Biology Class 12

12th Biology Chapter 2 Subjective

Sexual Reproduction in Flowering Plants Subjective

  1. What do you mean by protandry? Give its advantages?

Ans – Maturation of androecium (male part of flower) before maturation of Gynoecium (female part of flower) is known as protandry.

Ex : Salvia. This condition prevents self pollination.

  1. State the function of filiform apparatus found in mature embryo sac of an angiosperm.

Ans – The special cellular thickenings at the micropylar tip called filiform apparatus, found in mature embryo sac of an angiosperm helps in guiding the pollen tubes up to the synergids.

  1. Abilobed, dithecous anther has 100 microspore mother cells per microsporangium. How many male gametophytes this anther can produce?  

Ans – An anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe. Total microsporangium = 4 x 100 = 400. So, anther produces 400 male gametes

  1. An anther with malfunctioning tapetum often fails to produce viable male gametophytes. Give one reason.

Ans – The anther with malfunctioning tapetum cannot provide complete nutrition to the developing microspores or male gametophytes. So, it fails to produce viable male gametophyte.

  1. Name the mechanism responsible for the formation of seed without fertilisation in angiosperms. Give an example of a species of flowering plants with such seed formation.

Ans – Apomixis is the mechanism responsible for the formation of seeds without fertilisation in angiosperms, e.g. grasses.

  1. Name the part of flower that contributes to fruit formation in strawberry and guava respectively.

Ans – (i) In strawberry, the fruit develops from the ovary, other floral parts degenerate and fall off. Thalamus also contributes to fruit formation.
(ii) In guava, the wall of ovary develops into the wall of fruit called pericarp.

  1. Name the organic materials of which exine and intine of an angiosperm pollen grains are made up of. Explain the role of exine.

Ans – The angiosperm pollen grains comprises of outer exine made up of resistant organic material sporopollenin, while inner thin layer of intine is made up of cellulose and pectin.Since, the outer hard layer of exine is made up of sporopollenin which is one of the most resistant organic substance, it helps the pollen grains to resist high temperatures, strong acids and alkali and it also protects them from enzyme degradation.

  1. Differentiate between the two cells enclosed in a mature male gametophyte of an angiosperm.

Ans – Haploid pollen grain represents the male gametophyte. It contains two cells,i.e.vegetative cell and generative cell. The vegetative or tube cell is larger in size as with prominent nuclei that gives rise to two male gametes, while vegetative cell does not.
compare to generative cell and have vacuolated cytoplasm. The generative cell on the other hand have thin dense cytoplasm.

  1. How many haploid cells are present in mature female gametophyte of a flowering plant? Name them.

Ans – 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 antipodal, 2 synergids and 1 egg cell) and a large central cell with 2 pollen nuclei.

  1. Where is sporopollenin present in plants? State its significance with reference to its chemical nature.

Ans – Sporopollenin is present in exine of pollen grains in plants. It is one of the most resistant organic substances and can withstand high temperatures, strong acids and alkalis. It protect the pollen grains from enzymes and helps them to be well preserved.   

  1. What is pollen-pistil interaction and how is it mediated?  

Ans – Pollen-pistil interaction is a chain or group of events that takes place from the falling of pollen over the stigma to the formation of pollen tube and its entry into the ovule. It is basically the phenomenon of acceptance or rejection of pollen grains by the pistil (stigma), which is mediated by chemical components of pollen grain, interacting with that of pistil,

  1. Differentiate between xenogamy and geitonogamy?  

Ans – Xenogamy is transfer of pollen grains from anther of one flower to stigma of another flower of a different plant, while geitonogamy is transfer of pollen grains from anther of one flower to stigma of another flower on same plant.

  1. In angiosperms, zygote is diploid while primary endosperm cell is triploid. Explain.
    or Mention the reasons for difference in ploidy of zygote and primary endosperm nucleus in an angiosperm.

Ans – In angiosperms or flowering plants, one of the male gamete fuses with egg cell which results in formation of zygote. So, zygote is diploid.While primary endosperm cell is triploid because the nucleus of the second male gamete fuses with the two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus. The central cell is now called primary endosperm cell.

  1. State one advantage and one disadvantage of cleistogamy.

Ans – 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.

  1. Why should a bisexual flower be emasculated and bagged prior to artificial pollination?

Ans – 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’s why a bisexual flower should emasculated and bagged prior to artificial pollination

  1. Explain any two devices by which autogamy is prevented in flowering plants.  

Ans – The two devices to prevent autogamy in flowering plants are:
(i) Anthers and stigma of a flower are placed in such a way that pollen of the same flower cannot fall on the stigma.
(ii) Self-incompatibility is a genetic process that prevents germination of pollen from the same flower on the stigma

  1. 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]

Ans – The three outbreeding devices that flowering plants have developed, so as to encourage cross-pollination are:
(i)Receptivity of stigma and release of pollen grain is not synchronised, i.e. stigma becomes receptive much before pollens are released or after they are released to avoid self-pollination.
(ii) Self-incompatibility, a genetic mechanism to prevent pollens from fertilising ovules of same flower by inhibiting their germination on stigma or pistil.
(iii) Production of unisexual flowers so that male and female parts will be present on different plants (dioecious) or on different flowers in same plant (monoecious). It prevents both autogamy and geitonogamy.

  1. Explain the phenomenon of double fertilisation.  

Ans – The phenomenon of double fertilisation occur in following steps:
(i) In an angiospermic plant, two male gametes are discharged by a pollen tube into the embryo sac.
(ii) One of the male gametes fuses with the egg to form a zygote. This process is called syngamy.
(iii) Other male gamete fuses with the secondary nucleus to form the primary endosperm nucleus, this process is called triple fusion.
(iv) Since, there are two fusions (syngamy and triple fusion), inside an ovule during fertilisation, it is known as double fertilization

  1. Describe endosperm development in angiosperm.  

Ans – (i) Embryo development occurs after endosperm development in angiosperms.
(ii) The three methods of endosperm development are:
(a) nuclear type (b) cellular type
(c) helobial type
(iii) Nuclear type is the common method in which triploid Primary Endosperm Nucleus (PEN) undergoes repeated mitotic division without cytokinesis. This stage is called free-nuclear endosperm.
(iv) Cell wall formation starts from the periphery and the endosperm becomes completely cellular, e.g. coconut, rice, etc.
(v) Cells of the endosperm store food materials.
(vi) Endosperm may be completely utilised by the developing embryo before the maturation of seeds as in pea, bean, mustard, etc. These seeds are called non-albuminous or endospermic seeds.
(vii) In seeds like castor, maize, coconut, rice, etc., a portion of it may remain in the mature seeds, such seeds are called albuminous or endospermic seeds

  1. (i) Describe the endosperm development in coconut.
    (ii)Why is tender coconut considered as healthy source of nutrition?
    (iii)How are pea seeds different from castor seeds with respect to endosperm?    

Ans – (i) Coconut endosperm formation is nuclear type. The primary endosperm nucleus undergoes nuclear division without cell wall formation.
(ii) Soft coconut is an endosperm. It is rich in nutrients like fats, proteins, carbohydrates, minerals, vitamins, etc. Hence, it is considered as a healthy source of nutrition.
(iii)The seeds of pea are non-endospermic, while castor seeds are endospermic. The endosperm in pea seeds is consumed completely during embryo development, but endosperm is not utilised in castor seeds.

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