Sexual Reproduction in Flowering Plants
Angiosperms bearing unisexual flowers are said to be either monoecious or dioecious. Explain with the help of one example each. (All India 2016)
Plant bearing flowers of both sexes, i.e. staminate and pistillate flowers called monoecious, e.g. lea mays (maize).
When both sexes, i.e. staminate and pistillate flowers, are present on different plants; these plants are called dioecious, e.g. Carica papaya (Papaya).
These pictures show the gynoecium of (A) Papaver and (B) Michelia flowers. Write the difference in the structure of their ovaries. (Delhi 2015C)
The gynoecium of Papaver is multicarpellary and syncarpous (pistils are fused together), the ovary can be unilocular to multilocular. The gynoecium of Michelia is multicarpellary and apocarpus (pistils are free) and the ovary is always unilocular.
Name the parts of the flower which the tassels of corn cob represent. (All India 2014)
The parts of the flower that represent the tassels of corn cob are stigma and style which wave in the wind to trap pollen grains flowing with the wind. Question 4.
Draw a diagram of a mature microspore of an angiosperm. Label its cellular components only. (Foreign 2014)
Draw a labelled diagram of at mature pollen grain. (Delhi 2013C)
The labelled diagram of a mature microspore of an angiosperm with its cellular components is given below
State the function of filiform apparatus found in mature embryo sac of an angiosperm. (Foreign 2014)
The special cellular thickenings present in synergids at the micropylar tip called filiform apparatus, found in mature embryo sac of an angiosperm help in guiding the entry of pollen tubes up to the synergids.
Give an example of a plant which came into India as a contaminant and is a cause of pollen allergy. (All India 2014)
Parthenium or Carrot grass is a major contaminant which came to India and caused pollen allergy.
A bilobed, dithecous anther has 100 microspore mother cells per microsporangium. How many male gametophytes this anther can produce? (Delhi 2010)
An anther is a four-sided (tetragonal) structure consisting of four microsporangia.
Each microsporangium has 100 microspore
mother cells, so total number of microspore
mother cells in anther = 4 × 100 = 400
microspore mother cells. Meiosis in each microspore mother cell produces 4 male gametes, so 400 cells will produce = 4 × 400 = 1600 male gametes.
An anther with malfunctioning tapetum often fails to produce viable male gametophytes. Give one reason. (Delhi 2010)
Write the function of tapetum in anthers. (Delhi 2012)
The anther with malfunctioning tapetum cannot provide complete nutrition to the developing microspores or male gametophytes. So, it fails to produce viable male gametophyte. Question 9.
In the TS of a mature anther given below, identify ‘a’ and ‘b’ and mention their functions. (All India 2019)
In the given figure, a is sporogenous tissue and b is tapetum.
- Sporogenous tissue has cell which are potential Pollen Mother Cell (PMC) or microspore mother cell and give rise to microspore tetrad after meiotic cell division.
- Tapetum nourishes the developing microspores or pollen grains.
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? (All India 2017)
A pollen grain is partly germinated microspore representing the male gametophyte. It divides by unequal mitotic division and forms two cells. Thus, each mature pollen grain in angiosperms have a generative cell and a vegetative cell. In about 60% of angiosperms, pollen grains are shed at this 2-celled stage. However, in about 40% flowering plants, the generative cell may further divide mitotically to give rise to two male gametes and pollen grains are shed at this 3-celled stage.
The placement of cells within the pollen grain when shed at 2-celled stage can be visualised as shown below
A mature embryo sac in a flowering plant may possess 7-cells, but 8-nuclei. Explain with the help of a diagram only. (Delhi 2017)
A typical angiospermic embryo sac is 8-nucleated and 7-celled.
In a flowering plant, a microspore mother cell produces four male gametophytes while a megaspore mother cell forms only one female gametophyte. Explain. (Delhi 2017)
In flowering plants, microspore mother cells are found embedded in the spOrophytic tissue of anther. These cells undergo meiosis and give rise to four microspores that remain together in a microspore tetrad. After attaining maturity, these microspores separate from each other and each microspore develops into a male gametophyte or pollen grain.
On the other hand, megaspore mother cell develops in the ovary of a flower and divides by meiotic division to produce four megaspores. From these, three degenerate while, the one undergoes further development and mitotic divisions to produce female gametophyte. Thus, in a flowering plant, a microspore mother cell produces four male gametophytes while, megaspore mother cell produces one female gametophyte.
Gynoecium of a flower may be apocarpous or syncarpous. Explain with the help of an example each. (Delhi 2016)
Gynoecium of a flower is called as apocarpous when the carpels are free, e.g. apocarpus in Ranunculus. Whereas it is called syncarpous when the carpels are fused, e.g. syncarpous in Petunia.
Draw a diagram of a section of a megasporangium of an angiosperm and label funiculus, micropyle, embryo sac and nucellus. (All India 2016)
Diagrammatic view of a megasporangium and pollen grains are shed at this 3-celled stage. (anatropous ovule)
Differentiate between the two cells enclosed in a mature male gametophyte of an angiosperm. (All India 2013)
Haploid pollen grains represent the male gametophyte. It contains two cells, i.e. vegetative cell and generative cell.
The vegetative or tube cell is larger in size as compared to generative cell and have a vacuolated cytoplasm. The generative cell on the other hand have thin dense cytoplasm with prominent nuclei that give rise to two male gametes, while vegetative cell does not.
Name all the haploid cells present in an unfertilised mature embryo sac of a flowering plant. Write the total number of cells in it. (All India 2013)
How many haploid cells are present in mature female gametophyte of a flowering plant? Name them. (Delhi 2013C)
An unfertilised embryo sac of angiosperm is composed of 7 cells, i.e. 7-celled and 8-nucleated. Among 8-nuclei, 6 are enclosed by cell walls and organised into cells, which are haploid in number (3 antipodals, 2 synergids and 1 egg cell) and a large central cell with 2 polar nuclei. Question 17.
Draw a labelled schematic diagram of the transverse section of a mature anther of an angiosperm plant. (Delhi 2013)
Explain the function of germ pores. (All India 2012 )
Germ pores are prominent apertures of pollen grain exine where sporopollenin is absent. These are the regions where intine comes out forming a pollen tube to release male gamete in the embryo sac. Question 19.
Identify and label the parts in the given anatropous ovule. (All India 2010C)
A – Micropyle B – Outer integument
C – Inner integument D – Embryo sac
Pollen banks are playing a very important role in promoting plant breeding programme the world over.
How are pollens preserved in the pollen banks? Explain. How are such banks benefitting our farmers? Write any two ways. (Delhi 2019)
Pollen grains are stored for years in liquid nitrogen (-196°C) in pollen banks for later use in plant breeding programmes. Plant breeding is a technique of manipulation of plant species in order to create desired plant types that are better suited for cultivation, give better yield and are disease resistant.
The objectives of such pollen banks include incorporation of certain traits or characters into crop plants in order to enhance the food production such as
(i) Increased tolerance to environmental stresses such as salinity, extreme temperature, drought, etc.
(ii) Resistance to pathogens like viruses, fungi and bacteria.
(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? (All India 2016C)
(i) The organic material of exine of pollen grain is sporopollenin. This is most resistant biological material known so far. It protects pollen grains from damages.
(ii) Exine on pbllen grain is not a continuous layer. It is thin at places and pollen tube germinates by growth of intine through these thin parts of exine called germ pores.
(iii) Pollen banks are used to store pollen grains for short as well as long period of time in viable conditions. Question 22.
Draw a labelled diagram of a section of an enlarged view of microsporangium of an angiosperm. (All India 2016C)
Why are angiosperm anthers called dithecous? Describe the structure of its microsporangium. (Delhi 2014)
Describe the structure of a mature microsporangium of an angiosperm. (Delhi 2014)
The structure of a mature microsporangium can be described with the help of given diagram.
Since, the angiosperm’s anthers are bilobed, each lobe bearing two thecae, they are referred to as dithecous. Microsporangium appears circular in outline and is usually surrounded by four wall layers. The outer three layers epidermis, endothecium and middle layers are protective in function. They also help in dispersal of pollens by dehiscing themselves. While, the innermost layer tapetum is nutritive in function and nourishes the developing pollen grdins. The centre of the microsporangium comprises of compact
sporogenous tissue. The cells of this sporogenous tissue undergo meiotic divisions to form microspore tetrads, that further develop to form pollen grains.
Draw a labelled diagram of a typical anatropous ovule. (Delhi 2014)
For structure or labelled diagram of anatropous ovule,
Refer to Answer No. 14. Question 24.
Explain the process of microsporogenesis in angiosperms. (Delhi 2013C)
The formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
Microspores are arranged as tetrad. As the anthers mature and dehydrate they dissociate from each other and develop into mature pollen grains. Pollen grains or the male gametophytes are released by dehiscence of anther.
Draw a diagram of a mature embryo sac of an angiosperm and label the following parts in it. (Delhi 2013)
(i) Filiform apparatus
(iii) Central cells
(iv) Egg cell
(v) Polar nuclei
For mature embryo sac of angiosperm, Refer to figure in Answer No. 11.
Explain the process of megasporogenesis in angiosperms. (Delhi 2013C)
Trace the development of megaspore mother cell up to the formation of a mature embryo sac in a flowering plant. (Delhi 2012)
The process of formation of megaspores from the megaspore mother cell (2n) by the meiosis division in the ovule is called megasporogenesis. Refer to Answer No. 28.
Describe the structure of a 3-celled pollen grain of an angiosperm. (Delhi 2012C)
The 3-celled pollen grain structure in an angiosperm consists of two male gametes and one vegetative cell. The vegetative cell is bigger, has abundant food reserve and a large irregularly-shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus. In over 60% of angiosperms, pollen grains are shed at this 2-celled stage. In most of the angiospermic species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
Describe the process of megasporogenesis up to fully developed embryo sac formation in an angiosperm. (All India 2019)
Where does the process of megasporogenesis start in an angiosperm? Describe the process up to the formation of embryo sac. (Delhi 2019)
In angiosperms, the process of megasporogenesis starts inside the nucellus of the ovule. During megasporogenesis, the Megaspore Mother Cell (MMC) undergoes meiosis resulting in the production of four megaspores. Out of the four megaspores, only one is functional while the other three degenerate.
The functional megaspore undergoes mitosis to form two nuclei, which migrate to opposite poles, forming a 2-nucleate embryo sac.
Further, mitotic divisions lead to the formation of 4-nucleate and 8-nucleate stages of the embryo sac. In these mitotic divisions, nuclear division is not followed by cell division. After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embryo sac is formed.
Among the 8 nuclei, 6 are enclosed by cell walls and organised into cells, while the remaining 2 nuclei (polar nuclei) are situated above the egg apparatus in a large central cell. Out of the six cells, three are grouped at the micropylar end and constitute the egg apparatus. It is made up of two synergids and one egg cell. The other three cells are located at the chalazal end and are called antipodals. Thus, a typical angiosperm embryo sac after maturity is 8-nucleate and 7-celled.
(i) Draw a diagrammatic sketch of a transverse section of an anther of an angiosperm. Label its different walls and the tissue forming microspore mother cells.
(ii) Describe the process of microsporogenesis up to the formation of a microspore.
(iii) Write the function of ‘germ pore’ in a pollen grain of an angiosperm. (2018C)
(ii) For sporogenesis, Refer to Answer No. 24.
(iii) For function of germ pore, Refer to Answer No. 18.
(i) Describe the sequence of the process of microsporogenesis in angiosperms.
(ii) Draw a labelled diagram of a 2-celled final structure formed. (Delhi 2015C)
Trace pollen grain development from sporogenous tissue in the anther. (Delhi 2012)
(i) Development of pollen grain from Pollen Mother Cell (PMC)
- Pollen mother cell or microspore mother cell undergoes meiosis to form microspore tetrad or haploid microspores.
- As the anther matures, the microspores dissociate from the tetrad and develop into pollen grains.
- Nucleus of the microspores undergoes mitosis to form a large vegetative cell and small spindle-shaped generative cell.
- They develop a two-layered wall, the outer exine made of sporopollenin and the inner intine made of cellulose and pectin.
- Usually the pollen grains are liberated at this 2-celled stage.
In certain species, the generative cell divides mitotically to form two male gametes and the pollen grains are 3-celled during liberation.
(i) Draw a labelled diagram of the sectional view of microsporangium of an angiosperm. (Delhi 2015)
(ii) Explain the development of male gametophyte in the microsporangium. (Delhi 2015)
(i) Refer to Answer No. 22 for figure of microsporangium.
(ii) Refer to Answer No. 30 for development of male gametophyte.
(i) Describe the formation of mature female gametophyte within an ovule in angiosperms.
(ii) Describe the structure of cell that guides the pollen tube to enter the embryo sac. (All India 2014)
(i) The functional megaspore undergoes mitosis to form 2 nuclei, which migrate to opposite poles, forming a 2-nucleate embryo sac. Further, mitotic divisions lead to the formation of 4-nucleate and 8-nucleate stages of the embryo sac. In these mitotic divisions, nuclear division is not followed by cell wall formation. After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embryo sac is formed. Among the eight nuclei, six are enclosed by cell wall and organised into cells, while the remaining two nuclei (polar nuclei) are situated above the egg apparatus in a large central cell.
Out of the six cells, three are grouped at the micropylar end and constitute the egg apparatus made up of two synergids and one egg cell. The other three cells are located at the chalazal end and are called antipodals. Thus, a typical angiosperm embryo sac after maturity is 8-nucleate and 7-celled.
(ii) The egg apparatus present towards the micropylar end, comprises of two synergids and an egg cell.
These synergids possess special cellular thickenings at their micropylar tip and called filiform apparatus. This filiform apparatus guides the pollen tube to enter into embryo sac.
For figure. Refer to Answer No. 28 fig (e).
Draw a labelled diagram of sectional view of a mature embryo sac of an angiosperm. (Delhi 2014)
Refer to figure of embryo sac in Answer No. 11.
How does the megaspore mother cell develop into 7-celled and 8-nucleate embryo sac in an angiosperm? Draw a labelled diagram of a mature embryo sac. (Delhi 2012)
Explain with the help of diagram the development of mature embryo sac from a megaspore mother cell in angiosperm. (Foreign 2012, All India 2010C)
Describe the process of megasporogenesis in angiosperm until 8-nucleate stage. (All India 2013C)
For development of megaspore mother cell. Refer to Answer No. 32 (i).
(i) Draw a diagram of an enlarged view of TS of one microsporangium of an angiosperm and label the following parts
(b) Middle layers
(d) Microspore mother cell
(ii) Mention the characteristic features and function of tapetum.
(iii) Explain the following giving reasons
(a) Pollen grains are well-preserved as fossils.
(b) Pollen tablets are in use of people these days. (Foreign 2011)
(i) For diagram. Refer to Answer No. 22.
(ii) Tapetum is the inner nourishing layer of microsporangial wall. The cells of tapetum have dense cytoplasm and more than one nucleus. These cells nourish the developing pollen grains.
(iii) (a) The outer exine layer of pollen grain is highly resistant because of sporopollenin. It is an organic material which can withstand harsh conditions, action of alkalis and acids. No enzyme can degrade sporopollenin. Thus, pollen grains are well-preserved as fossils.
(b) Pollen grains are rich in nutrients. So, used by people as health tablets or food supplements
Write one advantage and one disadvantage of cleistogamy to flowering plants. (2018C)
The advantage of cleistogamy is that it ensures pollination in the absence of pollinators. Disadvantage of cleistogamy is that there is no chance of variation to occur.
What is pollen-pistil interaction and how is it mediated? (Foreign 2014)
Pollen-pistil interaction is a chain or group of • events that take place from the falling of pollen over the stigma to the formation of pollen tube and its entry into the ovule. It is mediated by chemical components of pollen grain, interacting with that of pistil.
Differentiate between xenogamy and geitonogamy. (Delhi 2014C)
Xenogamy is the transfer of pollen grains from anther of one flower to the stigma of another flower of a different plant, while geitonogamy is the transfer of pollen grains from anther of one flower to the stigma of another flower on same plant.
How do the pollen grains of Vallisneria protect themselves? (All India 2012)
Why do the pollen grains of Vallisneria have a mucilaginous covering? (Delhi 2010)
As the pollination of Vallisneria takes place by means of water, the pollen grains are covered by mucilaginous coating that protects them from damage and desiccation.
What is cleistogamy? Write one advantage and one disadvantage of it, to the plant. (All India 2019)
Cleistogamy is a type of self-pollination that occurs in a permanently closed flower. Advantage and disadvantage of cleistogamy are as follows
- Advantage Cleistogamous flowers produce assured seed-set even in the absence of pollinators.
- Disadvantage Cleistogamous flowers are invariably autogamous. So, there is no chance of cross-pollination. Hence, less variations are generated in the progeny.
You are conducting artificial hybridisation on papaya and potato. Which one of them would require the step of emasculation and why ? However for both you will use the process of bagging. Justify giving one reason. (Delhi 2019)
Papaya produces unisexual flowers and potato produces bisexual flowers. Therefore, the step of emasculation will be done on potato because emasculation is done on bisexual flower to avoid self-pollination. But, bagging is done on unisexual flowers, so to dust suitable pollen grains op the stigma when the stigma turns receptive and the flowers are rebagged.
Express the process of pollination in Vallisneria. (Delhi 2019)
Vallisneria is a water pollinated plant. In this plant, the process of pollination involves reaching of female flower at the surface of water by the long stalk and release of pollen grains onto the surface of water. These pollen grains are carried water currents to reach the stigma eventually.
A single pea plant in your kitchen garden produces pods with viable seeds, but the individual papaya plant does not. Explain. (All India 2016)
Out of mafty papaya plants growing in your garden, only a few bear fruits. Give reason.
A single pea plant produces pods with viable seeds because the pea plant is autogamous, i.e they have the ability of self-pollination. Whereas the individual papaya plant is prevented from both autogamy and geitonogamy. In this plant, male and female flowers are present on different plants, i.e. each plant is either male or female.
Explain the process of pollination in Vallisneria. (Delhi 2016)
Pollination in Vallisneria Refer to Answer No. 7.
List the different types of pollination depending upon the source of pollen grain. (Delhi 2016)
Differentiate between autogamy, geitonogamy and xenogamy. (All India 2012)
Depending on the source of pollen grain, pollination can be classified into
- Autogamy It is the transfer of pollen grain from anther to the stigma of the same flower.
- Geitonogamy It is the transfer of pollen grains from anther of one flower to the stigma of another flower on the same plant. Geitonogamy is functionally cross-pollination involving pollinating agent, but genetically it is equivalent to autogamy since the pollen grains come from the same plant.
- Xenogamy It is the transfer of pollen grains from anther to the stigma of different plants of same species. It brings genetically different types of pollen grains to the stigma.
In angiosperms, zygote is diploid, while primary endosperm cell is triploid. Explain. (All India 2013)
Mention the reasons for difference in ploidy of zygote and primary endosperm nucleus in an angiosperm. (Delhi 2010)
In angiosperms or flowering plants, zygote is diploid and primary endosperm nucleus is triploid. It is because in these plants, one of the male gametes fuses with egg cell, which results in the formation of zygote. So, zygote is diploid. While primary endosperm cell is triploid because the nucleus of the second male gamete (n) fuses with the two haploid polar nuclei or diploid secondary nucleus (2n) of the central cell to form a triploid primary endosperm nucleus (3n). This process is referred to as triple fusion. The central cell is now called primary endosperm cell.
Explain triple fusion in angiosperm. (Delhi 2013)
For triple fusion. Refer to Answer No. 11.
State one advantage and one disadvantage of cleistogamy. (All India 2012)
For cleistogamy. Refer to Answer No. 5.
Why should a bisexual flower be emasculated and bagged prior to artificial pollination? (Foreign 2010)
Why does a breeder need to emasculate a bisexual flower? Mention a condition in a flower where emasculation is not necessary. (Delhi 2011)
Emasculation in a bisexual flower is required to prevent contamination of the stigma with self-pollen grains. Bagging is done to prevent contamination of the stigma of the emasculated flower with any other unwanted pollen grains. That is why a bisexual flower should be emasculated and bagged prior to artificial pollination. Emasculation is not required in unisexual flowers.
(i) Draw a LS of pistil showing pollen tube entering into the embryo sac. Label the following. (All India 2019)
(ii) Write the functions of the following
(i) For pistil-pollen diagram, Refer to Answer No. 19 (ii).
(ii) (a) Synergids These possess special cellular thickenings at their micropylar tip called filiform apparatus. This filiform apparatus guides the pollen tube to enter embryo sac.
(b) Micropyle It facilitates the entry of pollen tube and thus fertilisation.
Emasculation and bagging are the two important steps carried during artificial hybridisation to obtain superior varieties of desired plants. Explain giving reasons, in which types of flowers and at what stages are the two processes carried out. (All India 2019)
For emasculation and bagging. Refer to Answer No. 14. If the plant bears bisexual flowers, emasculation and bagging are carried out before the anther dehisces.
If the plant bears unisexual flowers, emasculation is not required. The female flower buds are bagged before the opening of flowers are
|It is the transfer of pollen grains from the anther to the stigma of another flower of same plant.
|It is the transfer of pollen grains from the anther to the stigma of different plants.
|The pollen grains are genetically similar to the plant.
|The pollen grains are genetically different from the plant.
Geitonogamy result sun progenies which are animals and even humans. It is believed that pollination by wind (anemophily) is more coihmon because transport by wind-current can take pollens to distant places.
(i) Differentiate between geitonogamy and xenogamy.
(ii) Write the difference in the characteristics of the progeny produced as a result of the two processes. (Delhi 2019)
(i) Differences between geitonogamy and xenogamy purelines, e.g. homozygous. They are genetically similar. Xenogamy results in hybrids, e.g. heterozygous. They show variations in characters.
How does a bisexual flowering plant ensure cross-pollination ? Explain. (Delhi 2019)
Bisexual flowers ensuring cross-pollination Refer to Answer No. 22.
(i) Can a plant flowering in Mumbai be pollinated by pollen grains of the same species growing in New Delhi? Provide explanation to your answer,
(ii) Draw the diagram of a pistil where pollination has successfully occurred. Label the parts involved in reaching the male gametes to its desired destination. (All India 2017)
(i) Yes, a plant flowering in Mumbai can be pollinated by pollen grains of the same species growing in New Delhi. It is mainly because there are certain agents of pollination that can carry pollen grains to long distance. Plants can use either abiotic or biotic agents for pollination. Abiotic pollinators include wind and water while biotic pollinators are insects, birds, (ii) The parts involved in transferring the male gametes to its desired destination are stigma, style, micropyle, filiform apparatus and synergids. Longitudinal Section (LS) of a post-pollinated pistil is given below
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. (Delhi 2016)
When the pollen grains fall on the stigma, the pollen tube enters one of the synergids and releases two male gametes.
- One of the male gametes moves towards the egg cell and fuses with it to complete syngamy to form the zygote.
- The other male gamete fuses with the two polar nuclei and forms triploid Primary Endosperm Nucleus (PEN). This is termed as triple fusion.
- Since, two kinds of fusion syngamy and triple fusion takes place, the process is known as double fertilisation and is characteristics of flowering plants.
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. (All India 2016)
Cross-pollination is done to mix two desired characters of two different species of a plant. For example, purple and white flower of a pea.
- Select two pea plants one with white and other with purple flower.
- Label them as male (white flowered) and female (purple flowered) plant.
- Cut anthers from purple flower with the help of scissors before their dehiscence to avoid self-pollination and cover it with white paper bag.
- Now collect pollens from the white flower (male plant) with the help of brush.
- Dust the pollens on the stigma of female (purple fewer) flower.
- Cover it again with paper bag till seed formation.
Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. Explain three such devices. Delhi 2016C
Why do hermaphrodite angiosperms develop outbreeding devices? Explain any two such devices with the help of examples. (All India 2015)
Make a list of any three outbreeding devices that flowering plants have developed and explain how they help to encourage cross-pollination. (All India 2014)
Hermaphrodites or bisexual flowers develop outbreeding devices to ensure cross-pollination and avoid self-pollination. The three outbreeding devices that flowering plants have developed to discourage self-pollination are
(i) Unisexuality (Dicliny) 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 for preventing self-pollination.
(a) Protandry Anthers mature earlier than stigma of the same flower. The pollens thus btfcome 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,
(iii) The third device to prevent self-pollination is self-incompatibility. It is a genetic mechanism that prevents self-pollen from fertilising the ovules by preventing pollen germination or pollen tube growth in the pistil. All these methods encourage cross-pollination thus causing genetic variations among them.
Explain the phenomenon of double fertilisation. (Delhi 2014)
The phenomenon of double fertilisation occurs in following steps
- In an angiospermic plant, two male gametes are discharged by a pollen tube into the cytoplasm of a synergid of the embryo sac.
- One of the male gametes fuses with the egg to form a zygote. This process is called syngamy.
- Other male gamete fuses with the secondary nucleus to form the primary endosperm nucleus, this process is called triple fusion.
- Since, there are two fusions (syngamy and triple fusion) inside an ovule during fertilisation, it is known as double fertilisation.
Write the differences between wind pollinated and insect pollinated flowers. Give an example of each type. (Foreign 2014)
The differences between wind pollinated and insect pollinated flowers are
|Wind pollinated flowers
|Insect pollinated flowers
|These are small.
|They are either large or grouped to form large clusters.
|Usually inconspicuous due to dull colours.
|The presence of bright colours in corolla, calyx or bracts to attract insects.
|They are odourless and devoid of nectar.
|Strongly odoured and usually possess nectar or edible pollen.
|Pollens are produced in large numbers.
|Fewer pollen grains are produced.
|e.g. Urtica, Maize, Parthenium.
|e.g. Rose, Snapdragon, Calotropis.
Differentiate between geitonogamy and xenogamy in plants. Which one between the two will led to inbreeding depression and why? (Delhi 2011)
For difference. Refer to Answer No. 17 (i).
Geitonogamy will lead to inbreeding depression because the pollen grains are genetically similar resulting into inbreeding. Continuous inbreeding reduces fertility.
(i) Write the characteristic features of anther, pollen and stigma of wind pollinated flowers.
(ii) How do flowers reward their insect pollinator? Explain. (All India 2010)
(i) In wind pollinated flowers,
- Anthers are well-exposed for easy dispersal of pollen grains.
- Pollen grains are light and non-sticky, so that they can be transported by wind currents.
- Stigma is large and feathery to trap pollens.
(ii) Flower rewards their insect pollinators easily by offering
- Nectar and edible pollen grains.
- Safe place for insects to lay eggs by some flowers, e.g. Amorphophallus and Yucca.
(i) Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.
(ii) Explain the events up to double fertilisation after the pollen tube enters one of the synergids in an ovule of an angiosperm. (Delhi 2011)
(i) The two devices that prevent both autogamy and geitonogamy in flowering plants are as follows
(a) Self-incompatibility In some plants when pollen from same flower or other flower of the same plant comes on the stigma, it is incapable of bringing about fertilisation.
It is due to the presence of similar self-sterile gene, e.g. tobacco, potato, etc. It prevents autogamy and geitonogamy.
(b) Dioecy In several species such as papaya, male and female flowers are present on different plants. Thus, each plant is either male or female. This condition also prevents both autogamy and geitonogamy.
(ii) In the ovule, the pollen tube is attracted by secretions of synergids. Usually the pollen tube enters the embryo sac by passing into one of the two synergids which starts degenerating. The pollen tube bursts up by absorbing hydrolytic substances secreted by degenerating synergids. It is followed by double fertilisation in flowering plants.
Double fertilisation It is the fusion of two male gametes to two different cells of the same female gametophyte in order to produce two different structures.
Nawaschin (1898) was the first to show that both the male gametes are involved in fertilisation in Fritillaria and Lilium. Double fertilisation consists of two events that are as follows
- Syngamy or Amphimixis Fusion of the egg nucleus with one male gamete is called syngamy. This fusion results in the formation of diploid cell, the zygote.
- Triple fusion Along with syngamy, the other’male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid Primary Endosperm Mother (PEM) cell. In this way, fertilisation occurs in flowering plants.
Draw the longitudinal section of a flower showing growth of pollen tube up to the embryo sac. Label the following parts
(ii) Pollen tube
(iv) Chalazal end
For LS of flower. Refer to Answer No. 19 (ii).
(i) Plan an experiment and prepare a flow chart of the steps that you would follow to ensure that the seeds are formed only from the desired sets of pollen grains. Name the type of experiment that you carried out.
(ii) Write the importance of such experiments. (All India 2015)
(i) Artificial hybridisation is carried out to ensure that seeds are formed from the desired set of pollen grains. This is done by emasculation and bagging.
The flow chart below shows the steps to be followed
(ii) Importance of such experiments are (a) Creation of new genetic recombination with better qualities. (b) Incorporation of a large number of desirable characters into a single variety.
Question 65. Angiospermic 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. (All India 2014)
Answer: The characteristic features of angiospermic flowers (i) Monoecious flowers are unisexual, i.e. they have either the male reproductive or female reproductive part in separate flowers, both produced on same plant. The flowers (male and female) are separate. It prevents self¬pollination and promotes cross-pollination.
(ii) Cleistogamous flowers are those flowers in which anthers and stigmas lie close to each other and do not open at all, even at maturity. These flowers are invariably autogamous and promote inbreeding depression as there is no chance for cross-pollination at all.
(iii) Self-incompatible In angiospermic flowers, there is a genetic mechanism, wherein the flowers prevent the self-pollens from fertilising the ovules or inhibit their germination on stigma. This device or mechanism promotes outbreeding.
Question 66. (i) Draw a longitudinal section of a pistil of an angiosperm showing the growth of pollen tube up to the micropyle of ovule. Label (a) stigma, (b) embryo sac (c) pollen tube (d) micropyle.
(ii) Explain the events that occur, up to fertilisation, when the compatible pollen grain lands on the stigma. (Delhi 2014)
Answer: (i) For LS of flower. Refer to Answer No. 19 (ii). (ii) The events that occur when compatible pollen grains fall on stigma in the sequence are as follows
(a) Pollen-pistil interaction Once the compatible pollen grains fall on stigma which is receptive, it recognises and accepts the pollen with the aid of chemical components interacting with pollen.
(b) Germination of pollen grain Once the pollen is recognised, it germinates on the stigma of flower. The tube cell of pollen grain protrudes out through germ pores to form a pollen tube. The generative cell divides to form two male gametes that are released into the tube.
(c) Growth of pollen tube The pollen tube grows down through the tissues of stigma and style and enters ovule, usually through micropyle. Inside ovule, the filiform apparatus guides the pollen tube, carrying gametes to the egg cell.(d) Double fertilisation After releasing the two male gametes into the synergids, one of them fuses with egg to form a diploid zygote (syngamy) and other male gamete fuses with 2 polar nuclei to form triploid primary endosperm cell (triple fusion). Because of the occurrence of these two types of fusions, it is called double fertilisation.
Question 67. Why is fertilisation in an angiosperm referred to as double fertilisation? Mention the ploidy of the cells involved. (All India 2012) Answer: In fertilisation (in angiosperm), two types of fusion occur, i.e. syngamy and triple fusion, in the embryo sac. That is why it is called double fertilisation. Ploidy of cells involved in double fertilisation: Zygote is diploid (2n). It is formed as a result of syngamy, i.e. fusion of two haploid gametes (male gamete + egg). Primary endosperm nucleus (3M) is formed as a result of triple fusion, i.,e. fusion of two haploid polar nuclei with male gamete.