CBSE Class 12 Biology Sexual Reproduction in Flowering Plants Concept Map

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Long Answer Questions

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) Sporopollenin
It is most resistant material to high temperature, strong acids on alkali and no enzymes can degrade it.
(b) Germs pores are present to allow pollen tube to emerge out for pollen germination.
(c) Pollen banks help in storing pollen grains for years for crop breeding programmes.

Question. When and where do tapetum and synergids develop in flowering plants? Mention their functions. 
Answer. Tapetum develop during microsporogenesis in the microsporangium (anther). It nourishes the developing pollen grains.
Synergids develop during megasporogenesis in the megasporangium (ovule). Synergids have filiform apparatus to guide the pollen tube into it.

Question. (a) Name the structures which the parts ‘A’ and ‘B’ shown in the diagram below respectively develop into.
(b) Explain the process of development which ‘B’ undergoes in albuminous and exalbuminous seeds. Give one example of each of these seeds.

Answer. (a) A develops into an embryo; B develops into endosperm.
(b) Refer to Basic Concepts Point 9 (Endosperm Development).

Question. Where are the following structures present in a male gametophyte of an angiosperm? Mention the function of each one of them.
(a) Germ pore (b) Sporopollenin (c) Generative cell 
Answer. (a) Germ pore: Exine of pollen grain. It is the site from where pollen tube emerges.
(b) Sporopollenin: Exine of pollen grains. It protects the pollen grains from high temperature, strong acids and alkali, enzymes and adverse conditions.
(c) Generative Cells: These are present in pollen grains. These give rise to two male gametes.

Question. Draw the diagram of microsporangium of an angiosperm and label any four parts. State the function of its innermost wall layer.
Answer. 

Tapetum nourishes the developing pollen grains

Question. Why are angiosperm anthers called dithecous? Describe the structure of its microsporangium.
Answer. The anthers of angiosperms are called dithecous because they are bilobed and each lobe of anther has two theca.
Microsporangium is surrounded by four wall layers named as epidermis, endothecium, middle layer and tapetum. In young anther, a group of compactly arranged homogenous cells called sporogenous tissue occupies the centre of each microsporangium which produce microspores or pollen grains.

Question. The generative cell of a 2-celled pollen divides in the pollen tube but not in a 3-celled pollen. Give reasons. 
Answer. In a 3-celled pollen, the generative cell has already divided and formed 2 male gametes. Hence, it will not divide again in the pollen tube. As the generative cell has not divided in a 2-celled pollen, it divides in the pollen tube.

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. (a)

(b)

Question. (a) Draw a labelled sketch of a mature 7-celled, 8-nucleate embryo-sac.
(b) Which one of the cell in an embryo-sac produce endosperm after double fertilisation?
Answer. (a)

(b) Central cell

Question. What will be the ploidy of the cells of the nucellus, microspore mother cell, the functional megaspore and female gametophyte? 
Answer. Nucellus : Diploid
Microspore mother cell : Diploid
The functional megaspore : Haploid
Female gametophyte : Haploid

Question. Given below are the events that are observed in artificial hybridisation programme. Arrange them in the correct sequential order in which they are followed in the hybridisation programme.
(a) re-bagging; (b) selection of parents;
(c) bagging; (d) dusting the pollen on stigma;
(e) emasculation; (f) collection of pollen from male parent.
Answer. (b); e(); c(); f)(; d(); a().

Question. Explain the process of artificial hybridisation to get improved crop variety in (i) plants bearing bisexual flowers (ii) female parent producing unisexual flowers.
Answer. (i) In plants bearing bisexual flowers, the anthers are removed from the flower before they dehisce. This is called emasculation. The emasculated flowers are covered with a bag of butter paper to prevent contamination of stigma with unwanted pollen. This process is called bagging. When this stigma attains receptivity, mature pollen grains are dusted on the stigma and the flowers are rebagged to allow the fruits to develop.

Question. Write the differences between wind-pollinated and insect-pollinated flowers. Give an example of each type.
Answer. 

Question. (i) Write the characteristic features of anther, pollen and stigma of wind-pollinated flowers.
(ii) How do flowers reward their insect pollinators? Explain.
Answer. (i) The characteristics of wind-pollinated flowers are:
(a) Pollen grains are light in weight, non-sticky, dry and winged, so that they can be easily transported.
(b) Well-exposed stamens for easy dispersal of pollen grains in the wind.
(c) The stigma is sticky, large, feathery to trap pollen grains in air.
(d) Numerous flower are packed together to form inflorescence.
(d) The flowers are small and inconspicuous.
(ii) Insect pollinators are rewarded in following ways:
(a) The flowers offer floral reward like nectar and pollen grain.
(b) In some species floral reward provides safe place to lay eggs.

Question. (a) Explain the phenomenon of double fertilisation.
(b) Draw a labelled diagram of a typical anatropous ovule. 
Answer. (a) Double fertilisation includes syngamy where one of the male gametes fuses with egg cell to form zygote and triple fusion which includes fusion of second male gamete with two polar nuclei.
(b)

Question. (a) Seeds offer several advantages to angiosperms. Describe any three such advantages.
(b) Why is banana called a parthenocarpic fruit? Would you call banana a true fruit? Give reason in support of your answer.
Answer. (a) Reproductive processes such as pollination and fertilisation are independent of water.
Following are their advantages:
(i) Better adaptive strategies for dispersal to new habitats.
(ii) Hard seed coat provides protection to young embryo .
(iii) Sexual reproduction—new genetic combinations.
(iv) Sufficient food reserves for the seedling.
(v) Basis of agriculture–storage of seeds can occur due to seed habit-dehydration and dormancy. (Any three)
(b) Banana fruit develops without fertilisation therefore, it is called parthenocarpic fruit. Yes, it is a true fruit because it develoryp.

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