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Plants have no nerves or muscles, yet a seedling on a windowsill turns to face the light within days. Class 10 asks you to explain how, and to tell growth movements apart from quick ones like the touch-me-not's. This post covers the four tropisms in NCERT, works out how much unequal growth it takes to bend a shoot, and gives an experiment you can design for the board exam.
Is this in the 2026-27 syllabus?
Yes. The CBSE Class X Science curriculum for 2026-27 lists "Tropic movements in plants; Introduction of plant hormones" in Unit II, World of Living, which carries 25 of the 80 theory marks. The chapter is Control and Coordination in the NCERT Class 10 Science book, still the current book for 2026-27 (its pages are marked "Reprint 2026-27").
What are the two kinds of plant movement?
NCERT separates them clearly:
Movement without growth. When you touch the leaves of the sensitive plant (chhui-mui), they fold and droop quickly. Cells change shape by changing the amount of water in them.
Movement due to growth. A seedling's root goes down and its stem comes up. If it is prevented from growing, it will not show this movement.
Tropic movements are the second kind: slow, directional growth towards or away from a stimulus.
Which tropisms does NCERT name?
Phototropism: shoots bend towards light, roots bend away from it.
Geotropism: roots grow downwards and shoots upwards, in response to gravity.
Hydrotropism: growth in response to water.
Chemotropism: growth in response to chemicals. NCERT's example is pollen tubes growing towards ovules.
NCERT also describes pea tendrils: the part touching a support grows less rapidly than the part away from it, so the tendril coils around the support.
How does a shoot actually bend?
NCERT's explanation: auxin is made at the shoot tip. When light comes from one side, auxin diffuses towards the shady side, and those cells grow longer. The shady side grows faster, so the shoot curves towards the light.
Research has filled in more detail. A review in The Plant Cell by Liscum and colleagues (2014) describes phototropism as differential cell elongation in response to directional blue light, with light signals regulating the cell-to-cell movement of auxin to set up a gradient across the organ. For gravity, Friml and colleagues (2002) showed in Arabidopsis that a protein called PIN3, which controls auxin flow out of cells, relocates to the side of cells in the root tip after gravity stimulation. And for chemotropism, Okuda and colleagues (2009) identified small proteins called LUREs, secreted by synergid cells beside the egg cell, that attract pollen tubes in the plant *Torenia fournieri*. You do not need these names for the board exam, but they show NCERT's simple picture is the start of a real mechanism.
Worked example: how much extra growth bends a shoot?
Use sample numbers for a classroom model. A young shoot is 2 mm thick. In one day, its shady side grows 3.0 mm longer and its lit side 2.0 mm longer. How far does it bend?
Treat the bent part as two arcs with the same centre, one on each side of the shoot, 2 mm apart. From Class 10 maths, an arc of angle θ on a circle of radius r has length (θ/360) × 2πr. The difference between the two arcs is (θ/360) × 2π × 2 mm, and that must equal 3.0 − 2.0 = 1.0 mm.
So θ = 360 × 1.0 ÷ (2π × 2) ≈ 28.6°.
A difference of just 1 mm turns the tip by almost 30 degrees. If the shady side grew 2 mm more instead, the bend would double to about 57°.
How would you design a hydrotropism experiment?
NCERT asks you to design one. A simple plan: fill a tray with dry soil, push a small porous clay pot filled with water into one side, and sow a few germinated seeds in the middle. After several days, gently dig them up. If the roots have grown towards the pot, that suggests hydrotropism. To make it fair, keep a second tray with an empty pot, and keep both trays in the same light and temperature.
Which mistakes cost marks?
Saying auxin moves towards the light. It moves to the shady side.
Calling the touch-me-not's folding a tropism. No growth is involved.
Saying roots are positively phototropic. NCERT says roots bend away from light.
Forgetting that the growth is directional, which is what makes a movement tropic.
A practice routine
Try NCERT Activity 6.2 at home with bean seeds and a box open on one side, and note what happens after you turn the flask.
Write a four-line answer for each tropism: stimulus, organ, direction, example.
Answer NCERT's in-text questions on the sensitive plant and the tendril without the book.
Read our Class 10 control and coordination guide for the nervous system half of the chapter.
If the link between hormones and growth is still hazy, the Fermi Concept Library covers every concept in biology with its own lesson, so you can revise auxin and then come back to tropisms.
This week, set up the bean-seed box and draw the shoots on day one and day three.
