Q.1. Define growth, differentiation, development, de-differentiation, redifferentiation, determinate growth, meristem, and growth rate.
(a) Growth: It is an irreversible and permanent process, accomplished by an increase in the size of an organ or organ parts or even of an individual cell.
Growth in Plants
(b) Differentiation: It is a process in which the cells derived from the apical meristem (root and shoot apex) and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions.
(c) Development: It refers to the various changes occurring in an organism during its life cycle – from the germination of seeds to senescence.
(d) De-differentiation: It is the process in which permanent plant cells regain the power to divide under certain conditions.
(e) Re-differentiation: It is the process in which de-differentiated cells become mature again and lose their capacity to divide.
(f) Determinate growth: It refers to limited growth.
Example: Animals and plant leaves stop growing after having reached maturity.
(g) Meristem: In plants, growth is restricted to specialized regions where active cell divisions take place. Such a region is called meristem.
There are three types of meristems: Apical meristem, Lateral meristem, and Intercalary meristem.
(h) Growth rate: It can be defined as increased plant growth per unit time.
Q.2. Why is not anyone parameter good enough to demonstrate growth throughout the life of a flowering plant?
Ans. Growth is a consequence of the increase in the quantity of protoplasm. Measuring the protoplasmic growth includes several parameters, to name a few – increase in height, weight, number of cells, fresh tissue sample, length, area, volume, etc. Hence it is difficult to demonstrate any one parameter of growth throughout the life of a flowering plant. Measurement of Plant
Q.3. Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
(a) Arithmetic growth: In arithmetic growth, one of the daughter cells continues to divide, while the other differentiates into maturity. The elongation of roots at a constant rate is an example of arithmetic growth.
It can be mathematically expressed as follows:
Lt = Lo + rt
Lt = length of time ‘t’
Lo = length at time ‘zero’
r = growth rate/elongation per unit time
(b) Geometric growth: Geometric growth is characterized by slow growth in the initial stages and rapid growth during the later stages. The daughter cells derived from mitosis retain the ability to divide but slow down because of a limited nutrient supply.
(c) Sigmoid growth curve: The growth of living organisms in their natural environment is characterized by an S-shaped curve called a sigmoid growth curve. This curve is divided into three phases – lag phase, log phase, or exponential phase of rapid growth, and stationary phase.
(d) Absolute and relative growth rates: Absolute growth rate refers to the measurement and comparison of total growth per unit time. Relative growth rate refers to the growth of a particular system per unit of time, expressed on a common basis.
Q.4. List five main groups of natural plant growth regulators. Write a note on the discovery, physiological functions, and agricultural/horticultural applications of any one of them.
Ans. Plant growth regulators are the intercellular intrinsic factors (chemical substances) that are responsible for the growth and development of plants.
The five main groups of natural plant growth regulators (PGR) are as follows:
(iv) Abscisic acid
These PGRs are synthesized in different plant parts, governing various differentiation and developmental events that take place in the life cycle of a plant.
Q.5. What do you understand by photoperiodism and vernalisation? Describe their significance.
Q.6. Why is Abscisic acid also known as stress hormone?
Abscisic acid is called stress hormones as it induces various responses in plants against stress conditions:
Q.7. 'Both growth and differentiation in higher plants are open'. Comment.
Ans. Growth and development in higher plants are 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.
Ans. The flowering response in short-day plants and long-day plants is dependent on the duration 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.
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.
(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
Q.10. Would a defoliated plant respond to the photoperiodic cycle? Why?
Ans. No, a defoliated plant will not respond to the photoperiodic cycle. This is because the leaves are the sites of perception of dark or light duration. Hence, if leaves were not present, plants 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.
(a) If GA3 is applied to rice seedlings, then the rice seedlings will exhibit internode elongation and increase in height.
GA3 effect on plants(b) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cells is called a 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.