NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants
These Solutions are part of NCERT Solutions for Class 12 Biology. Here we have given NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants
Name the parts of an angiosperm flower in which development of male and female gametophyte take place.
Development of male gametophyte takes place from microspore or pollen grains which develop inside the microsporangium or pollen sac of an anther.
Development of female gametophyte takes place in the nucellus of ovule.
Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.
Differences between microsporogenesis and megasporogenesis are as follows :
During microsporogenesis and megas-megas¬porogenesis meiotic cell division occurs which results in haploid gametes – the microspores or pollen grains and megaspores.
Arrange the following terms in- the correct developmental sequence : Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.
Sporogenous tissue – pollen mother cell – microspore tetrad – pollen grains – male gametes.
With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
An angiospermic ovule consists of the following parts :
- The ovule is attached to placenta by means of a stalk called funicle or funiculus.
- The point of attachment of funiculus to the body of ovule is called hilum.
- The main body of ovule is made of parenchymatous tissue called nucellus.
- Nucellus is covered on its outside by one or two coverings called integuments and hence ovule is rightly called as integumented megasporangium.
- The integuments cover entire nucellus except a small pore at upper end, which is called micropyle. Micropyle is formed generally by inner integument or by both integuments.
- The place of junction of integuments and nucellus is called chalaza.
- In inverted ovules (most common type), the stalk or funiculus is attached to the main body of ovule for some distance to form a ridge like structure, called- raphe.
- In the nucellus of ovule, a large oval cell is present at micropylar end, which is known as embryo sac (female gametophyte), which develops from megaspore.
What is meant by monosporic development of female gametophyte?
In many flowering plants, only one out of the four megaspores enlarges and develops into female gametophyte or embryo sac. The other three megaspores degenerate. This type of embryo sac formation is called as monosporic type of development.
With a neat diagram explain the 7-celled, 8 nucleate nature of the female gametophyte
Embryo sac is an oval multicellular structure. It is covered by a thin membrane derived from the parent megaspore wall. The typical or Polygonum type of embryo sac contains 8 nuclei but 7 cells – 3 micropylar, 3 chalazal and one central. The three micropylar cells are collectively known as egg apparatus. One cell is larger and is called egg or oosphere. It bears a central or micropylar vacuole and a nucleus towards the chalazal end. The remaining two cells are called synergids or help cells. Each of them bears a filiform apparatus in the micropylar region, a lateral hook, chalazal vacuole and a central nucleus. The egg or oosphere represents the single female gamete of the embryo sac. The synergids help in obtaining nourishment from the outer nucellar cells, guide the path of pollen tube by their secretion and function as shock absorbers during the penetration of pollen tube into the embryo sac.
The three chalazal cells of the embryo sac are called antipodal cells. They are the vegetative cells of the embryo sac which may degenerate soon or take part in absorbing nourishment from the surrounding nucellar cells. Internally, they are connected with central cell by means of plasmodesmata.
The central cell is the largest cell of the embryo sac. It has a highly vacuolated cytoplasm which is rich in reserve food and Golgi bodies. In the middle, the cell contains two polar nuclei which often fuse to form a single diploid secondary nucleus. Thus, all the cells of the embryo sac are haploid except the central cell which becomes diploid due to fusion of polar nuclei.
What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.
Chasmogamous flowers or open flowers in which anther and stigma are exposed for pollination. Cross-pollination cannot occur in cleistogamous flowers. These flowers remain closed thus causing only self-pollination. In cleistogamous flowers, anthers dehisce inside the closed flowers. So the pollen grains come in contact with stigma. Thus there is no chance of cross¬pollination, e.g., Oxalis, Viola.
Mention two strategies evolved to prevent self pollination in flowers.
Continued self pollination results in inbreeding depression. So flowering plants have developed following devices to discourage self pollination and to encourage cross pollination:
- 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).
- Dichogamy: Anthers and stigmas mature at different times in a bisexual flower so as to prevent self pollination.
- 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.
- Protogyny – Stigmas mature earlier so that they get pollinated before the anthers of the same flower develop pollen grains, e.g., Mirabilis jalapa (four o’ clock), Gloriosa, Plantago.
What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
When the pollen grains of an anther do not germinate on the stigma of the same flower, then such a flower is called self-sterile or incompatible and such condition is known as self¬incompatibility or self-sterility.
The transference of pollen grains shed from the anther to the stigma of the pistil is called pollination. This transference initiate the process of seed formation. Self-pollination is the transfer of pollen grain shed from the anther to stigma of pistil in the same flower. But in some flower self¬pollination does not lead to the formation of seed formation because of the presence of same sterile gene on pistil and pollen grain.
What is bagging technique? How is it useful in a plant breeding programme?
If the female parent bears bisexual flowers, removal of anthers from the flower bud before the anther dehisces using a pair of forceps is necessary. This step is referred to as emasculation. 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. When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma, and the flowers are rebagged, and the fruits allowed to develop.
This process allows plant breeders to use desired varieties of pollen to obtain desired seeds.
What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
Inside the embryo sac, one male gamete fuses with egg cell to form zygote (2n) and this is called syngamy or true act of fertilisation. This result of syngamy, i.e., zygote (2n) ultimately develops into embryo.
The second male gamete fuses with 2 polar nuclei or secondary nucleus to form triploid primary endosperm nucleus and this is called triple fusion. The result of triple fusion, i.e., primary endosperm nucleus (3n) ultimately develops into a nutritive tissue for developing embryo called endosperm.
The nuclei involved in this triple fusion are the two polar nuclei or secondary nucleus and the second male gamete.
Why do you think the zygote is dormant for sometime in a fertilised ovule?
Fertilised egg is known as zygote which gives rise to embryo. Before development, the zygote undergoes a resting period. This is because the zygote waits for the formation of certain amount of endosperm for the nourishment of embryo. This is an adaptation to provide assured nutrition to the developing embryo.
- Epicotyl and hypocotyl;
- Coleoptile and coleorhiza;
- Integument and testa;
- Perisperm and pericarp
- Differences between epicotyl and hypocotyl are as follows :
- Differences between coleoptile and coleorhiza are as follows :
- Differences between integument and testa are as follows :
- Differences between perisperm and pericarp are as follows :
Why is apple called a false fruit? Which part (s) of the flower forms the fruit?
Most fruits develop only from ovary and are called true fruits. When fruit develops from other floral parts other than ovary it is called false fruit. Apple is a false fruit where thalamus contributes to the fruit formation.
What is meant by emasculation? When and why does a plant breeder employ this technique?
Emasculation is the removal of stamens mainly the anthers from the flower buds before their dehiscence. This is mainly done to avoid self-pollination. Emasculation is one of the measures in the artificial hybridization. Plant breeders employed this technique to prevent the pollination within same flower or to pollinate stigmas with pollens of desired variety.
lf one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why ?
Parthenocarpic fruits are fruits which develop without fertilisation and hence are seedless. Parthenocarpy can be induced through the application of growth hormones. Important fruits like banana, papaya, orange, grapes, guava, watermelon etc. can be made seedless by applying growth substances as they are economically important fruits and if made seedless they will be more valuable.
Explain the role of tapetum in the formation of pollen-grain wall.
Tapetum is the innermost layer of the microsporangium. The tapetal cells are multinucleated and polyploid. They nourish the developing pollen grains. These cells contain ubisch bodies that help in the ornamentation of the microspores or pollen grains walls. The outer layer of the pollen grain is called exine and is made up of the sporopollenin secreted by the ubisch bodies of the tapetal cells. This compound provides spiny appearance to the exine of the pollen grains.
What is apomixis and what is its importance ?
Normal type of sexual reproduction having two regular features i.e., meiosis and fertilisation is called amphimixis. But in some plants this normal sexual reproduction is replaced by some abnormal type of sexual reproduction called apomixis.
The term apomixis was first given by Winkler (1908). Apomixis may be defined as, abnormal kind of sexual reproduction in which egg or other cells associated with egg (synergids, antipodals, etc.) develop into embryo without fertilisation and with or without meiosis. Hybrid varieties of several food and vegetable crops are being extensively cultivated. Cultivation of hybrids tremendously increased productivity. One of the problems of hybrids is that hybrid seeds have to be produced every year. If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters. Production of hybrid seeds is costly and hence the cost of hybrid seeds becomes too expensive for the farmers. If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year and do not have to buy hybrid seeds every year. Embryos formed through apomixis are generally free from infections.
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