NCERT Class 12 Biology: Chapter –2 Sexual Reproduction in Flowering Plants Part 6

Glide to success with Doorsteptutor material for NSO-Level-2 Class-8: fully solved questions with step-by-step explanation- practice your way to success.

Question 18:

What is the function of the two male gametes produced by each pollen grain in angiosperms?

Answer:

The two male gametes produced by each pollen grain in angiosperms helps in double fertilisation.

i. One of the male gametes moves towards the egg cell and fuses with its nucleus called Syngamy. This results in the formation of a diploid cell, the zygote.

ii. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (Figure 2.13a).

As this involves the fusion of three haploid nuclei it is termed triple fusion.

Two types of fusions, Syngamy and triple fusion, take place in an embryo sac the phenomenon is termed double fertilisation, an event unique to flowering plants.

Short Answer Type Questions

Question 1:

List three strategies that a bisexual Chasmogamous flower can evolve to prevent self-pollination (autogamy).

Answer:

Autogamy is pollination where pollination is achieved within the same flower, i.e., it is type of self -pollination. In this the pollens are transferred from anther to stigma of same flower.

Chasmogamous flower are flowers with exposed anthers and stigma.

The strategies that a bisexual Chasmogamous flower can evolve to prevent self-pollination are:

i. As pollen release and stigma reception should be synchronised for autogamy to take place. Avoiding synchronisation of pollen release and stigma reception can prevent autogamy that is avoiding the simultaneous maturing of anther and stigma. Such type of pollination is known as Dichogamy where stigma and anther matures at different time. Examples of such type of flowers are Sunflower, Datura, Solarium, etc.

ii. 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.

iii. 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.

Question 2:

Given below are the events that are observed in an artificial hybridization 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:

The correct sequential order in which the steps should be followed in the artificial hybridisation programme are:

(a) Selection of parents is done first.

(b) Emasculation – after parents are selected, in bisexual female flower anthers are removed from flowed bud before the anther dehisces, this is called Emasculation. In case of female parent producing unisexual female flowers Emasculation step is not needed.

(c) Bagging – Now the stigma is bagged with butter paper bag to prevent its contamination with unwanted pollen. This is called Bagging.

(d) Collection of pollen from male parent – Now mature pollens are collected from other male parent is done.

(e) Dusting the pollen on stigma – When the stigma of bagged flower attains receptivity, the collected mature pollen grains are dusted on the stigma.

(f) Re-bagging – Now again the flower is re-bagged till the fruits are developed.

Question 3:

Vivipary automatically limits the number of offspring’s in a litter. How?

Answer:

i. Vivipary is defined, where the seed grows first to break through the seed coat then out of the fruit wall while still attached to the parent plant. But not many seeds can grow on single parent plant, hence they will fall to ground around the parent plant. Here in a struggle to get enough oxygen from the soil (mostly marshy lands), many seedlings die. Out of the left out seedlings as they grow they struggle to get enough sunlight, and many others don’t survive. So the numbers of offsprings’ are limited.

ii. Vivipary in animals – here embryo develops inside the body of female. A litter is the offspring at one birth of animals from the same mother. A litter usually are 3-8 offsprings together. The vivipary automatically limits the number of offsprings in a litter due to the reason that limited number of eggs are produced and fertilized during female reproductive cycle.