NCERT Class 12 Biology: Chapter –2 Sexual Reproduction in Flowering Plants Part 9

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Question 2:

What are the possible types of pollinations in chasmogamous flowers. Give reasons.

Answer:

Chasmogamous flower are flowers with exposed anthers and stigma.

All the following types of pollinations are possible in Chasmogamous flowers are:

i. Autogamy or self-pollination - for autogamy to occur in Chasmogamous flower both pollen and ovules should mature simultaneously. As pollen release and stigma reception should be synchronised for autogamy to take place.

ii. Dichogamy - For this type of pollination synchronisation of pollen release and stigma reception should be avoided that is by avoiding the simultaneous maturing of anther and stigma. In this type of pollination stigma and anther matures at different time. Examples of such type of flowers are Sunflower, Datura, Solarium, etc.

iii. Self-incompatibility or self-sterility is a genetic mechanism that prevents self-pollination (i.e., pollination from same flower or other flowers of same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

iv. In Chasmogamous flower length of anther and stigma plays a secondary role in autogamy. Therefore by placing anther and stigma at different position, in such a way that the pollen of a flower doesn’t come in contact with stigma of the same flower can prevent autogamy.

v. Geitonogamy – here pollen and pistil mature at different times but are self – compatible.

vi. Xenogamy – here pollen grains from the flower of one plant to the stigma of another plant. This pollination brings genetically different types of pollen grains.

Question 3:

With a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.

Answer:

Parts of mature angiosperm embryo sac:

i. Antipodal: They are 3-celled at the chalazal end of the ovule. The large central cell is formed by the fusion of 2-polar nuclei. They are also referred as vegetative cells of the embryo sac.

ii. Polar Nuclei: One nucleus from each end of the chalazal end and Micropyle end are called Polar Nuclei.

iii. Central cell: It is the largest cell, formed by the fusion of two polar nuclei. It is also known as secondary nucleus. It shows central vacuole and 2 haploid polar nuclei.

iv. Egg Apparatus: It includes one large egg cell, 2 synergids and fusiform apparatus. Three cells present towards the Micropyle end grouped together constitute egg apparatus.

The antipodal is 3-celled with 3 nucleus. Polar nuclei and Central cell constitute 2 nucleus and it is 1-celled. Egg apparatus is 3-celled with 3 nucleus. So a mature angiosperm embryo sac is 7-celled and 8-nucleate.

Role of synergids:

i. Synergids are essential for angiosperm reproduction.

ii. Synergids helps in growth of pollen tubes towards egg.

iii. Synergids helps in the nutrition of embryo sac by absorption and transport of food from nucleus through their fusiform apparatus.

iv. The fusiform apparatus (special cellular thickening of synergids) helps in guiding the pollen tube to enter one of the synergids.

Question 4:

Draw the diagram of a microsporangium and label its wall layers. Write briefly on the role of the endothecium.

Answer:

The diagram of a microsporangium and labelling of its wall layers is as follows:

The diagram of a microsporangium and labelling

The Diagram of a Microsporangium and Labelling

The diagram of a microsporangium and labelling

Role of endothecium:

i. Endothecium helps in dehiscence of anther.

ii. It is fibrous layer of cells lying below the epidermis of the wall of the anther.

iii. At maturity these cells loose water, contract and help in dehiscence of pollen sac.

Question 5:

Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition.

Answer:

- Apomixis is development of a seeds without prior fertilization. Example: replacement of the flower by bulbils and replacement of seed by a plant.

- It is observed in few flowering.

- Apomixis is a form of asexual reproduction that mimics sexual reproduction.

- Apomixis avoids the processes of meiosis and fertilization, leading to embryo development.

- The offsprings produced in this case are genetically identical to the parent plant.

- Embryo sacs of some apomictic species appear normal but contain diploid cells due to absence of meiotic division at the megaspore mother cell stage.

- As the megaspore mother cell has a diploid nucleus, if it undergoes mitosis instead of meiotic, all the resulting nuclei and cells will be diploid in nature.

- Apomixis is common in plants like Asteraceae, Poaceae.

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