NCERT Solutions for Class 11 Biology Chapter 15 - Plant Growth and Development

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“Plant Growth and Development” chapter of Biology Class 11 includes an explanation of topics such as growth differentiation, plant growth regulators, seed germination, redifferentiation and dedifferentiation, phases of plant growth and plant growth rate, vernalisation, photoperiodism, development, seed democracy, etc.

Question 1:

Define growth, differentiation, development, dedifferentiation, rediffere- ntiation, determinate growth, meristem and growth rate.


Growth. Growth is an increase in the amount of protoplasm, usually accompanied by an irreversible increase in size and weight involving the division, enlargement and the differentiation of cells.
Determinate Growth. It is a type of growth in which growth follows a precise pattern such that each part has its characteristic position and an unalterable fate and grow for a limited period.
Meristem. Specific areas in higher plants which take part in the formation of new cells are called meristems. The cells of such meristems have the capacity to divide and self-perpetuate.
Growth rate. The increased growth per unit time is termed as growth rate. Growth rate can be expressed mathematically.

Question 2:

Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?


Growth is indeterminate in higher plants and it may not be and is generally not sustained at a high rate throughout the life of a plant. Growth at a cellular level is principally a consequence of increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it. Growth is, therefore, measured by a variety of parameters

  1. Increase in fresh weight
  2. Increase in dry weight
  3. Increase in length, area and volume
  4. No increase in cell number.
Question 3:

Describe briefly :

  1. Arithmetic growth
  2. Sigmoid growth curve
  3. Absolute and relative growth rates

(a) Arithmetic growth. In arithmetic growth, following mitotic cell division, only one daughter cell continuous to divide while the other differentiates and matures. The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate. On plotting the length of the organ against time, a linear curve is obtained. Mathematically, it is expressed as :
Lt = L0 + rt
Lt = length at time ‘t’
L0 = length at time ‘zero’
r = growth rate / elongation per unit time.
(b) Sigmoid growth curve. Fig. 15.4. When the rate of growth of a cell, organ or an entire plant is plotted against time, an S-shaped curve, called the sigmoid curve is obtained. It shows lag phase, log (exponential) phase, diminishing phase and stationary phase. The rate of growth varies in different species and in different organs. Growth begins slowly, then enters a period of rapid enlarge- ment, following which it gradually decrease, till no further enlargement. The mathematical curve which represent this variation in growth rate is ‘S’ -shaped or sigmoid growth curve.
Three phases of growth curve
1. Lag phase—phase of cell division.
2. Exponential phase (Log phase)— phase of cell elongation.
3. Stationary phase (steady phase)—phase of cell maturation.
(c) Absolute and relative growth rate. Quantitative comparisons between the growth of living system can be made in two ways : (i) measure- ment and the comparison of total growth per unit time is called the absolute growth rate. (ii) The growth of the given system per unit time expressed on a common basis e.g. per unit initial parameter is called the relative growth rate.

Question 4:

Why is abscisic acid also known as stress hormone ?


Abscisic acid carries out the physiological function of increasing the tolerance of plants to various kinds of stresses. It leads to loss of turgor and closure of stomata. Hence it is also called stress hormone. Its production is stimulated by drought, water logging and other adverse conditions.

Question 5:

‘‘Both growth and differentiation in higher plants are ‘open’ ’’. Comment.


In higher plants, when growth occurs new cells are always being added to the plant body by the activity of the meristem. Such a type of growth is referred to as open form of growth. Even differentiation in plants is open because cells or tissues arising out of the same meristem have different structures at maturity. The final structure at maturity of a cell/tissue is determined by the location of the cell within.

Question 6:

‘‘Both a short day plant and a long day plant produce flower simultaneously in a given place’’. Explain.


Flowering in certain plants at a given place depends not only on a combination of hight and dark exposures but also their relative durations. For flowering both the duration of light and the duration of darkness are equally important. Some plants require the exposure to light for a period exceeding a well defined critical duration , such plants are short day plants. Certain other plants must be exposed to light for a period less than the critical duration before flowering is initiated and such plants are long day plants.

Question 7:

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

  1. induce rooting in a twig,
  2. quickly ripen a fruit,
  3. delay leaf senescence,
  4. induce growth in axillary buds,
  5. ‘bolting’ a rosette plant,
  6. induce immediate stomatal closure in leaves ?

(a) auxins (IAA) (b) ethylene (c) cytokinins (d) cytokinins (e) gibberellins (f) abscisic acid (ABA)

Question 8:

What would be expected to happen if :

  1. GA3 is applied to rice seedlings,
  2. dividing cells stop differentiating,
  3. a rotten fruit gets mixed with unripe fruits,
  4. you forget to add cytokinin to the culture medium ?
  1. Application of GA3 to rice seedlings shall cause an intense increase in cell growth of stem young leaves and other aerial parts.
  2. If dividing cells stop differentiating then there is the possibility of formation of a large mass of cells in the form of tumour that may cause cancerous condition.
  3. If a rotten fruit gets mixed with unripe fruit then all the unripe fruits ripen due to effect of ripening activity of ethylene being continuously produced by the rotten fruit.
  4. If we forget to add cytokinin to the culture medium then the cell division process may halt as cytokinin is a plant growth regulator that is responsible for the induction of cytokinesis.
Question 9:

Would a defoliated plant respond to photoperiodic cycle ? Why ?


A defoliated plant does not respond to photoperiodic cycle at all. This is because the site of perception of light or dark stimulus are the leaves. Leaves have the hormone florigen that makes them capable of responding to stimulus of photoperiodicity. So in the absence of any foliage over the plant, there is no response towards photoperiodic cycle.