Very Short Answer Type Questions
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. 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. 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. 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. A bilobed anther has 100 microspore mother cells per microsporangium. How many male gametophytes can this anther produce.
Answer : 1600.
Detailed Answer:
Each bilobed, dithecous anther has 4 microsporangia.
Number of pollen mother cell (PMC) in one microsporangia is 100.
Therefore total number of PMC in 4 microsporangia would be 400.
Each PMC produces 4 male gametophyte (pollen grain), thus total number would be 4 × 400 = 1600 male gametophytes.
Short Answer Type Questions
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.
Detailed Answer :
Gynoecium represents the female reproductive part of the flower. If a gynoecium has multiple, distinct (free, unfused) carpels, it is apocarpous. The example includes strawberry, buttercup etc. If a gynoecium has multiple carpels fused into a single structure, it is syncarpous. The example includes Tulip and most flowers.
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 :
| S. No. | Geitonogamy | Xenogamy |
| (i) | Transfer of pollen grains from the anther to stigma of another flower of the same plant. | Transfer of pollen grains from anther to the stigma of a different plants. |
| (ii) | It does not cause genetic variation. | It causes genetic variations. |
Geitonogamy will lead to inbreeding depression because genetically it is similar to autogamy.
Question. State one advantage and one disadvantage of cleistogamy.
Answer : Advantage – Assured seed set / maintain purelines
Disadvantage – No variation / only parental characters are preserved / it can lead to inbreeding depression
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. In angiosperms the zygote is diploid while primary endosperm nucleus is triploid. Explain.
Answer : In angiosperms, the zygote is diploid (2n). It is formed by the fusion of haploid (n) egg and a haploid male gamete. This diploid zygote gives rise to diploid embryo.
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.
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. 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. (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. 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. (Any three)
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. 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) 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.
Detailed Answer :
(i) Post-pollination changes leading to seed production are :
(a) Germination of pollen tube which will ultimately transfer two male gametes to the embryo sac.
(b) Double fertilization : In this, one male gamete will fuse with the egg forming zygote (syngamy).
Male gamete (n) + Egg (n) → Zygote (2n) Other male gamete will fuse with central cell (2 polar nuclei) forming triploid Primary Endosperm Nucleus (PEN).
Male gamete (n) + Central cell (2n) → PEN (3 n) (Triple fusion)
(c) Zygote will develop into an embryo while PEN gives rise to endosperm.
(d) Integuments get hard and form the seed coat.
(e) Ovule is thus converted into a seed.
(f) Seeds are the fertilised ovules that are developed inside a fruit.
(ii) Depending on the source of pollen grains, pollination are of following types :
(a) Autogamy : Where pollen of a flower reaches the stigma of the same flower.
(b) Geitonogamy : Pollen grain of one flower reaches the stigma of another flower of same plant.
(c) Cross pollination or Xenogamy : When pollen grain of a flower from one plant pollinates the stigma of a flower on another plant.
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 (Any three)
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.
Detailed Answer :
(i) When the pollen grain falls on the stigma, they germinate and give rise to the pollen tube that passes through the style and enter into the ovule.
(ii) After this, the pollen tube enters one of the synergids and releases two male gametes.
(iii) Out of the two male gametes, one gamete fuses with the nucleus of the egg cell and forms the zygote. The process is known as syngamy.
(iv) The other male gamete fuses with the two polar nuclei located in the central cell to form a triploid primary endosperm nucleus (PEN).
Since, the process involves the fusion of three haploid nuclei, it is known as triple fusion.
(v) Thus, triple fusion is the fusion of male gamete with two polar nuclei inside the embryo sac of the angiosperm.
(vi) Since two types of fusions (syngamy and triple fusion) take place in an embryo sac it is known as double fertilisation.
(vii) Double fertilisation is unique to flowering plants.
(viii) After triple fusion, the central cell becomes the primary endosperm cell (PEC).
(ix) Primary endosperm nucleus develops into the endosperm while the zygote develops into an embryo.
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 : (i)
(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.
Detailed Answer :
(ii) Here, two gametic fusions or fertilization in the female gametophyte or embryo sac of angiosperms takes place, i.e. one male gamete fuses with the egg to form zygote and the other male gamete fuses with the secondary nucleus to form the primary endosperm nucleus. The former fertilization is called as syngamy and the latter as the triple fusion. Thus, fertilization occurs twice in the same embryo sac, therefore, this is called as the double fertilization. This phenomenon is unique in angiosperms and is not found in any other group of plant kingdom.
Question. (i) Draw a diagram of a fertilized embryo sac of a dicot flower. Label all its cellular components.
(ii) Explain the development of a mature embryo from this embryo sac.
Answer : (i)
(ii) Zygote starts mitotic division and gives rise to pro-embryo → globular and heart shaped → mature embryo with radical – plumule and two cotyledons, primary endosperm nucleus divides and forms endosperm, which may persist or used up in nourishing the embryo.
Detailed Answer :
(ii) The pollen tube releases two male gametes.
One fuses with the egg cell and forms a diploid zygote. Other male gamete fuses with secondary nucleus in the central cell to form a triploid primary endosperm nucleus (PEN).
The primary endosperm nucleus develops into endosperm. The embryo formation starts after a certain amount of endosperm is formed to nourish it. The zygote divides mitotically to form a proembryo, which later results in the formation of globular, heart shaped embryo with one or two cotyledons. The mature embryo consists of two cotyledons and the embryonal axis between them.
Question. (i) Why is fertilization in an angiosperm referred to as double fertilization ? Mention the ploidy of the cells involved.
(ii) Draw a neat labelled sketch of L.S. of an endospermous monocot seed.
Answer : (i) Fertilization of haploid egg cell by one haploid male gamete, to form diploid zygote, is called syngamy.
Fertilization of two polar nuclei by the other haploid male gamete to from triploid primary endosperm nucleus, This is called triple fusion.
The fertilization thus occurs twice in the embryo sac. Therefore it is called as double fertilization.
Endosperm cell – triploid
Fertilized egg cell – diploid Synergids – haploid Antipodals – haploid
(ii)
Question. (i) Describe the endosperm development in coconut.
(ii) Why is tender coconut considered a healthy source of nutrition ?
(iii) How are pea seeds different from castor seeds with respect to endosperm ?
Answer : (i) Coconut endosperm development is free nuclear type :
The primary endosperm nucleus divides by free nuclear division. The nucleus divides repeatedly into at last 128 nuclei. Then by the process of cytokinesis it partly changes in to cellular endosperm.
(ii) The water of tender coconut, technically the liquid endosperm is rich in nutrients like fat, proteins, carbohydrates, minerals, vitamins as endosperm provides nutrition to developing embryo.
(iii) Pea seed is non-endospermic as endosperm is absent because endosperm is consumed completely during embryo development, while castor seed is endospermic as endosperm is present.
Question. (i) With labelled diagrams, depict stages in embryo development in a dicotyledenous plant.
(ii) Endosperm development precedes embryo development. Why ?
Answer :
(i)
(ii) Endosperm is filled with reserve food materials which are used for as nutrition of the developing embryo.
Question. (i) When a seed of an orange is squeezed, many embryos, instead of one are observed. Explain how it is possible.
(ii) Are these embryos genetically similar or different?
Comment.
Answer : (i) Polyembryony, nucellar cells surrounding embryo sac start dividing, protrude into the embryo sac and develop into many embryos.
(ii) These embryos are genetically similar, as produced from nucellar cells by mitotic division / formed without fertilisation (but different from the embryo formed by fertilization)
Question. A flower of brinjal has 520 ovules in its ovary.
However, it produces a fruit with only 480 viable seeds.
(i) What could have prevented the rest of the 40 ovules from maturing into viable seeds ? Explain giving a reason.
(ii) Describe the development of a dicot embryo in a viable seed.
(iii) Why certain angiospermic seeds are albuminous while others are ex-albuminous ? Explain.
Answer : (i) Less number of pollen grains / less number of male gametes were available / all pollen grains] did not germinate / all pollen grains did not form pollen tubes / many pollen were noncompatible / 40 ovules did not get fertilised / only 480 ovules were fertilised
(ii) Zygote divides (mitotically) to give rise to pro embryo, globular, heart shaped, mature embryo
(iii) Albuminous : Endosperm is not completely used up during embryo development / residual endosperm found in the seed Ex-albuminous : Endosperm is completely consumed / no residual endosperm is left in seed
Question. In the given figure of embryo sac label the parts
(1), (2), (3) and (4).
Answer : In the given figure, (1) – Antipodal cells,
(2) – Polar nuclei, (3) – Central cell, (4) – Egg cell.
Question. Identify and label the parts in the given anatropous ovule
Answer : In the given fogure, A – Micropyle, B – Outer integument, C – Nucellus, D – Antipodal cell.
Question. In the T.S. of a mature anther given below identify ‘a’ and ‘b’ and mention their functions.
Answer : In the given figure, ‘a’ represents sporogenous tissue and ‘b’ represents tapetum. Sporogenous tissue fills the whole interior of the microsporangium. Its cells divide with the growth of anther and increase their number. Ultimately they are transformed into microspore or pollen mother cells (PMC). Microspore mother cells undergo meiosis to produce
haploid microspores or pollen grains.
Question. Draw the longitudinal section of pistil showing pollen tube.
Answer : Longitudinal section of pistil showing growth of pollen tube is shown below.
Question. (a) Draw a schematic diagram of T.S. of a mature anther. Label only the layers that help in dehiscence of the anther to release pollen grains.
(b) Why is exine of the pollen grain not a continuous layer?
Answer : (a) T.S. of mature dehisced anther is as follows :
(b) Exine of pollen grain is not a continuous layer.It bears prominent apertures called germ pores.
These are the places from where intine comes out as pollen tube, which carries male gametes required for fertilisation in angiosperms. If the exine is present as a continuous layer, it would render the pollen grain non-functional.
Question. The flower of brinjal is referred to as chasmogamous while that of beans is cleistogamous.
How are they different from each other?
Answer : Differences between flowers of brinjal and beans are as follows :
Question. Draw the structure of anatropous ovule.
Answer : Sectional view of anatropous ovule is given below.
Question. What is geitonogamy? Give its one similarity to
(a) autogamy and
(b) xenogamy.
Answer : Geitonogamy is a type of pollination in which pollen grains of one flower are transferred to the stigma of another flower belonging to the same plant.
(a) Geitonogamy is genetically similar to autogamy since the pollen grains come from the same plant.
(b) Geitonogamy is functionally similar to xenogamy as it requires pollinating agent.
Question. (a) Name the organic material exine of the pollen grain is made up of. How is this material advantageous to pollen grain?
(b) Still it is observed that it does not form a continuous layer around the pollen grain.Give reason.
(c) How are ‘pollen banks’ useful?
Answer : (a) Exine is made up of a highly resistant fatty substance called sporopollenin. Sporopollenin is highly protective layer not degraded by any enzyme.
It is not afected by high temperature, strong acid or strong alkali. Because of sporopollenin, exine provides protection during the hazardous journey of pollen from anther to stigma. Also, pollen grains are well preserved as microfossils and protected from external adversities due to the presence of sporopollenin.
Question. Make a list of any three outbreeding devices that flowering plants have developed and explain how they help to encourage cross-pollination.
OR
Why are angiosperm anthers called dithecous? Describe the structure of its microsporangium.
Answer : 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 plants may 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., 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.
(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.
OR
Angiosperm anthers consist of two anther lobes therefore are called dithecous.
A typical angiospermic microsporangium appears near circular in outline. It consists of two parts, outer wall and central homogenous tissue. Microsporangial wall has four types of layers:
– Epidermis of single layer for protection.
– Endothecium of single layer for dehiscence of anther.
– Middle layers are 1-4 in number.
– Tapetum, the innermost parietal layer with cells having dense cytoplasm and large nuclei.
Question. Write the differences between wind-pollinated and insect-pollinated flowers. Give an example of each type.
Answer : Differences between anemophilous (wind pollinated) and entomophilous (insect pollinated) flowers are as follows:
Question. Describe the structure of a mature microsporangium of an angiosperm.
Answer : 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.
Question. Explain the process of megasporogenesis in angiosperms.
Answer : 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 :
Question. Write the mode of pollination in Vallisneria and water lily. Explain the mechanism of pollination in Vallisneria.
Answer : The mode of pollination in Vallisneria is water whereas mode of pollination in water lily is wind or insects.
Question. Describe the process of megasporogenesis in angiosperms until 8 nucleate stage.
Answer : 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 oosphere. The egg cell on fusing with one male gamete (fertilisation) gives rise to zygote.
Question. Describe the structure of a 3-celled pollen grain of an angiosperm.
Answer : 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.
Question. Explain the processes of emasculation and bagging of flowers. State their importance in breeding experiments.
Answer : Emasculation is the removal of the anthers of a bisexual flower in order to prevent self-pollination.
Bagging involves covering of emasculated flowers by butter paper or polythene in order to protect them from contamination by foreign or undesirable pollen.
Emasculation and bagging are important steps of artificial hybridisation technique used in crop improvement programmes.
Question. Draw a diagram of a male gametophyte of an angiosperm. Label any four parts. Why is sporopollenin considered the most resistant organic material?
Answer : Labelled diagram of mature pollen grain is as follows:
Sporopollenin is considered the most resistant organic material because it is not degraded by any enzyme, not afected by high temperature, strong acid or strong alkali.
Question. (a) Mention the exact location or the site in a flowering plant where the following developments take place.
(i) Deposition of sporopollenin
(ii) Megasporogenesis
(b) Draw a labelled diagram of a male gametophyte of an angiosperm.
Answer : (a) (i) Deposition of sporopollenin occurs in exine layer of pollen grains.
(ii) Megasporogenesis occurs inside the nucellus of developing ovule of angiosperms.
Question. Explain the characteristic features of flowers that facilitate wind-pollination.
Answer : The characteristic features of flowers that facilitate wind pollination are as follows:
(i) Both the stigmas and anthers are exserted.
(ii) Pollen grains are light, small and winged or dusty. They can be blown by wind to distances of upto 1300 km.
(iii) Pollen grains are dry, smooth, non-sticky and unwettable.
(iv) Stigma is hairy, feathery or branched to catch the wind-borne pollen grains. The large thread-like stigmas and style of maize cob hang in air to catch wind borne pollens.
(v) Pollen grains are produced in very large number. For example, a single flower of Cannabis produces 5,00,000 pollen grains. Consequently, the pollen grains spread over large tracts so that even isolated plants get pollinated.
(vi) Calyx and corolla are either reduced or absent.Anthers are usually versatile.
(vii) Flowers are small and inconspicuous.
(viii) When flowers are unisexual, male flowers are more abundant than female flowers. In bisexual flowers, the stamens are generally numerous.
Question. 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.
Answer : Longitudinal section of post pollinated pistil showing entry of pollen tube into a mature embryo sac is shown as follows
Question. Given below is an enlarged view of one microsporangium of a mature anther
(a) Name ‘A’, ‘B’ and ‘C’ wall layers.
(b) Mention the characteristics and function of the cells forming wall layer ‘C’.
OR
State the significance of pollination. List any four differences between wind-pollinated and animal-pollinated flowers.
Answer : (a) A – Endothecium
B – Middle layers
C – Tapetum
(b) The characteristics of labelled part C 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 differentiation.
OR
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 :
Question. Describe the Polygonum type of embryo sac.Why is it generally referred to as monosporic?
Answer : The most common type of embryo sac, found in 80% flowering 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.
Question. (a) Draw a labelled diagram of the sectional view of microsporangium of an angiosperm.
(b) Explain the development of male gametophyte in the microsporangium.
Answer : (a) Sectional view of microsporangium of an angiosperm is as follows:
(b) 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 microsporangium or 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.
Question. Angiosperm flowers may be monoecious, cleistogamous or show self-incompatibility.
Describe the characteristic features of each one of them and state which one of these flowers promotes inbreeding and outbreeding respectively.
Answer : Monoecious flowers are bisexual i.e., they bear both male and female reproductive organs, e.g., pea.Cleistogamous flowers are those flowers which do not open at all. These flowers are intersexual and remain closed causing self pollination. In cleistogamous flowers, the anthers dehisce inside closed flowers. Growth of style brings the pollen grains in contact with stigma. Pollination and seed setting are assured. Pollinators are not required. E.g.Commelina benghalensis, balsam.
Self incompatibility is inability of pollen of a plant to fertilise the pistil of the same plant, e.g., Primula. Monoecious and cleistogamous flowers promote inbreeding whereas self incompatibility in plants promotes outbreeding.
Question. (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.
Label (i) st igma, (ii) embr yo sac,
(iii) pollen tube (iv) micropyle.
(b) Explain the events that occur, upto fertilisation, when the compatible pollen grain lands on the stigma.
Answer : Longitudinal section of pistil showing growth of pollen tube is shown below.
(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).
Question. (a) Draw a labelled schematic diagram of the transverse section of a mature anther of an angiosperm plant.
(b) Describe the characteristic features of an insect pollinated flower.
Answer : (a) T.S. of mature dehisced anther is as follows :
(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.
Question. (a) Draw a labelled diagram of a transverse section of a mature anther of an angiosperm showing different wall layers.
(b) Mention the function of each of these wall layers during pollen grain development
Answer : (a) T.S. of mature dehisced anther is as follows :
(b) The different wall layers of an anther and their functions are as follows:
(i) Epidermis – One cell thick and protective in function.
(ii) Endothecium – Second wall layer. Usually single layered. Cells have a cellulose thickening with a little pectin and lignin in some cases. It helps in anther dehiscence.
(iii) Middle layers – The number of middle layers ranges from 2-3. The middle layers degenerate at the time of maturity of the anther to provide nourishment to the growing microspore mother cells.
(iv) Tapetum – This is the innermost layer of anther wall which surrounds the sporogenous tissue.
Tapetum has a number of functions :
– Nourishment of the developing microspore
mother cells and pollen grains.
– It produces lipid rich ubisch granules containing sporopollenin for exine formation, pollenkitt special proteins for the pollen grains which recognise compatibility and hormone IAA.
– It secretes enzyme callase responsible for the degradation of callose wall around pollen tetrad.