NCERT Class 12 Biology: Chapter –2 Sexual Reproduction in Flowering Plants Part 8

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Question 8:

Why does the zygote begin to divide only after the division of Primary endosperm cell (PEC)?

Answer:

The zygote begins to divide only after the division of Primary endosperm cell (PEC) because of the following reasons:

i. Zygote needs nourishment during its development.

ii. The primary endosperm cell (PEC) forms the endosperm where food materials are stored for the developing embryo.

iii. A mature, fertilised embryo sac offers very little nourishment to the zygote.

iii. PEC divides and generates the endosperm tissue which nourishes the zygote.

So the zygote begins to divide only after the division of Primary endosperm cell (PEC) is an adaptation to provide assured nutrition to the developing embryo.

Question 9:

The generative cell of a two-celled pollen divides in the pollen tube but not in a three-celled pollen. Give reasons.

Answer:

The generative cell in pollen grain divides mitotically to give rise to the two male gametes before pollen grains are shed. This is 3-celled stage i.e., one vegetative cell and two male gametes. And as the pollen grains are already divided this will not divide again in the pollen tube. So the pollen tube carries two male gametes from the beginning.

Pollen grains in 2-celled stage just have 2 cells namely vegetative cell and generative cell. As here division of generative cell did not take place, hence the generative cell of two-celled pollen divides in the pollen tube.

Question 10:

In the figure given below label the following parts: male gametes, egg cell, polar nuclei, synergid and pollen tube

Male gametes, egg cell, polar nuclei, synergid

Male Gametes, Egg Cell, Polar Nuclei, Synergid

Answer:

The required labeled diagram is as shown below with necessary labels.

The required labeled diagram

The Required Labeled Diagram

Long Answer Type Questions

Question 1:

Starting with the zygote, draw the diagrams of the different stages of embryo development in a dicot.

Answer:

The different stages of embryo development in a dicot are as shown below:

The different stages of embryo development in a dicot

The Different Stages of Embryo Development in a Dicot

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