EXERCISES

Q1. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
Answer:

• Growth is the permanent and irreversible increase in size, mass, or number of cells, usually accompanied by metabolic activity.
  Differentiation is the process by which new cells mature to perform specific functions.
• Development includes all the changes a plant undergoes during its life, from germination to death.
• Dedifferentiation means when mature cells regain the ability to divide again, like the formation of meristem from parenchyma cells.
• Redifferentiation is when dedifferentiated cells again become specialized to perform specific roles.
• Determinate growth is limited growth that stops after reaching a particular stage, as seen in leaves and flowers.
• Meristem is a group of continuously dividing cells found mainly at the tips of roots and shoots.
• Growth rate refers to the increase in growth per unit time, measured using arithmetic or geometric methods.

Q2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer: No single parameter can show growth in all stages of a plant’s life because different parts grow in different ways. For example, the length of roots, area of leaves, and volume of fruits all increase differently. Therefore, multiple parameters like length, weight, or volume are needed to describe overall growth accurately.

Q3. Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates

(a) Arithmetic growth
Answer: In arithmetic growth, one daughter cell continues to divide while the other differentiates. Growth occurs at a constant rate and produces a straight-line graph. It is represented by the equation
Lₜ = L₀ + rt,
where Lₜ is the length at time t, L₀ is the initial length, and r is the growth rate.

(b) Geometric growth
Answer: In geometric growth, both daughter cells divide and increase rapidly. Growth starts slowly in the lag phase, increases rapidly in the log phase, and then slows down in the stationary phase. The curve formed is S-shaped and expressed as
W₁ = W₀eʳᵗ,
where W₁ is the final size, W₀ is the initial size, r is the growth rate, and t is time.

(c) Sigmoid growth curve
Answer: A sigmoid growth curve shows three stages: a slow beginning (lag phase), a rapid middle phase (log phase), and a slow final stage (stationary phase). It represents natural growth in living organisms.

(d) Absolute and relative growth rates
Answer: Absolute growth rate measures total growth per unit time, while relative growth rate measures growth per unit of the initial size per unit time. For example, a smaller leaf may show a higher relative growth rate even if both leaves grow by the same amount.

Q4. List five main groups of natural plant growth regulators (PGR). Write a note on discovery, physiological functions and applications of any one of them.
Answer: The five main groups of natural plant growth regulators are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

For example, Auxins were first discovered by Charles Darwin and later studied by F.W. Went. The main natural auxin is indole-3-acetic acid (IAA). It helps in root formation, apical dominance, fruit development, and xylem differentiation. It also prevents premature fruit and leaf drop. Auxins are used in agriculture to promote rooting, induce seedless fruit formation, and control weeds using synthetic auxins like 2,4-D.

Q5. Why is abscisic acid also known as stress hormone?
Answer: Abscisic acid is called the stress hormone because it helps plants survive during unfavorable conditions such as drought. It causes the stomata to close and prevents water loss. It also induces seed dormancy so that seeds can survive dry or harsh conditions.

Q6. Both growth and differentiation in higher plants are open. Comment.
Answer: Growth and differentiation in plants are open because they continue throughout the life of the plant. Meristematic cells divide repeatedly, so growth remains open. The same cells can also differentiate into various types of tissues depending on their position and need, making differentiation open too.

Q7. Both a short-day plant and a long-day plant can produce flowers simultaneously in a given place. Explain.
Answer: Both types of plants can flower together if their light and dark requirements are satisfied. This can happen naturally when the duration of light and darkness matches both their needs or artificially by controlling the period of light and darkness in the environment.

Q8. 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.
Answer:
(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 – Gibberellins
(f) Induce immediate stomatal closure in leaves – Abscisic acid

Q9. Would a defoliated plant respond to photoperiodic cycle? Why?
Answer: No, a defoliated plant cannot respond to a photoperiodic cycle because leaves are responsible for detecting light duration. Without leaves, the plant cannot sense day length, and flowering will not be induced.

Q10. 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
Answer:

(a) GA₃ is applied to rice seedlings – The seedlings will grow abnormally tall, showing symptoms similar to the ‘foolish seedling disease’.

(b) Dividing cells stop differentiating – The plant will develop an undifferentiated mass of cells called a callus without forming proper organs.

(c) A rotten fruit gets mixed with unripe fruits – The unripe fruits will ripen faster because ethylene gas from the rotten fruit triggers ripening.

(d) You forget to add cytokinin to the culture medium – The callus will not grow shoots, as cytokinins are required for cell division and shoot formation.