Question 1

Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.

Accepted Answer

Growth: An irreversible permanent increase in size and volume of an organ or organism.
Differentiation: The process by which cells or tissues undergo a change toward a more specialized form or function.
Development: The sum of all changes an organism undergoes from germination to maturity.
Dedifferentiation: The process by which differentiated cells lose their specialization and revert to a more primitive form.
Redifferentiation: The process by which dedifferentiated cells regain their specialized structure and function.
Determinate growth: Growth that stops once a genetically pre-determined structure has formed.
Meristem: Regions of actively dividing cells in plants responsible for growth.
Growth rate: The rate at which growth occurs, often measured as increase in size or mass per unit time. — Each term defines a fundamental concept in plant growth and development. Growth refers to size increase, differentiation to specialization, development to overall changes, dedifferentiation and redifferentiation to changes in cell specialization states, determinate growth to limited growth, meristems to growth tissues, and growth rate to the speed of growth.

Question 2

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

Accepted Answer

Because growth in plants is a complex process involving increase in cell number, cell size, and differentiation, no single parameter such as height, weight, or cell number alone can fully represent growth throughout the plant's life. Different parameters may be relevant at different stages or parts of the plant. For example, increase in height may not reflect biomass increase, and dry weight may not capture cell division rates. Therefore, multiple parameters are needed to comprehensively demonstrate growth. — Growth involves multiple facets including cell division, elongation, and differentiation. Since these occur at different rates and times in various plant parts, relying on one parameter can be misleading. Hence, a combination of parameters is necessary for accurate growth assessment.

Question 3

Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates

Accepted Answer

(a) Arithmetic growth: Growth in which the increase in size or volume is by a constant amount over equal time intervals, resulting in a straight line when plotted against time.

(b) Geometric growth: Growth in which the increase is proportional to the current size, leading to exponential increase and a curve when plotted against time.

(c) Sigmoid growth curve: A growth curve that is S-shaped, showing a lag phase (slow growth), exponential phase (rapid growth), and stationary phase (growth ceases).

(d) Absolute growth rate: The increase in size or mass per unit time.
Relative growth rate: The increase in size or mass per unit size per unit time, often expressed as growth per unit biomass per day. — These terms describe different patterns and measurements of growth. Arithmetic growth is linear, geometric is exponential, sigmoid curve combines phases of growth, and absolute and relative growth rates quantify growth in different ways.

Question 4

List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Accepted Answer

Five main groups of natural plant growth regulators are:
1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic acid
5. Ethylene

Note on Auxins:
Discovery: First discovered by F.W. Went in 1926 through experiments on coleoptile curvature.
Physiological functions: Promote cell elongation, root initiation, apical dominance, and fruit development.
Agricultural/horticultural applications: Used to induce rooting in cuttings, prevent fruit drop, and as herbicides (e.g., 2,4-D). — Plant growth regulators control various aspects of growth and development. Auxins were the first discovered and have multiple roles including cell elongation and rooting. Their applications in agriculture include rooting agents and weed control.

Question 5

Why is abscisic acid also known as stress hormone?

Accepted Answer

Abscisic acid (ABA) is called the stress hormone because it accumulates in plants under stress conditions such as drought, salinity, and cold. It helps the plant to survive stress by inducing stomatal closure to reduce water loss, promoting seed dormancy, and inhibiting growth processes that are unfavorable under stress. — ABA mediates plant responses to adverse environmental conditions, helping conserve water and protect tissues, thus earning the name stress hormone.

Question 6

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

Accepted Answer

In higher plants, growth and differentiation are termed 'open' because they continue throughout the life of the plant. Meristems remain active and produce new cells that differentiate into various tissues, allowing continuous growth and development. This contrasts with animals where growth and differentiation are often determinate and cease after maturity. — The term 'open' indicates that growth and differentiation are not limited to a fixed period but are ongoing processes in plants due to persistent meristematic activity.

Question 7

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

Accepted Answer

Both short day plants (SDP) and long day plants (LDP) can flower simultaneously in a given place because flowering depends on the photoperiod (length of day/night) and the plant's critical day length. If the environmental day length matches the requirement of both types, or if the plants have different critical day lengths that are met simultaneously, both can flower together. Additionally, other factors like temperature and genetics also influence flowering. — Flowering is regulated by photoperiod sensitivity. Different plants have different critical day lengths, so under certain natural day lengths, both SDPs and LDPs can flower at the same time.

Question 8

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.

Accepted Answer

(a) Induce rooting in a twig: Auxins (e.g., IAA, IBA)
(b) Quickly ripen a fruit: Ethylene
(c) Delay leaf senescence: Cytokinins
(d) Induce growth in axillary buds: Cytokinins
(e) 'Bolt' a rosette plant: Gibberellins
(f) Induce immediate stomatal closure in leaves: Abscisic acid (ABA) — Each plant growth regulator has specific roles: auxins promote rooting, ethylene induces fruit ripening, cytokinins delay senescence and promote axillary bud growth, gibberellins promote bolting, and ABA causes stomatal closure under stress.

Question 9

Would a defoliated plant respond to photoperiodic cycle? Why?

Accepted Answer

No, a defoliated plant would not respond to the photoperiodic cycle because leaves are the primary sites for perceiving photoperiod (light and dark periods). Photoreceptors in leaves detect day length and initiate flowering signals. Without leaves, the plant cannot sense photoperiod and thus cannot respond appropriately. — Leaves contain photoreceptors that detect light duration, which is essential for photoperiodic response. Defoliation removes these sensors, preventing the plant from responding to day length cues.

Question 10

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.

Accepted Answer

(a) GA₃ applied to rice seedlings would promote stem elongation, breaking of dormancy, and faster growth.

(b) If dividing cells stop differentiating, growth would be abnormal as cells would not mature into functional tissues, leading to developmental defects.

(c) A rotten fruit mixed with unripe fruits would release ethylene gas, accelerating ripening of the unripe fruits.

(d) Forgetting to add cytokinin to the culture medium would result in poor or no shoot formation, as cytokinins promote cell division and shoot initiation. — GA₃ (gibberellin) promotes elongation and growth; differentiation is essential for functional tissue formation; ethylene from rotten fruits acts as a ripening hormone; cytokinins are necessary for shoot development in tissue culture.

Question 11

Which of the following best defines growth in plants?

Accepted Answer

An irreversible permanent increase in size of an organ or cell accompanied by metabolic activity — Growth in plants is defined as an irreversible and permanent increase in size of an organ, part, or cell, accompanied by metabolic processes such as anabolic and catabolic reactions. Temporary swelling, such as wood swelling in water, is reversible and not considered growth.

Question 12

Identify the location of the root apical meristem, shoot apical meristem, and vascular cambium in a dicot plant and explain their roles in growth.

Accepted Answer

Root apical meristem is located at the tip of the root and is responsible for root elongation (primary growth). Shoot apical meristem is at the tip of the shoot and causes elongation of the shoot axis (primary growth). Vascular cambium is a lateral meristem located between xylem and phloem in stems and roots, responsible for secondary growth (increase in girth). — The root apical meristem and shoot apical meristem are responsible for primary growth by producing new cells that elongate the root and shoot respectively. The vascular cambium appears later and produces secondary xylem and phloem, increasing the thickness of the plant organs.

Question 13

Which of the following parameters is NOT commonly used to measure plant growth?

Accepted Answer

Decrease in water content — Growth is measured by parameters proportional to increase in protoplasm such as fresh weight, dry weight, length, surface area, volume, and cell number. A decrease in water content is not a measure of growth.

Question 14

Describe the three phases of growth in plant roots and their characteristics.

Accepted Answer

The three phases of growth in plant roots are: (1) Meristematic phase, where cells actively divide and have thin primary walls and large nuclei; (2) Elongation phase, where cells increase in size due to vacuolation and new cell wall deposition; (3) Maturation phase, where cells attain final size and differentiate to perform specific functions, such as forming root cap and vascular tissues. — Growth in roots occurs in a sequence of phases. The meristematic zone contains actively dividing cells rich in protoplasm. The elongation zone follows where cells enlarge considerably. The maturation zone is where cells differentiate and mature, developing specialized structures needed for root function.

Question 15

The growth of a root elongating at a constant rate can be expressed by the formula $L_t = L_0 + r t$. If the initial length ($L_0$) is 2 cm and the growth rate ($r$) is 0.5 cm/day, what will be the length after 6 days?

Accepted Answer

5 cm — Given: L_0 = 2 cm, r = 0.5 cm/day, t = 6 days
Find: L_t
Formula: L_t = L_0 + r × t
Solution:
Step 1: Substitute values: L_t = 2 + 0.5 × 6
Step 2: Calculate: L_t = 2 + 3 = 5 cm
Answer: 5 cm
Note: Students often forget to multiply the growth rate by time.

Plant Growth and Development

Plant Growth and Development — Study Notes

13.1 Growth

13.1.1 Plant Growth Generally is Indeterminate

13.1.2 Growth is Measurable

Practice Questions — Plant Growth and Development

All 19 Chapters in Biology