This chapter covers the “Energy Release” phase of plant life—how the sugars made during photosynthesis are broken down to power the plant’s growth and survival.

Growth and Development: Mastering the Plant Life Cycle

Plant growth is a unique phenomenon. Unlike animals, plants retain the ability for “open growth” throughout their lives, thanks to specialized tissues called meristems. But growth isn’t just about getting bigger; it’s about Development—the sum of growth and differentiation.

In this chapter, we explore how a plant “decides” to grow, how it responds to its environment (Plasticity), and the “Chemical Messengers” (Phytohormones) that tell a tree when to drop its leaves or a flower when to bloom.

The Core Pillars of Plant Development

1. The Phases of Growth

Growth follows a predictable Sigmoid Curve in most living organisms under natural conditions:

• Lag Phase: Initial slow growth.
• Log (Exponential) Phase: Rapid, maximum growth.
• Stationary Phase: Growth slows down due to limited resources.

2. Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: Cells from meristems settle into specific roles (e.g., becoming a tracheary element).
• Dedifferentiation: Living differentiated cells regain the capacity to divide (e.g., formation of cork cambium).
• Redifferentiation: Dedifferentiated cells lose their ability to divide again to take on mature roles.

3. Plant Growth Regulators (PGRs)

These are the “Big Five” hormones that control everything:

• Auxins: Apical dominance and rooting.
• Gibberellins: Stem elongation and breaking seed dormancy.
• Cytokinins: Cell division and delaying senescence.
• Abscisic Acid (ABA): The “Stress Hormone”; closes stomata.
• Ethylene: Fruit ripening and abscission (leaf fall).

The Gauntlet: 10 Challenging Aptitude Questions

Question 1: Arithmetic vs. Geometric Growth

In Arithmetic growth, only one daughter cell continues to divide while the other differentiates. In Geometric growth, both daughter cells divide. Which of these is represented by the formula L_t = L₀ + rt?

Question 2: The “Plasticity” Example

Plants like Cotton, Coriander, and Larkspur show different leaf shapes in the juvenile stage vs. the adult stage. What is this phenomenon called, and how does it differ in the Buttercup (Ranunculus)?

Question 3: The Discovery of Auxin

Charles Darwin and his son Francis observed that the coleoptile of canary grass bent towards light. Who was the scientist that finally isolated “Auxin” from the tips of oat seedlings?

Question 4: Bolting and Gibberellins

If you apply Gibberellins to “rosette” plants like Cabbage or Beet, they undergo a process called Bolting. What is bolting, and why is it commercially useful?

Question 5: The Coconut Milk Factor

Cytokinins were discovered during the search for factors that stimulate cell division. What was the first natural cytokinin isolated from corn kernels and coconut milk called?

Question 6: The “Stress Hormone”

Why is Abscisic Acid (ABA) called the stress hormone? Mention its specific role during a severe drought.

Question 7: The Fruit Ripening Gas

Ethylene is the only gaseous plant hormone. It causes a “Respiratory Climactic” in ripening fruits. What does this term mean for the metabolic rate of the fruit?

Question 8: Photoperiodism

Some plants require a specific duration of light to flower. Distinguish between Short Day Plants (SDP) and Long Day Plants (LDP) based on their “Critical Duration.”

Question 9: Vernalisation

Some plants are “chilled” to promote flowering. What is this process called, and which specific plant organ perceives the cold stimulus?

Question 10: Seed Dormancy

A seed may fail to germinate even when environment conditions (water, temp) are favorable. Name two internal chemical inhibitors that cause this seed dormancy.

Detailed Explanations & Solutions

1. Arithmetic Growth

The formula L_t = L₀ + rt represents a straight line graph.

Result: Arithmetic Growth. (Geometric growth is represented by W₁ = W₀ e^rt).

2. Plasticity

This is called Heterophylly. In Cotton/Larkspur, it’s due to the phases of life.

Result: In Buttercup, it’s due to the environment (different leaf shapes in air vs. water).

3. Isolation of Auxin

Result: F.W. Went. (He isolated it using Agar blocks).

4. Bolting

Bolting is the sudden elongation of internodes just prior to flowering.

Result: It increases the yield of crops where the stem or flower head is the product.

5. Cytokinins

Result: Zeatin. (The first one discovered was actually Kinetin from herring sperm DNA, but Zeatin is the natural plant version).

6. The Stress Hormone

ABA inhibits plant metabolism and promotes seed dormancy.

Result: During drought, it triggers the rapid closure of stomata to prevent water loss.

7. Respiratory Climactic

Result: It refers to a sudden, massive rise in the rate of respiration during the ripening of the fruit.

8. Photoperiodism

• LDP: Flower only when light exposure exceeds the critical duration.
• SDP: Flower only when light exposure is less than the critical duration.
• Result: It’s actually the duration of the dark period that is often more critical.

9. Vernalisation

Result: The stimulus of low temperature is perceived by the Shoot Apex or the Embryo, not the leaves.

10. Seed Dormancy

Result: Abscisic acid (ABA), Phenolic acids, and Para-ascorbic acid.

Pro-Tip: The “Antagonistic” Pair

When studying for exams, remember that Gibberellins and Abscisic Acid are often opposites. Gibberellins break dormancy and start growth; ABA promotes dormancy and stops growth!