flowers
Question. (i) Geitonogamy and xenogamy, both require pollinating agents, yet they are very different from each other. Explain how.
(ii) Describe the characteristics of flowers that are pollinated by wind.
Answer : (i) Geitonogamy is transfer of pollen grains from the anther to stigma / pollination of another flower of same plant // self-pollination and genetically same pollen to the stigma.
Xenogamy is transfer of pollen grain from anther of one flower to stigma of another flower of another plant of the same species / pollination of a flower of a different plant // cross pollination and genetically different type of pollens to the stigma.
(ii) (a) Pollen grains are light, non-sticky
(b) Well exposed stamens
(c) Large and feathery stigma
(d) Flowers often have a single ovule in each ovary / inflorescence
Question. A flower of tomato plant following the process of sexual reproduction produces 240 viable seeds.
Answer the following questions giving reasons :
(i) What is the minimum number of pollen grains that must have been involved in the pollination of its pistil ?
(ii) What would have been the minimum number of ovules present in the ovary ?
(iii) How many megaspore mother cells were involved ?
(iv) What is the minimum number of microspore mother cells involved in the above case ?
(v) How many male gametes were involved in this case ?
Answer : (i) 240, one pollen grain participates in fertilization of one ovule.
(ii) 240, one ovule after fertilization forms one seed.
(iii) 240, each MMC forms four megaspores out of which only one remain functional.
(iv) Atleast 60, as each microspore mother cell meiotically divides to form four pollen grains (240/4 = 60)
(v) 480, each pollen grain carries two male gametes (which participate in double fertilization)
Question. Explain the events upto fertilization that occur in a flower after the pollen grain has landed on its compatible stigma.
Answer : The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores, the content of the pollen grain moves into the pollen tube, pollen tube grows through the tissues of the stigma and style and reaches the ovary, the generative cell divides and forms two male gametes during the growth of pollen tube (in the stigma), the pollen tube enters the ovule through micropyle and then enters one of the synergids (through filiform apparatus), the pollen tube releases two male gametes (in the cytoplasm of synergids), one of the male gamete fuses with egg cell to form zygote (2n) (syngamy), the other male gamete fuses with two polar nuclei (in central cell) to form primary endosperm nucleus (PEN-3n)/PEC.
Question. (i) Draw a labelled schematic diagram of the transverse section of a mature anther of an angiosperm plant.
(ii) Describe characteristic features of an insect pollinated flower.
Answer : (i) For figure- Refer Topic I/Revision Notes/
Important Diagrams/ Fig 2.1
(ii) Characteristic features of insect pollinated flowers :
(a) The flowers are brightly coloured, showy, large and if small they becomes conspicuous by grouping as in capitulum and umbel etc.
(b) The flowers are sweetly scented so as to attract the insects for pollination.
(c) The flowers have nectar secreting glands which secrete abundant nectar which attract the pollinating insects.
(d) Flowers may have edible pollen e.g., rosa and clematis.
(e) The flowers have stamens and stigma inserted.
(f) Flowers possesses pollen kit as an yellowish sticky substance.
Question. (i) Differentiate between : autogamy, geitonogamy, and xenogamy.
(ii) Explain the events that occur during pollen-pistil interaction.
Answer : (i) (a) Autogamy : Pollination is achieved within the same flower. Transfer of pollen grains from the anther to stigma of the same flower.
Autogamy in flowers requires synchrony in pollen release and stigma receptivity.
(b) Geitonogamy : Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant. Although geitonogamy is functionally cross-pollination involving pollinating agents but genetically, it is similar to autogamy since the pollen grains come from the same plant.
(c) Xenogamy : Transfer of pollen grains from anther to the stigma of a different plant i.e., cross pollination. This is the pollination that bring genetically different types of pollen grains to the stigma.
(ii) Pollen-pistil Interaction :
(a) It is a dynamic process involving pollen recognition followed by promotion and inhibition of the pollen.
(b) This interaction takes place through the chemical components produced by them.
(c) If the pollen is compatible, then the pistil accepts it and promotes post-pollination events.
(d) The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
(e) The contents of the pollen grain move into the pollen tube.
(f) The pollen tube grows through the tissues of the stigma and style and reaches the ovary.
(g) If the pollen is incompatible, then the pistil rejects the pollen by preventing pollen germination on the stigma or the pollen tube growth in the style.
(h) In some plants, the pollen grains are shed at two-celled stage, the generative cell divides and forms the two male gametes during the growth of pollen tube on the stigma.
(i) The plants which shed pollen in the threecelled stage, the pollen tube carries two male gametes from the beginning.
(j) The pollen tube, after reaching the ovary, enters the ovule through the micropyle, chalaza/integument and then enters one of the synergids through the filiform apparatus.
(k) The filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube.
(l) A plant breeder can manipulate pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids.
Question. (i) Draw the longitudinal section of a flower showing growth of pollen tube upto the embryo sac. Label the following parts :
(a) Stigma (b) Pollen tube
(c) Integument (d) Chalazal end
(e) Nucellus (f) Synergids.
(ii) What is double fertilisation in Angiosperms ? Why is it so called ?
Answer :
(ii) One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing syngamy. This results in the formation of a diploid cell—the zygote. The other male gamete moves towards the two polar nuclei located in central cell and fuses with them to produce a triploid primary endosperm nucleus. Since two types of fusions take place in an embryo sac, the phenomenon is known as double fertilization.
Question. (a) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.
(b) Explain the events upto double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm.
Answer : (a) (i) Dioecy/Production of unisexual flowers (in different plants)
(ii) Self incompatibility
(b) (i) Pollen tube releases 2 male gametes in the cytoplasm of synergids.
(ii) One male gamete fuses with egg cell, syngamy, resulting in diploid zygote.
(iii) Other male gamete fuses with polar nuclei /triple fusion, to form triploid PEN (Primary endosperm nucleus)/PEC (Primary endosperm cell).
TOPIC-3
Post-Fertilization Changes and Special Modes ofReproduction
Question. Name the type of fruit apple is categorised under and why ? Mention two other examples which belong to the same category as apple.
Answer : False fruit, thalamus contributes to fruit formation: Strawberry, Cashew (any other correct examples)
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.
OR
If you squeeze a seed of orange you might observe many embryos of different sizes. How is it possible? Explain.
Answer : Some nucellar cells / surrounding the embryo sac start dividing and protrude into the embryo sac and develop into embryos. In such species each ovule contains many embryos.
Question. Given below is a section of a Maize grain. Identify ‘A’ and state its function.
Answer : Coleoptile, protecting the shoot apex/plumule.
Question. Identify ‘A’ in the figure showing a stage of embryo development in a dicot plant and mention its function.
Answer : Cotyledons–Store food (for growth of embryo of the seed)
Question. In the following figure of a fruit, label the part that is protective in function and the part that is responsible for producing new plants.
Answer :
Question. In case of polyembryony, an embryo A develops from the synergids and the embryo B develops from the nucellus. State the ploidy of embryo A and B.
Answer : A- Haploid ; B- Diploid
Question. Meiocyte of rice has 24 chromosomes. Write the number of chromosomes in its endosperm.
Answer : Number of chromosomes in meiocyte = 24 = 2n.
Haploid number (n) = 12.
Therefore, the number of chromosomes will be n × 3 = 36, because the endosperm in angiosperms is triploid (3n) as it is formed by the fusion of 2 polar nuclei (n) and one gamete.
Question. Apple and cashew are not called true fruits, why ?
Answer : These fruits are not called true fruits because thalamus which is a part of flower other than ovary takes part in fruit formation. A true fruit is one which develops only from the ovary. Apple and cashew are thus false fruits
Question. Mention the fate of the components of the embryo sac after fertilization.
Answer : (i) The egg cell forms the zygote (2n) which give rise to embryo.
(ii) Synergids and antipodals degenerate.
(iii) Polar nuclei form primary endosperm nucleus (3n), which forms the endosperm.
Short Answer Type Questions – l
Question. Differentiate between pericarp and perisperm.
Answer : Pericarp – wall of the fruit (which develops from the wall of ovary)
Perisperm – persistent residual nucellus
Question. Explain the function of each of the following :
(i) Coleorrhiza
(ii) Germ pores
Answer : (i) Protects, the radicle of (monocot) embryo.
(ii) Allow germination of pollen grain / formation of pollen tubes.
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 : For diagram: Refer to Topic 3/ Revision Notes/ Fig 2.15
Thalamus also contributes to fruit formation
Question. How do plants produce seeds through apomixis? Explain with the help of an example.
Answer : In apomixis or agamospermy, seeds are formed without the fusion of gametes. Diploid cells of the nucellus or integuments develop into an embryo, giving diploid seeds with a genetic constitution identical to the parent. Apomixis takes place in orange and onion.
Question. Some angiosperm seeds are said to be albuminous, whereas a few others are said to have a perisperm. Explain each with the help of an example.
Answer : Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development. e.g., wheat, maize, barley, castor, sunflower.
When remnants of nucellus are persistent it is said to have a perisperm. Example; black pepper, Sugar beet.
Question. For a layman, both apples and mangoes are ‘fruits‘. Do you agree ? Give reasons in support of your answer.
Answer : No. 1
Apple – thalamus, (false fruit)
Mango – Ovary, (true fruit)
The parthenocarpic fruits never contain seeds.
Question. Why are some seeds of citrus referred to as polyembryonic ? How are they formed ?
Answer : Some seeds of citrus are referred to as polyembryonic because they contain more than one embryo. This phenomenon is called as polyembryony. In citrus, one embryo develops normally as a result of sexual reproduction and other additional embryos are produced from the cells of nucellus or integument
apomictically. The cells of nucellus or integument surrounding the embryo sac protrude into it, divide and produce the embryos.
Question. Differentiate between albuminous and nonalbuminous seeds, giving one example of each.
Answer : Albuminous – (with residual) endosperm is not completely used up during embryonic development. e.g., wheat / maize / castor / sunflower. Non albuminous – (with out residual) endosperm is completely consumed during embryonic evelopment. e.g., pea / groundnut.
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 : Fruit is a ripened ovary where seed develops from ovule. Mango is a true fruit because it develops only from the ovary of the flower. Apple is a false fruit because here in this case along with ovary thalamus of flower also takes part in the formation of fruit. Banana is a seedless fruit as it develops without the stimulus of pollination and fertilization. Such fruits are also called as parthenocarpic fruits.
Question. Banana is a parthenocarpic fruit, whereas oranges show polyembryony. How are they different from each other in respect to seeds ?
Answer : The banana is a parthenocarpic fruit as it develops without fertilization, whereas in oranges, additional embryos develop directly from diploid cells other than the egg, like nucellus and integument. In banana, the ovary may develop into the fruit without fertilization. The parthenocarpic fruits never contain seeds.
Short Answer Type Questions – ll
Question. (i) How does a farmer use the dormancy of seeds to his advantage ?
(ii) What advantages a seed provides to a plant ?
Answer : (i) For storage (dehydration) of seeds to be used as food, to raise the crop in the next season.
(ii) Seed formation is more dependable, better adaptive strategy for dispersal to new habitat, hard seeds provide protection to the young embryo, being a product of sexual reproduction they generate new genetic combinations / genetic variations / sufficient food reserve for the young seedling to be nourished.
Question. In the figure of a typical dicot embryo, label the parts (1), (2) and (3). State the function of each of the labelled part.
Answer : Label 1 : Origin of plumule; plumule grows into shoot.
Label 2 : Cotyledons; food storage.
Label 3 : Origin of radicle; radicle grows into root.
Question. List the changes that occur when an ovule matures into seed.
Answer : (a) Integuments of ovules harden as tough protective seed coats.
(b) The micropyle remains as a small pore in the seed coat.
(c) As the seed matures, its water content is reduced.
(d) Seeds become relatively dry (10-15 per cent moisture by mass).
(e) The general metabolic activity of the embryo slows down.
(f) The embryo may enter a state of inactivity called dormancy.
Question. Describe the development of endosperm after double fertilization in an angiosperm. Why does endosperm development precedes that of zygote?
Answer : (1) After triple fusion, the central cell develops to form primary endosperm cell which contain triploid primary endosperm nucleus.
(2) The primary endosperm cell undergoes successive cell divisions to form triploid endosperm which has abundant food reserves.
(3) The primary endosperm nucleus undergoes successive nuclear divisions to form many free nuclei. This types of endosperm development is called free nuclear endosperm, after which cell walls are laid and the endosperm becomes cellular endosperm. E.g. coconut water is nuclear endosperm (containing many free nuclei). While white kernel around is cellular endosperm. The endosperm development precedes that of zygote to ensure that endosperm containing abundant food reserves is formed earlier and can nourish the developing embryo.
Question. (i) How are parthenocarpic fruits produced by some plants and apomictic seeds by some others? Explain.
(ii) When do farmers prefer using apomictic seeds?
Answer : (i) Ovary develops into fruit without fertilisation.
Formation of seeds without fertilisation without reductional division/develop into embryo without fertilisation. 1
(ii) To maintain hybrid characters (year after year in a desired plant), to avoid buying hybrid seeds every year (expensive seeds).
Question. Differentiate between perisperm and endosperm, giving one example of each.
Answer : Differences between perisperm and endosperm :
Question. Apomixis resembles asexual reproduction, as well as mimics sexual reproduction in plants. Explain with the help of a suitable example.
Answer : Since there is no fertilisation in apomixis, it resemble asexual reproduction and development of embryo / seed / fruit formation is mimicing sexual reproduction.
In Citrus / Mango, some of the nucellar cells surrounding the embryo sac, act as diploid egg cell, which are formed without reduction division and develop into embryo, without fertilisation.
Question. List the post-fertilization events in angiosperms.
Answer : The various post-fertilization events occurring in angiosperms are :
(i) Primary endosperm nucleus gives rise to endosperm. It is formed before the development of the embryo, as it provides nourishment to the developing embryo.
(ii) The zygote at the micropylar end undergoes successive divisions to form mature embryo.
(iii) Ovule is converted into seed.
(iv) Ovary develops into fruit.
Question. (i) Give one example each of albuminous and nonalbuminous seeds.
(ii) Name the parts of the ovule and the embryo sac of an angiosperm that develop into :
(a) perisperm, (b) seed coats,
(c) endosperm, (d) embryonal axis.
Answer : (i) Example :
Non-albuminous seed : Pea, bean, mustard.
Albuminous seed : Castor, maize, coconut.
(ii) (a) Perisperm : Residual persistent nucellus.
(b) Seed coats : Integuments.
(c) Endosperm : Primary endosperm nucleus.
(d) Embryonal axis : Tigellum.
Question. Do you think apomixis can be compared with asexual reproduction ? Support your answer giving one reason. How is apomixis beneficial to farmers? Explain.
Answer : Yes, seeds are produced without fertilisation. Production of hybrid seeds is costly, if hybrids with desirable characteristics can be made into apomicts, there is no segregation of characters in the hybrid progeny, farmer can continue using hybrid seeds year after year and not buy new seeds.
Question. Describe with the help of three labelled diagrams the different embryonic stages that include the mature embryo of dicot plants.
Answer : Following are the steps that occur during the development of an embryo :
First, the zygote starts dividing and gives rise to proembryo. The cells of this proembryo further divide, forming a globular, heart-shaped and finally mature embryo. A typical dicot embryo consists of an embryonal axis and two lateral cotyledons.
The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the cotyledons is called hypocotyl. It contains the radicle (root tip).
The root tip is covered by the root cap.
For diagram: Refer Topic 3/ Revision Notes/ Important Diagrams/ Fig 2.9 3
Question. Explain any three advantages the seeds offer to angiosperms.
Answer : (i) Since reproductive processes such as pollination and fertilization are independent of water, seed formation is more dependable.
(ii) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
(iii) As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.
(iv) The hard seed coat provides protection to the young embryo.
(v) Being products of sexual reproduction, they generate new genetic combinations / variations.
(vi) Dehydration and dormancy of mature seeds are crucial for survival under adverse conditions.
Question. In angiosperms, the zygote is diploid, while the primary endosperm cell is triploid. Explain.
Answer : A zygote is the product of syngamy, it is formed by the fusion of a haploid male gamete and a haploid female gamete, i.e., egg cell.
Male gamete + Egg → Zygote (n) (n) (2n)
A primary endosperm cell is the product of the fusion of a secondary nucleus (2n) and a haploid male gamete (n).
Secondary + male gamete → Primary endosperm nucleus nucleus (2n) (n) (3n)
Question. Double fertilization 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 fertilization events that are responsible for it.
Answer : Development of endosperm (preceding the embryo) takes place in both, developing embryo derives nutrition from endosperm
Endosperm is retained / persists / not fully consumed in castor, endosperm is consumed in groundnut. In non-albuminous or non-endospermic seeds, the endosperm may be completely utilized by the developing embryo before the maturation of seeds as in pea, bean, groundnut etc. In albuminuous or endospermic seeds, a portion of endosperm persists in the mature seeds. e.g., castor.
Question. Explain how false, true and parthenocarpic fruits are different from each. Give one example of each.
Answer : (i) True fruit : It is the fruit which develops only from the ovary part of flower. e.g., maize, wheat.
(ii) False fruit : It is the fruit which develops from ovary but in addition other parts of the flower like thalamus, calyx or corolla also take part in fruit formation. e.g., apple, strawberry.
(iii) Parthenocarpic fruits : These are the fruits which develop without the act of fertilization. e.g., Banana.
Question. (i) Draw a diagram of a sectional view of monocot seed (grain).
(ii) Label and write the functions of coleoptile, coleorhiza, endosperm.
Answer : Refer to Topic 3/ Revision Notes/ Important
Diagrams/ Fig 2.13
Functions :
Coleoptile – sheath of plumule / protection of plumule
Coleorhiza – sheath of radicle / protection of radicle
Endosperm – filled with reserve food materials for nutrition of developing embryo
Question. With the help of an example of each explain the following ?
Apomixis, Parthenocarpy, Polyembryony.
Answer : (i) Apomixis : It is a type of an asexual process which imitates the sexual reproduction by formation of seed without fertilization. It can be referred to as special type of sexual reproduction which does not involve the process of meiosis and syngamy.
(ii) Parthenocarpy : The formation of fruit without fertilization is called parthenocarpy. Such fruits are seedless and called as parthenocarpic fruits. e.g., Banana.
(iii) Polyembryony : The occurrence of plural embryos in a seed is called polyembryony, e.g., citrus fruits like orange. Normally, one embryo is formed in a seed as a result of fertilization but additional embryos may develop apomictic.
Question. Parthenocarpy and apomixis have been observed in some plants. Give an example of each. State a similarity and a difference observed between the two processes.
Answer :
Similarity : In both the processes, development takes place without fertilisation = 1
Question. If the meiocyte of a maize plant contains 20 chromosomes, write the number of chromosomes in the endosperm and embryo of the maize grain and give reasons in support of your answer.
Answer : Endosperm = 30 , Embryo = 20
Diploid meiocyte (20 chromosomes) form haploid gametes (10 chromosomes). \ Two haploid gametes fuse to form diploid (20) zygote which develops into a (diploid = 20) embryo / syngamy of two haploid gametes to form a diploid zygote. One haploid gamete (chromosome 10) fuses with two polar nuclei (chromosome 10 + 10) to form (triploid – 30) endosperm nuclei (which divides to form endosperm) / Triple fusion of three haploid nuclei (1 gamete + 2 polar nuclei) to form a triploid endosperm
