NCERT Class 11-Biology: Chapter –15 Plant Growth and Development Part 4

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Question 8:

Where are the following hormones synthesized in plants

a. IAA

b. Gibberellins

c. Cytokinins


a. IAA: in the tips of shoot and apical buds from where they travel to the regions of their actions.

b. Gibberellins: in young leaves and tips of roots and later from the terpenoid pathway in plastids where the food is made, they travel to the ER where they are modified for further use.

c. Cytokinins: meristematic zones of the root like the root tip i.e. it is synthesized in regions where rapid cell division occurs.

Question 9:

In botanical gardens and tea gardens, gardeners trim the plants regularly so that they remain bushy. Does this practice have any scientific explanation?


In plants especially the higher plants, apical dominance or growth of apical buds forbids the growth of lateral buds. When the gardeners trim the plants regularly what they are doing is decapitation or removal of shoot tips (apical dominance is stopped) that results in growth of the lateral buds. This allows the plants to remain bushy in the botanical gardens and tea gardens.

Question 10:

Light plays an important role in the life of all organisms. Name any three physiological processes in plants which are affected by light.


Three physiological processes in plants which are affected by light are; photosynthesis transpiration and flowering.

Question 11:

In the figure of Sigmoid growth curve given below, label segments 1, 2 and 3.

Chapter 15 - Question 11 - Images of Sigmoid Growth Curve Gi …

Sigmoid Growth Curve Given Below

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The segments labelled 1,2,3 are as follows:

1: Lag phase: During this period, the cells are active metabolically only increasing their cell size and synthesizing enzymes required for cell division under environmental conditions that are given.

2: Log phase: In this phase, the cell increases in number in a logarithmic pattern resulting in balanced increase of cell constituents. This phase shall continue unless nutrients are depleted and toxic materials accumulation reaches a threshold level.

3: Stationary phase: This phase occurs when the nutrients get depleted and toxic by-products accumulate thus not leading to anymore increase in cell number.

Question 12:

Growth is one of the characteristic of all living organism? Do unicellular organism also grow? If so, what are the parameters?


Characteristics of growth are increase in mass of the cell and increase in cell number.

Yes, unicellular organisms also grow. When it comes to a multicellular and unicellular organisms, it grows by dividing its cells or undergoing cell division by increase in cell mass as well as increase in number of cells. This can be observed by observing the cells under microscope and counting the increase in number of cells.

Question 13:

The rice seedlings infected with fungus Gibberlla fujikuroi is called foolish seedlings? What was the reason behind it?


The property of Gibberellin that causes foolish seedling disease in rice is the elongation of internodes.

E. Kurosawa reported the symptoms of the disease in uninfected rice seedlings when they were treated with gibberellic acid.

Short Answer Type Questions

Question 1:

Nicotiana tabacum, a Short Day Plant, when exposed to more than critical period of light fails to flower. Explain.


Short day plants need a prolonged dark period to flower because during this time all the chemical transformations occur in the plant which aids in the flowering of the plant. It requires a short day exposure to light in order to flower but beyond a certain point when exposed to more than the critical period of light, short day plants will not be able to flower. So Nicotiana tabacum being a short day plant is unable to flower because of the above stated reasons.