How Does a Plant “Feel” Sunlight? Coordination in Plants

Place a potted plant by a window for a week, and its stem leans towards the glass. Turn the pot around, and within days the stem bends back again. The plant has no eyes, no nerves and no brain, yet it clearly knows where the light is.

This guide explains how: the chemical messengers that do the job nerves do in animals, the mechanism that bends a stem, the different kinds of plant movement, and the five hormones the Class 10 chapter expects you to know.

No Nerves, but Chemical Messengers

Animals coordinate their bodies with nerves and hormones. Plants have no nervous system, so they rely on plant hormones, chemicals made in one part of the plant that travel to another part and change how it grows. Because hormones work slowly, plant responses are mostly slow, and most of them are changes in growth.

Auxin and the Bending Stem

The hormone behind the bending is auxin. It is made in the tip of the shoot and helps cells elongate. Here is what happens when light falls on a plant from one side.

1. Light from one side reaches the shoot tip.
2. Auxin moves away from the light, so more of it collects on the shaded side of the stem.
3. The cells on the shaded side receive more auxin and elongate more than the cells on the lit side.
4. Because the shaded side grows longer, the stem bends towards the light.

That is the whole of phototropism: the plant is not leaning on purpose, it is simply growing faster on one side.

Tropisms: Growing Towards or Away

A directional growth response to a stimulus is called a tropism. If the plant grows towards the stimulus, the tropism is positive. If it grows away, it is negative.

Tendrils are a good example of how auxin works in more than one way. The side of the tendril touching the support grows less than the side away from it, so the tendril coils round the support and holds it.

Movements That Are Not Growth

Not every plant movement is slow growth. The leaves of the touch-me-not plant (Mimosa pudica) fold up within seconds when you touch them. No growth can happen that fast. Instead, special cells at the base of the leaf quickly lose water and shrink, so the leaf droops.

Five Hormones at a Glance

A memory aid is to pair each hormone with a verb: auxin elongates, gibberellins lengthen the stem, cytokinins divide, abscisic acid stops, and ethylene ripens.

A Simple Experiment

Plant a bean seedling in a pot and place it near a window. Mark the direction of the bend after three days. Then rotate the pot by 180 degrees and watch the stem bend back over the next few days. You are watching auxin redistribute itself. Compare this with the coordination in animals, such as in why pupils change size in bright and dark rooms and reflex actions explained with everyday examples, where nerves make the response almost instant.

The Experiment That Found the Sensor

A classic experiment from the 1880s, by Charles Darwin and his son Francis, showed where a seedling senses light. They used the young shoots of canary grass and tested three set-ups with light falling from one side.

The conclusion was that something produced in the tip travels down to the region below, where the bending happens. That something was later found to be auxin. The experiment is a good model of scientific thinking: change one thing at a time, and see what is needed.

Hormones Put to Work

Farmers and gardeners use plant hormones every day, often without calling them that.

• Rooting: cuttings dipped in a rooting hormone powder, which contains auxin, grow roots faster.
• Seedless fruit: treating flowers with certain hormones can make fruit develop without fertilisation, so there are no seeds.
• Ripening: ethylene is used to ripen fruits in a controlled way. A familiar home version is to put a raw mango in a paper bag with a ripe banana, which gives out ethylene.
• Sugarcane: gibberellins can make the stems grow longer, which means more juice.
• Tissue culture: cytokinins, together with auxin, help plant cells to divide and grow into whole plants in the laboratory.

Every one of these uses the hormone’s normal job, only with the timing and the amount chosen by the grower.

Growth Is Not Always Upward

A plant is sensing several things at once: light from above, gravity from below, water in the soil and touch from supports. The result is the shape you see. In a dark cupboard, a bean seedling grows tall and pale, searching for light. In sunlight, it grows shorter and greener. A climbing plant spends most of its energy in tendrils that find something to hold. Each plant body is a record of the signals it has received.

Plants and Animals: Two Designs for Coordination

The comparison shows that both groups solve the same problem, receiving information and responding to it, but with very different tools.

Coordination in Your Textbooks

Coordination in plants is part of the chapter on Control and Coordination, which is Chapter 6 in the CBSE Class 10 Science textbook for 2026-27 and Chapter 7 in the Karnataka SSLC textbook, according to published listings. Revise with these notes on coordination in plants, the chapter MCQs with answers and the full chapter notes. In Class 11, the topic becomes plant growth and development, with more hormones and more detail, which is part of NEET Biology. For that level, see these notes on plant hormones. Confirm the current portions with your own board.

Frequently Asked Questions

Q1. How do plants respond to light without nerves?

They use a hormone called auxin. When light falls on one side of a shoot, auxin collects on the shaded side, makes those cells elongate more, and the shoot bends towards the light.

Q2. What is phototropism?

Phototropism is the growth of a plant in response to light. Shoots usually show positive phototropism and grow towards light, while roots show negative phototropism.

Q3. What is the difference between phototropism and geotropism?

Phototropism is a growth response to light. Geotropism is a growth response to gravity, in which roots grow downward and shoots grow upward.

Q4. Why do the leaves of a touch-me-not plant fold when touched?

Special cells at the base of the leaf quickly lose water when touched, so they shrink, and the leaf droops. It is a rapid movement caused by changes in the water content of cells, not by growth.

Q5. Which plant hormone helps fruits to ripen?

Ethylene, a gaseous plant hormone, helps fruits to ripen.

Slow Messages, Steady Growth

A plant’s way of sensing the world is not a weaker version of ours. It is a different design, built on chemical messages and uneven growth. Learn the sequence once, light on one side, auxin on the other, a bend towards the light, and every question in this part of the chapter becomes a variation on it.