In This Post we are providing Chapter-2 SEXUAL REPRODUCTION IN FLOWERING PLANTS NCERT MOST IMPORTANT QUESTIONS for Class 12 BIOLOGY which will be beneficial for students. These solutions are updated according to 2021-22 syllabus. These MCQS can be really helpful in the preparation of Board exams and will provide you with a brief knowledge of the chapter.
NCERT MOST IMPORTANT QUESTIONS ON SEXUAL REPRODUCTION IN FLOWERING PLANTS
1. Trace the development of microsporocyte into mature pollen grains.
Ans. i) When the anther is young, the microsporangium contains compactly arranged homogenous cells forming the Sporogeneous tissues.
- Every cell of the sporogenous tissue is a potential Pollen mother cell (PMC) & give rise to microspore tetrad or Pollen grains.
- But Some of them forego this Potential & become differentiated into pollen or microspore mother cell (MMC)
- Each microspore mother cell undergoes meiosis to form a cluster of four haploid cells called microspore tetrad.
- As the anther matures, microspores dissociate from tetrad & develop into pollen grains.
- The nucleus of microspore undergoes mitosis to form large vegetative cell & small generative cell. They develop a two layered wall – outer exine made up of sporopollenin & inner intine made up of cellulose & pectin. Usually Pollen grains are liberated at two celled stage.
2. i) Explain the structure of a maize grain with the help of a diagram
ii) Why cannot we use the term maize seeds for maize grains?
Ans. (i) In grass family ( eg. Maize ) fruit is single seeded where pericarp & seed coat are fused together to form the husk. Just below husk, there is a layer of cells called aleurone layer, with stores proteins. There is a large endosperm that stores starch. The embryo lies on one side of endosperm & consists of a single cotyledon called scutellum & embryonal axis. The region of embryonal axis that points down ward from point of attachment of cotyledons is radicle & is covered by protective sheath called coleorhiza. The region of embryonal axis that points upward from point of attachment of cotyledon is plumule, it is covered by foliaceous sheath called coleoptite
(ii) We cannot use the term seeds for maize grain because seed is not completely developed from embryo but retains a part of endosperm.
3. Trace the development of megasporocyte into mature ovule.
Ans. i. A single Megaspore mother cell is differentiated in the micropylar region of nucleus of an ovule & undergoes meiosis & forms a cluster of haploid cells called megaspore tetrad. Of these, soon three degenerates & only one megaspore becomes functional
ii. Functional megaspore enlarges to form embryo sac. Its nucleus undergoes mitotic division & two nuclei move to opposite poles forming 2-nucleate embryo Sac.
iii. Two successive mitotic divisions in each of these two nuclei results in formation of 8-nucleate embryo sac.
iv. Three cells are grouped together at micropylar end to form egg apparatus. consisting of two synergids & a female egg cell .
v. Three cells are grouped together at the chalazal end, they are called antipodal cells.
vi. The remaining two nuclei are called Polar nuclei, they move to centre of embryo sac & fuse to form Secondary nucleus.
Thus a typical angiospermic embryo sac is 8-nucleate 7-celled
4. “Incompatibility is the natural barrier in fusion of gamete”. Justify this statement.
Ans. Pollen grains of a plant species cannot germinate on stigma of other unrelated species because both the species are incompatible & process is called pollen – pistil incompatibility. In many angiospermic plants, it is seen that pollen grains germinate on stigma of unrelated species but male gametes produced in pollen tube cannot fertilize egg. This is called gametic incompatibility Self incompatibility can be achieved by any of the following ways :-
- Pollen Stigma interaction: – In this phenomenon, pollen grains fails to germinate on Stigma because of incompatibility.
- Pollen tube style interaction: – In this phenomena, pollen grains become able to germinate on stigma & pollen tube penetrate stigmatic surface but due to incompatibility growth of pollen tube within stigma & style is inhibited.
- Pollen – ovule interaction: – pollen tube successfully pierces & grows within style & its growth is inhibited at micropyle of ovule.
5. How dose pollination takes place in salivia. List any four adaptations required for such type of pollination.
Ans. In salivia, entomophily or pollination lay insects occurs. The flowers of salivia are bilipped. Its upper lip consists of two petals & lower lip consists of three petals. The lower lip functions as sitting pad for insects. In normal conditions, the connective remains upright. When insect enters the tube of corolla towards nectar sitting on lower lip, it pushes sterile anther lobe which automatically brings about fertile anther to touch the back of insects gets the blow of fertile lobe. Pollen grains are dusted on back feather & legs of insects.
ADAPTAIONS EOR ENTOMOPHILY :-
- Flowers are brightly coloured.
- Flowers possess nectar glands.
- pollen grains are usually sticky & spiny
- flowers are large – sized & stout
6. Explain the formation of an embryo sac with diagrams.
Ans.
- The functional megaspore grows in size.
- The nucleus divides mitotically to form two nuclei which move to opposite poles.
- Each nucleus at the poles undergoes two mitotic divisions to form four nuclei in each pole or a total of 8 nuclei.
- two nuclei from each pole move to the centre to form the polar nuclei.
- the other nuclei, three at each pole get surrounded by bit of cytoplasm to form cells.
- the female gametophyte or the embryo sac thus has 7 cells and eight nuclei.
7. Explain the development of embryo in a dicotyledonous plant with neatly labeled diagrams.
Ans. The embryo develops at the micropylar end where the zygote is located. The zygote starts developing only after certain amount of endosperm is formed to assure nutrition to the embryo. The zygote divides mitotically to form various stages including pro- embryo, globular, heart shaped and finally the mature embryo
8.Describe the post-fertilization changes taking place in a flowering plant?
Ans. The major events taking place in a flowering plant after fertilization:-
(i) DEVELOPMENT OF ENDOSPERM:- Endosperm development proceeds embryo development . The most common method of endosperm development is nuclear type where triploid endosperm (PEN) undergoes repeated mitotic divisions without cytokinesis – Subsequently cell wall formation occurs from periphery & endosperm store food materials which is later used up by embryo.
(ii) DEVELOPMENT OF EMBRYO :- The zygote divides lay mitosis to for a pro-embryo first . Later development results in formation of globular & heart shaped embryo & that ultimately become horseshoe – shaped embryo with one or more cotyledons. In dicot embryo, the portion of embryonal axis about the level of attachment is epicotyl & it terminates into plumule while portion of embryonal axis below the level of attachment is hypocotyl & terminates into radicle.
9.Trace the events that would take place in flower from the time of Pollen grain of species fall on stigma up To completion of fertilization.
Ans. GERMINATION OF POLLEN GRAINS ON STIGMA
The pollen grains absorb fluid present on stigma & swell up. The exine ruptures at the place of germ pore & intine comes out in the form of tube with its internal contents. This small tubular structure is called pollen tube & process is called pollen germination.
ii) Entry of pollen tube into Ovule: – The entry of pollen tube into ovule occurs through micropyle or chalaza or through lateral sides of ovule. Only one pollen tube enters inside the embryo sac of an ovule. Normal two synergids are destroyed while entry of pollen tube into embryo sac.
iii) Discharge of Mate Gametes :- After enter of pollen tube both the male gametes discharged into embryo sac by either forming two pores into pollen tube & each male gamete is discharged through every pore or sometime pollen tube may burst & release the male gametes into embryo sac.
iv) Fertilization:- The fusion of first male gamete (n) with egg (n) is called fertilization. It results in formation of a diploid zygote (2n). The second male gamete fuses with secondary nucleus (2n) to form triploid endosperm nucleus (3n). This fusion between second male gamete & secondary nuclei is triple fusion. Since process of fertilization occurs twice. It is called double fertilization.
10. i)Why is zygotes dominant for sometime in fertilized ovule.
ii) What is polyembryony? Give an example.
iii) In fruits, what is formed from following parts :-
a) Ovary wall
b) Outer integument
c) Inner integument
d) zygote
e) primary endosperm
f) Ovary
g) Nucellus
Ans. (i) Zygote remain dominant for sometime in a fertilized ovule because embryo develops after formation of endosperm therefore zygote wants for formation of endosperm which supplies food material for developing embryo
(ii) The presence of more than one embryo in a seed is called polyembryony eg. Sometimes more than one embryo is formed within an embryo sac either by cleavage or splitting of egg, synergid, antipodal or endosperm.
(iii) In fruits, the following is formed from given parts:-
| a) | Ovary wall | Per carp |
| b) | Outer integument | Testa |
| c) | Inner integument | Tegmen |
| d) | zygote | embryo |
| e) | primary endosperm | endosperm |
| f) | Ovary | fruit |
| g) | Nucellus | perisperm. |
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