NCERT Class 11-Biology: Chapter –15 Plant Growth and Development Part 3

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Question 3:

A primary root grows from to in a week. Calculate the growth rate and relative growth rate over the period.


Absolute Growth rate can be calculated as follows:

Where, size (weight, height, number etc.)

Initial size at the beginning of the period

of natural logarithms

Here, r is the relative growth rate







Or, r

Relative growth rate can be calculated as follows:

Absolute growth rate

Relative growth rate

Question 4:

Gibberellins were first discovered in Japan when rice plants were suffering from bakane (the foolish seedling disease) caused by a fungus Gibberella fujikuroi.

a. Give two functions of this phytohormone.

b. Which property of Gibberellin caused foolish seedling disease in rice?


(a) Two functions of the phytohormone (plant hormones that are responsible for signalling in plants and occur in small concentrations) Gibberellin are:

(i) Gibberellins cause fruits to elongate and improve in its shape. It is also responsible to delay senescence.

(ii) Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the sugar production that increases the yield by almost 20 tonnes per acre.

(iii) Speeds up the malting process in brewing industry (GA3 is one of the kinds of gibberellins).

(b) 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.

Question 5:

Gibberellins promote the formation of ______ flowers on genetically _____ plants in Cannabis whereas ethylene promotes formation of _______ flowers on genetically ______ plants.


Gibberellins promote the formation of male flowers on genetically female plants in Cannabis whereas ethylene promotes formation of female flowers on genetically male plants.

GA does not directly promote stamen differentiation in the bud but suppress the formation of female flowers due to which male flowers eventually develop.

Ethylene induces flowering so can be used to increase the number of female flowers in the cucumber plants. It selectively induces flowering of female plants suggesting a link between ethylene signalling and primordial anther specific DNA damage in female cucumber flowers.

Question 6:

Classify the following plants into Long-Day Plants (LDP), Short Day Plants (SDP) and Day Neutral Plants (DNP) Xanthium, Henbane

(Hyoscyamus Niger), Spinach, Rice, Strawberry, Bryophyllum, Sunflower, Tomato, Maize.


Question 7:

A farmer grows cucumber plants in his field. He wants to increase the number of female flowers in them. Which plant growth regulator can be applied to achieve this?