Biology Class 11 NCERT Solutions: Chapter 15 Plant Growth and Development Part 1

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Q: 5. What do you understand by photoperiodism and vernalisation? Describe their significance.

Answer

Photoperiodism refers to the response of plants with respect to the duration of light (i.e., period of day and night). On the basis of its response to the duration of light, a plant is classified as a short-day plant, a long-day plant, or a day-neutral plant. Short-day plants flower when they are exposed to light for a period less than the critical day-length (for example: Chrysanthemum). Long-day plants flower when they are exposed to light for a period more than the critical day-length (for example: radish). When no marked correlation is observed between the duration of exposure to light and the flowering response, plants are termed as day-neutral plants (for example: tomato).

It is hypothesised that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Photoperiodism helps in studying the response of flowering in various crop plants with respect to the duration of exposure to light.

Image of the Importance of photoperiodism

Image of the Importance of Photoperiodism

Image of the Importance of photoperiodism

Vernalisation is the cold-induced flowering in plants. In some plants (such as the winter varieties of wheat and rye and biennials such as carrot and cabbage), exposure to low temperature is necessary for flowering to be induced. The winter varieties of rye and wheat are planted in autumn. They remain in the seedling stage during winters and flower during summers. However, when these varieties are sown in spring, they fail to flower. Similar response is seen in cabbage and radish.

Q: 6. Why is Abscisic acid also known as stress hormone?

Answer

Abscisic acid is called stress hormones as it induces various responses in plants against stress conditions.

It increases the tolerance of plants toward various stresses. It induces the closure of the stomata during water stress. It promotes seed dormancy and ensures seed germination during favourable conditions. It helps seeds withstand desiccation. It also helps in inducing dormancy in plants at the end of the growing season and promotes abscission of leaves, fruits, and flowers.

Q: 7. 'Both growth and differentiation in higher plants are open'. Comment.

Answer:

Growth and development in higher plants is referred to as being open. This is because various meristems, having the capacity for continuously dividing and producing new cells, are present at different locations in these plant bodies.

Q: 8. 'Both a short day plant and a long day plant can flower simultaneously in a given place'. Explain.

Answer

The flowering response in short-day plants and long-day plants is dependent on the durations for which these plants are exposed to light. The short-day plant and long- day plant can flower at the same place, provided they have been given an adequate photoperiod.

Short day plant and a long day plant

Short Day Plant and a Long Day Plant

Short day plant and a long day plant

Q: 9. Which one of the plant growth regulators would you use if you are asked to:

(A) Induce rooting in a twig

(B) Quickly ripen a fruit

(C) Delay leaf senescence

(D) Induce growth in axillary buds

(E) 'Bolt' a rosette plant

(F) Induce immediate stomatal closure in leaves.

Answer:

(A) Induce rooting in a twig - Auxins

(B) Quickly ripen a fruit - Ethylene

(C) Delay leaf senescence - Cytokinins

(D) Induce growth in axillary buds - Cytokinins

(E) 'Bolt' a rosette plant - Gibberellic acid

(F) Induce immediate stomatal closure in leaves - Abscisic acid

Image of the plant growth regulators

Image of the Plant Growth Regulators

Image of the plant growth regulators

Q: 10. Would a defoliated plant respond to photoperiodic cycle? Why?

Answer:

A defoliated plant will not respond to the photoperiodic cycle.

It is hypothesised that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Therefore, in the absence of leaves, light perception would not occur, i.e., the plant would not respond to light.

Q: 11. What would be expected to happen if:

(A) GA3 is applied to rice seedlings

(B) Dividing cells stop differentiating

(C) A rotten fruit gets mixed with unripe fruits

(D) You forget to add cytokinin to the culture medium.

Answer

(A) If is applied to rice seedlings, then the rice seedlings will exhibit internode elongation and increase in height.

(B) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cell is called callus.

(C) If a rotten fruit gets mixed with unripe fruits, then the ethylene produced from the rotten fruits will hasten the ripening of the unripe fruits.

(D) If you forget to add cytokinin to the culture medium, then cell division, growth, and differentiation will not be observed.