Sexual Reproduction in Flowering Plants

Q1. What is apomixis and polyembryony?

Ans: The phenomenon of asexual reproduction that imitates sexual reproduction by the formation of seed without fertilization is called apomixis or agamospermy.
The occurrence of more than one embryo in a seed is called polyembryony, e.g., orange.

Q2. What is emasculation?

Ans: The process of removal of anther from bisexual flowers without affecting the pistil is called emasculation.

Q3. Mention any five features of insect pollinated flowers.

Ans:
o The form of pollination in which insects distribute pollens is called entomophily.
o Flowers are large, sticky and brightly coloured.
o They have honey and nectar glands, which are highly fragrant to attract insects.
o The pollen grain surface is sticky due to exine layer and stigma is sticky due to mucilaginous secretion.
o The flowers offer floral rewards like nectar and pollen grains for pollination to insects.
o In some species, floral rewards provide safe place to lay eggs, e.g., Amorphophallus.
o The flower sometimes secretes foul odour to attract insects like flies and beetles.

Q4. Mention two strategies evolved to prevent self-pollination (autogamy) in flowers.

Ans:
i. Unisexuality: Male and female flowers are present on different plants.
ii. Dichogamy: The condition in which the stamens and stigma of a bisexual flower mature at different times.
iii. Protandry: This is the condition where anthers mature earlier than the stigma and release pollens.
iv. Protogyny: This is the condition where the stigma matures earlier than the anther.
v. Self-sterility or self-incompatibility: It is a genetic mechanism that prevents self-pollination.
vi. Chasmogamous flowers: These are open flowers with exposed stamens and stigma which facilitate cross-pollination.

Q5. Differentiate between perisperm and pericarp.

Ans:
Perisperm:
1. It represents the persistent nucellus in the seed.
2. It is a part that belongs to seed.
3. It is usually dry.

Pericarp:
1. It represents the fruit wall formed by the ovarian wall.
2. It is a part of fruit.
3. It can be dry or fleshy.

Q6. Describe the structure of pollen grains.

Ans:
They develop from Pollen Mother Cell by meiotic division.
➢ They represent the male gametophyte.
➢ Pollen grains are generally spherical in structure.
➢ They possess two prominent layered walls - outer exine and inner intine.
➢ The exine is a hard layer made of sporopollenin which is one of the most resistant organic material present in nature.
➢ The inner thin layer of intine is made up of cellulose and pectin.
➢ The exine has an aperture where sporopollenin is absent, called germ pore.
➢ The newly differentiated pollen grain has a central nucleus and dense cytoplasm.
➢ The protoplast mitotically divides into two unequal cells - bigger vegetative cell which is rich in food reserve and smaller spindle-shaped generative cell with dense cytoplasm and a nucleus. This is called 2-celled stage.

Q7. Explain the process of megasporogenesis or explain the events of development of female gametophyte.

Ans:
• The female gametophyte (embryo sac) develops from a single functional megaspore.
• This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs.
• The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other nucleus moves towards the chalazal end.
• Then, these nuclei divide at their respective ends and re-divide to form eight nucleate stages. As a result, there are four nuclei each at both the ends i.e., at the micropylar and the chalazal end in the embryo sac.
• At the micropylar end, out of the four nuclei only three differentiate into two synergids and one egg cell. Together they are known as the egg apparatus.
• Similarly, at the chalazal end, three out of four nuclei differentiates as antipodal cells.
• The remaining two cells (of the micropylar and the chalazal end) move towards the centre and are known as the polar nuclei, which are situated in a large central cell.
• Hence, at maturity, the female gametophyte appears as a 7-celled structure, though it has 8 nucleate.

Q8. What are autogamy and xenogamy?

Ans:
Autogamy: Transfer of pollen grains from the anthers to the stigma of the same flower.
e.g., pea, rice, wheat, etc.…

Xenogamy/allogamy: Transfer of pollen grains from the anthers to the stigma of a different plant of the same species.
e.g., papaya, maize

Q9. Describe the structure of anatropous ovule or megasporangium with a neat labelled diagram.

Ans:
➢ The ovule is a small structure attached to the placenta by means of a stalk called funicle.
➢ The junction between an ovule and a funicle is called hilum. Sometimes, the funicle extends beyond the hilum to form a ridge called raphae.
➢ The ovule is surrounded by one or two protective envelopes called integuments.
➢ Integument encircles the ovule entirely except at the tip, resulting in a small opening called micropyle.
➢ The basal part of an ovule opposite to micropyle is called chalaza.
➢ The cells with high or abundant reserve food material enclosed within integument is called nucellus.
➢ The female gametophyte located within the nucellus is called an embryo sac.

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Q10. Draw a neat labelled diagram of T. S. of anther (Microsporangium).

Ans: T. S. of anther (Microsporangium)

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Q11. Draw a neat labelled diagram of a mature embryo sac or female gametophyte.

Ans: A mature embryo sac or female gametophyte

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