CBSE Class 12 Biology Sexual Reproduction in Flowering Plants Flow Chart

SEXUAL REPRODUCTION IN FLOWERING PLANTS

(FLOW CHART)

Long Answer Questions

Question. (a) Mention any four strategies adopted by flowering plants to prevent self-pollination.
(b) Why is geitonogamy also referred to as genetical autogamy? [
Answer. (a) Refer to Basic Concepts Point 5 [Contrivances for self-pollination (Autogamy)].
(b) Geitonogamy is the transfer of pollen grains from the anther to the stigma of another flower of the same plant. Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy, since the pollen grains come from the same plant.

Question. State the significance of pollination. List any four differences between wind-pollinated and animal pollinated flowers.
Answer. Pollination is the phenomena of transfer of pollen grains from anthers to the stigma of a pistil.
Pollination is prerequisite for the beginning of fertilisation.

Question. Write the functions of:
(a) Coleoptile (b) Tapetum (c) Scutellum
Answer. (a) Coleoptile: It protects the plumule of the monocot embryo.
(b) Tapetum: It provides nourishment to developing pollen.
(c) Scutellum: It provides nourishment and protection to the developing embryo.

Question. (a) Describe the endosperm development in coconut.
(b) Why is tender coconut considered a healthy source of nutrition?
(c) How are pea seeds different from castor seeds with respect to endosperm?
Answer. (a) The primary endosperm nucleus (PEN) undergoes successive nuclear divisions to give rise to free nuclei. Subsequently, cell wall is formed towards the periphery and endosperm becomes cellular, leaving free nuclear endosperm in the central part. This division is followed by cytokinesis and thus endosperm becomes cellular and is called cellular endosperm.
(b) It is rich in many nutrients like fats, proteins, carbohydrates, minerals, vitamins. Hence,tender coconut is considered a healthy source of nutrition.
(c) In peas, the endosperm is used up and there is no endosperm present in the mature seed. In castor, the endosperm remains intact in the mature seed.

Question. Differentiate between perisperm and endosperm by giving one example of each.
Answer.

Question. Explain any three advantages the seeds offer to angiosperms.
Answer. (i) Since reproductive process such as pollination and fertilisation are independent of water,seed formation is more dependable.
(ii) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
(iii) As they have sufficient food reserves young seedlings are nourished until they are capable of photosynthesis on their own.
(iv) The hard seed coat provides protection to the young embryo.
(v) Being products of sexual reproduction, they generate new genetic combinations or variations.

Question. State what is apomixis. Comment on its significance. How can it be commercially used?
Answer.  Ans. Apomixis is a type of asexual reproduction that mimics sexual reproduction to form seeds without fertilisation.
In apomictic seeds, parental characters are maintained in the progeny/offspring as there is no meiosis or segregation of characters.
If desired hybrid seeds are made apomictics the farmers can keep on using the hybrid seeds to raise new crops year after year.

Question. Differentiate between parthenocarpy and parthenogenesis. Give one example of each.
Answer.

Question. (a) How are parthenocarpic fruits produced by some plants and apomictic seeds by some others? Explain.
(b) When do farmers prefer using apomictic seeds? 
Answer. (a) Parthenocarpic fruits are formed when ovary develops into fruit without fertilisation.
Apomictic seeds are formed when formation of seeds take place without fertilisation.
(b) To maintain hybrid characters (year after year in a desired plant) and to avoid buying hybrid seeds every year (expensive seeds) farmers prefer using apomictic seeds.

Question. (a) How does a farmer use the dormancy of seeds to his advantage?
(b) What advantages a seed provides to a plant? 
Answer. (a) Dormancy of mature seeds are important for storage of seeds which can be used as food throughout the year and also to raise crop in the next season.
(b) Seeds offer several advantages to angiosperms. Firstly, since reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable.
Also seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas. As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own. The hard seed coat provides protection to the young embryo. Being products of sexual reproduction, they generate new genetic combinations leading to variations.

Question. Make a list of any three outbreeding devices that flowering plants have developed and explain how they help to encourage cross-pollination.
Answer. (i) Time of pollen release and stigma receptivity are different (not synchronised). This prevents self-pollination.
(ii) Anther and stigma are placed at different positions, so the pollens cannot come in contact with the stigma of the same flower.
(iii) Self-incompatibility, which is a genetic mechanism to prevent the pollen germination on the stigma of the same flower.

Question. ‘Fertilisation is not an obligatory event for fruit production in certain plants’. Explain the statement. 
Answer. This can be observed in parthenocarpic fruits. The ‘seedless fruits’ that are available in the market,such as pomegranate, grapes, etc., are good examples. Flowers of these plants are sprayed by a growth hormone that induces fruit development even though fertilisation has not occurred. The ovules of such fruits, however, fail to develop into seeds.

Question. Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition. 
Answer. Many apomictic species have been seen to have normal looking embryo sacs. The only possibility of the embryo sac possessing diploid cells will be due to failure of meiotic division at the megaspore mother cell stage. Since the megaspore mother cell has a diploid nucleus, if it undergoes mitosis instead of meiosis, all the resulting nuclei and cells will be diploid in nature.

Question. Draw a labelled diagram of an anther lobe at microspore mother cell stage. Mention the role of different wall layers of anther.
Answer. 

Roles of different wall layers of anther:
(i) Epidermis, endothecium and middle layers perform the function of protection and help in dehiscence of anther to release the pollen.
(ii) Tapetum is the innermost wall layer and it provides nourishment to the developing pollen grains.

Question. A flower of tomato plant following the process of sexual reproduction produce 240 viable seeds. Answer the following questions giving reasons:
(a) What is the minimum number of pollen grains that must have been involved in the pollination of its pistil?
(b) What would have been the minimum number of ovules present in the ovary?
(c) How many megaspore mother cells were involved?
(d) What is the minimum number of microspore mother cells involved in the above case?
(e) How many male gametes were involved in this case? 
Answer. (a) 240 pollen grains. One pollen grain participates in fertilisation of one ovule.
(b) 240 ovules. One ovule after fertilisation forms one seed
(c) 240 MMC werien volved. Each MMC forms four megaspofr esw hoiucht onolnye remains functional.
(d) 60 MMCs (240/4 = 60). Each microspore mother cell meiotically divides to form four pollen grains.
(e) 480 male gametes (240 × 2 = 480). Each pollen grain carries two male gametes (which participate in double fertilisation)

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