In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant

NCERT Exemplar · Q33

Q. In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant hormones formed? How are the hormones translocated to the site of activity?

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Concept understanding — Plant Growth Regulators

Plant Growth Regulators: The Plant's Chemical Messengers

Plants can't run away from trouble. They can't chase food or find a mate. So how do they decide when to grow, when to flower, when to drop leaves, or when to defend themselves? They use chemical signals — tiny molecules that act like a postal system inside the plant, carrying instructions from one cell to another.

These chemical signals are called Plant Growth Regulators (PGRs). Think of them as the plant's hormones, but with a twist: unlike animal hormones that mostly travel through blood, plant regulators can work right where they're made, move to nearby cells, or travel through the plant's vascular system. They work in tiny amounts — parts per million or even parts per billion — but their effects are dramatic.

The Two Big Families

All PGRs fall into two broad groups based on what they do:

Growth Promoters — these encourage cell division, cell enlargement, flowering, fruit formation. They make the plant grow bigger and develop faster.

Growth Inhibitors — these slow down growth, promote dormancy, help the plant survive stress, and cause aging and ripening.

Important

The balance between promoters and inhibitors — not the absolute amount of any one regulator — determines what the plant actually does. A seed stays dormant when inhibitors dominate; it germinates when promoters take over.

The Five Classical Regulators

Indian exam syllabi (CBSE, ICSE, state boards) focus on five major PGRs. Here they are, grouped by function:

Regulator	Type	Where it's made	Key effects
Auxin	Promoter	Shoot tips, young leaves	Cell elongation, apical dominance, root initiation
Gibberellin	Promoter	Young tissues, seeds	Stem elongation, seed germination, fruit growth
Cytokinin	Promoter	Root tips	Cell division, delay aging, shoot formation
Abscisic acid	Inhibitor	Leaves, fruits	Stomatal closure, seed dormancy, stress response
Ethylene	Inhibitor (mostly)	Ripening fruits, aging tissues	Fruit ripening, leaf fall, senescence

Note

Ethylene is the only gaseous PGR. It's a simple hydrocarbon (C₂H₄) — two carbons, four hydrogens — yet it controls when your banana turns yellow and your apple gets sweet.

How They Work Together: A Real Example

Take a seed buried in soil. It's full of abscisic acid — the inhibitor that keeps it dormant. The seed waits. When enough water soaks in and temperature rises, the balance shifts: gibberellins are produced, which break down the stored food and push the shoot upward. Auxin then takes over at the tip, making cells elongate and guiding the shoot toward light. Meanwhile, cytokinins from the growing root tip encourage cell division in the shoot. Later, when fruit forms, ethylene triggers ripening and eventually leaf fall.

No single regulator works alone. They interact, oppose each other, and amplify each other's effects.

Why This Matters for Exams

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Unlike animals, plants have no dedicated glands for hormone production — plant growth regulators are made directly by growing tissues such as the shoot and root apices and then transported internally to wherever they are needed.

Animals rely on specialised endocrine glands to secrete their hormones, but plants have no equivalent structures. Instead, the intercellular chemical messengers known as plant growth regulators (PGRs) are synthesised directly by ordinary plant tissues, in the parts of the plant body where they are needed.

Auxins are generally produced by the growing apices of the stems and roots.
Cytokinins are synthesised specifically in regions of rapid cell division, such as root apices, developing shoot buds and young fruits.
Gibberellins are synthesised in various parts of the plant. …

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Q.In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant hormones formed? How are the hormones translocated to the site of activity?

Concept understanding — Plant Growth Regulators

Plant Growth Regulators: The Plant's Chemical Messengers

The Two Big Families

The Five Classical Regulators

How They Work Together: A Real Example

Why This Matters for Exams

Method: Tracing a Signal From Its Site of Synthesis to Its Site of Action

Steps

Q.In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant hormones formed? How are the hormones translocated to the site of activity?

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