Which one among the following is a plant hormone?

1. Introduction to Chemical Coordination (basic)

In the vast world of biology, coordination is the process through which various organs and systems of a living organism work together in a synchronized manner to maintain a stable internal environment and respond to external stimuli. While complex animals like humans possess a nervous system to send lightning-fast electrical signals, plants and simpler organisms lack these specialized tissues. Instead, they rely primarily on chemical coordination to manage growth, development, and responses to the environment Science, Class X, Control and Coordination, p. 100.

At the heart of chemical coordination are hormones—chemical messengers produced in minute quantities by specific cells or glands. Unlike electrical impulses that require a "wire" (a nerve fiber) to reach a destination, hormones are released into the organism's transport system (like blood in animals or through diffusion and vascular tissues in plants) to reach distant target sites. Once they arrive, they trigger specific physiological changes Science, Class X, Control and Coordination, p. 111. For instance, in humans, the thyroid gland produces thyroxin to regulate metabolism, ensuring the body has the right balance for growth Science, Class X, Control and Coordination, p. 110.

To understand why organisms use chemical signaling instead of just electrical signals, we can compare the two systems:

Feature	Nervous Coordination	Chemical Coordination
Medium	Electrical impulses	Chemical messengers (Hormones)
Speed	Very rapid/instantaneous	Slower and steady
Reach	Only to cells connected by nerves	Can reach potentially every cell in the body
Duration	Short-lived effects	Often long-lasting effects (e.g., growth)

In plants, this chemical system is the only way to manage life processes because they do not have a nervous system Science, Class X, Control and Coordination, p. 112. These plant-specific chemicals are called phytohormones. They guide everything from the direction a shoot grows toward the light to the timing of fruit ripening and leaf fall. Thus, chemical coordination acts as the "biological software" that directs the hardware of the organism's body to function harmoniously Science, Class VII, Adolescence: A Stage of Growth and Change, p. 84.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.100; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.110; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.111; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.112; Science, Class VII (NCERT Revised ed 2025), Chapter: Adolescence: A Stage of Growth and Change, p.84

2. The Human Endocrine System (basic)

In our journey through biological coordination, we now move from plants to the complex world of the Human Endocrine System. While plants use hormones to control directional growth (like bending toward light), animal bodies use hormones as chemical messengers to coordinate growth and physiological functions in carefully controlled locations. Unlike the nervous system, which uses electrical impulses for rapid responses, the endocrine system uses hormones—chemicals secreted by ductless glands directly into the blood—to create long-lasting and widespread effects throughout the body Science, Class X, Chapter 6, p. 109.

The system is governed by a sophisticated hierarchy. At the top is the Hypothalamus, which acts as the bridge between the nervous system and the endocrine system. It releases "releasing factors" that signal the Pituitary Gland (the master gland) to secrete hormones like Growth Hormone. If these levels are imbalanced during childhood, it can lead to dwarfism or gigantism Science, Class X, Chapter 6, p. 110. Other critical glands include the Thyroid, which requires iodine to produce Thyroxin. This hormone is essential because it regulates the metabolism of carbohydrates, proteins, and fats to ensure balanced growth Science, Class X, Chapter 6, p. 110.

Gland	Hormone	Primary Function
Thyroid	Thyroxin	Regulates metabolism for growth.
Pancreas	Insulin	Regulates blood sugar levels Science, Class X, Chapter 6, p. 111.
Adrenal	Adrenaline	Prepares the body for "fight or flight" situations.
Testes/Ovaries	Testosterone/Oestrogen	Changes associated with puberty and reproduction Science, Class X, Chapter 7, p. 123.

A vital feature of this system is the Feedback Mechanism. Hormones must be secreted in precise quantities; too much or too little can be harmful. For instance, when blood sugar levels rise, the Pancreas detects this and increases insulin production. As sugar levels fall, insulin secretion is automatically reduced. This ensures the body maintains a stable internal environment Science, Class X, Chapter 6, p. 111.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.109; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.110; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.111; Science, Class X (NCERT 2025 ed.), Chapter 7: How do Organisms Reproduce?, p.123

3. The Human Nervous System: An Adjacent Control Mechanism (intermediate)

In the animal kingdom, survival often depends on split-second coordination—whether it is escaping a predator or pulling your hand away from a hot surface. To achieve this, animals have evolved a specialized Nervous System. Unlike the slower, growth-based responses seen in plants, the nervous system uses electrical impulses to transmit messages almost instantaneously. This system acts as an 'adjacent control mechanism' because it works alongside the endocrine (hormonal) system to ensure every part of the body reacts harmoniously to external stimuli Science, Class X (NCERT 2025 ed.), Control and Coordination, p.111.

The structural architecture of this system is divided into two main parts: the Central Nervous System (CNS), consisting of the brain and spinal cord, and the Peripheral Nervous System (PNS). The PNS is made up of cranial nerves (arising from the brain) and spinal nerves (arising from the spinal cord), which facilitate communication between the CNS and the rest of the body Science, Class X (NCERT 2025 ed.), Control and Coordination, p.103. While the brain is responsible for complex integration and 'thinking' actions, certain emergency responses—known as reflex arcs—bypass the 'thinking' part of the brain initially, allowing for a much faster output action via the spinal cord to prevent injury Science, Class X (NCERT 2025 ed.), Control and Coordination, p.102.

It is important to note the fundamental difference in how organisms respond to their environment. While animals use this complex network of nerves and muscles to move quickly, plants have neither a nervous system nor muscles. When a plant moves—such as the folding of leaves in a 'touch-me-not' (Mimosa pudica) or a stem growing toward light—it is usually due to changes in cell shape through water regulation or chemical signaling, rather than nervous conduction Science, Class X (NCERT 2025 ed.), Control and Coordination, p.105.

Feature	Nervous System (Animals)	Control in Plants
Primary Signal	Electrical Impulses	Chemical/Hormonal Signals
Speed	Very Rapid	Slow/Growth-dependent
Hardware	Nerves and Muscles	No nerves or muscles

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.102; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.103; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.105; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.111

4. Plant Movements and Tropisms (intermediate)

In the plant world, movement isn't about running or jumping; it is a sophisticated response to environmental triggers. Since plants lack a nervous system, they use directional growth to navigate their surroundings. These directional growth movements are known as tropisms. A tropism can be either 'positive' (towards the stimulus) or 'negative' (away from the stimulus). For instance, shoots generally show positive phototropism by bending towards light, while roots exhibit positive geotropism by growing downwards into the soil in response to gravity Science, Class X (NCERT 2025 ed.), Chapter 6, p.107. This ensures that the plant maximizes its energy intake from the sun and its nutrient/water intake from the earth.

One of the most fascinating examples of tropism is thigmotropism, or the response to touch. You might have noticed how pea plants use tendrils to climb fences. These tendrils are highly sensitive to contact. When a tendril touches a support, the part of the tendril in direct contact does not grow as rapidly as the part away from the object. This differential growth rate causes the tendril to circle the object and cling to it tightly Science, Class X (NCERT 2025 ed.), Chapter 6, p.106. It is a slow, calculated movement that allows the plant to reach for the sky without having a rigid woody trunk.

Beyond light and gravity, plants respond to water (hydrotropism) and chemicals (chemotropism). A classic example of chemotropism is the growth of a pollen tube towards an ovule during fertilization. These movements are essential for the plant's survival and reproduction. While we often think of plants as stationary, they are constantly 'moving' in slow motion to optimize their position in the ecosystem.

Type of Tropism	Stimulus	Example of Movement
Phototropism	Light	Shoots bending towards a window
Geotropism	Gravity	Roots growing downwards into the earth
Thigmotropism	Touch	Tendrils twining around a support
Chemotropism	Chemicals	Growth of pollen tubes towards ovules

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.106-107

5. Classification of Plant Growth Regulators (Phytohormones) (intermediate)

In the plant kingdom, growth and development are not random; they are meticulously coordinated by chemical messengers known as Plant Growth Regulators (PGRs) or Phytohormones. Unlike animals, plants lack a nervous system, so they rely entirely on these chemical signals to respond to their environment, such as bending toward a window or shedding leaves in autumn Science, Class X (NCERT 2025 ed.), Chapter 6, p.106. These substances are produced in minute quantities but have profound effects on the plant's physiology.

Broadly, we classify phytohormones into two functional groups based on their primary action: Growth Promoters and Growth Inhibitors. Promoters are responsible for activities like cell division, cell enlargement, and flowering. Inhibitors, on the other hand, act as the plant's "brakes," helping it respond to stress or enter dormancy when conditions are unfavorable Science, Class X (NCERT 2025 ed.), Chapter 6, p.108.

Hormone Group	Key Examples	Primary Functions
Growth Promoters	Auxins, Gibberellins, Cytokinins	Stimulate stem elongation, promote cell division in fruits/seeds, and facilitate directional growth (tropism).
Growth Inhibitors	Abscisic Acid, Ethylene*	Induce dormancy, promote leaf wilting (abscission), and inhibit growth during stress.

*Note: Ethylene is unique as it can act as both, but is often associated with fruit ripening and senescence (aging).

It is fascinating to note that while animal hormones (like insulin or thyroxin) regulate metabolic stability, plant hormones are deeply involved in directional growth. You will never see an animal grow specifically toward light, but a plant's hormonal balance ensures its stems reach for the sun (phototropism) and its roots dive for water Science, Class X (NCERT 2025 ed.), Chapter 6, p.109. Understanding this classification helps us comprehend how a stationary organism manages to be so dynamic in its development.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.106, 108-109

6. Gibberellins: Functions and Physiological Effects (exam-level)

Gibberellins are a group of naturally occurring plant hormones (phytohormones) that act as powerful growth regulators. While they are found throughout the plant body, they are particularly influential in the elongation of stems and the initiation of germination. Historically, they were discovered through the study of the "foolish seedling" disease in rice, where a fungus called Gibberella fujikuroi caused plants to grow excessively tall and thin, eventually collapsing. This discovery revealed that plants produce their own version of these chemicals to manage height and development. Science, Class X (NCERT 2025 ed.), Chapter 6, p. 108

The physiological effects of gibberellins are diverse but generally focus on promoting growth. Their primary functions include:

• Stem Elongation: Like auxins, gibberellins promote the growth of the stem, but they specifically stimulate the lengthening of the internodes (the regions between leaves). This is often used commercially to increase the length of grape stalks or to cause "bolting" (rapid stem growth) in rosette plants like cabbage.
• Breaking Dormancy: Gibberellins are the "wake-up call" for seeds. They trigger the production of digestive enzymes (like amylase) that break down stored starch into sugars, providing the energy needed for a seedling to emerge. Science, Class X (NCERT 2025 ed.), Chapter 7, p. 121
• Fruit Growth: They contribute to the development and ripening of fruits, often working alongside other hormones to increase fruit size and delay senescence (aging).

In the delicate balance of plant life, gibberellins act as an antagonist to Abscisic Acid (ABA). While ABA acts as a growth inhibitor—inducing seed dormancy and the wilting of leaves to conserve resources—gibberellins drive the plant forward into active growth and reproduction. Science, Class X (NCERT 2025 ed.), Chapter 6, p. 108

Feature	Gibberellins	Abscisic Acid (ABA)
Primary Role	Growth Promoter	Growth Inhibitor
Effect on Seeds	Promotes Germination	Maintains Dormancy
Stem Effect	Stimulates Elongation	No direct elongation effect

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.108; Science, Class X (NCERT 2025 ed.), Chapter 7: How do Organisms Reproduce?, p.121

7. Solving the Original PYQ (exam-level)

Now that you have mastered the distinction between chemical coordination in plants and animals, this question serves as a direct application of that knowledge. In your previous modules, you learned that plants lack a nervous system and instead rely on phytohormones to regulate growth and development. This question requires you to sift through a list of biochemicals and categorize them based on their biological origin—specifically identifying which one belongs to the plant growth regulator family.

To arrive at the correct answer, (C) Gibberellin, you should recall that it is one of the five primary plant hormones responsible for stem elongation and breaking seed dormancy. As highlighted in Science, class X (NCERT 2025 ed.), Gibberellins help in the growth of the stem, much like Auxins, but with a specific focus on internodal expansion. If you remembered the "foolish seedling" disease in rice mentioned in your biology concepts, you would instantly connect that fungal discovery to the hormone that makes plants grow excessively tall.

UPSC often uses common animal hormones as distractors to test your fundamental classification skills. Options (A), (B), and (D)—Insulin, Thyroxin, and Estrogen—are all part of the human endocrine system. These are produced by specialized glands like the pancreas, thyroid, and ovaries to regulate glucose, metabolism, and reproduction respectively. By recognizing these as animal-specific regulators, you can use the process of elimination to confidently select the only phytohormone listed.