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Select the INCORRECT statement related to tropism.
Phototropism – movement towards water
Chemotropism – movement towards chemical
Hydrotropism – movement towards water
Geotropism – in response to the pull of earth or gravity
Phototropism – movement towards water
Tropism refers to the directional growth movement of a plant in response to an external stimulus. 'Phototropism' specifically describes growth towards or away from light, while movement towards water is called 'Hydrotropism'.
Tropism refers to the directional growth movement of a plant in response to an external stimulus. 'Phototropism' specifically describes growth towards or away from light, while movement towards water is called 'Hydrotropism'.
Think of a sunflower 'following' the sun across the sky during the day—that is phototropism in action. Roots growing deep and wide towards a leaking pipe is the plant's clever way of hunting for water.
PHOTO (Light), HYDRO (Water), GEO (Earth/Gravity), CHEMO (Chemical).
Fgeotropism∝g — geotropism response is proportional to gravitational acceleration
In phototropism, auxin (a plant hormone) accumulates on the shaded side of the stem, causing cells there to elongate faster than those on the illuminated side, which results in the stem bending towards the light. Similarly, roots exhibit positive hydrotropism, where they grow towards a moisture gradient to ensure the plant's survival.
Phototropism: Response to light (e.g., shoot tips).
Geotropism: Response to gravity (roots = positive, shoots = negative).
Hydrotropism: Response to water/moisture.
Chemotropism: Growth in response to chemical stimuli (e.g., pollen tube growth towards the ovary).
Ensures optimal light exposure for photosynthesis.
Helps roots find water and stabilize the plant.
Plant survival in dense forests by growing towards light gaps.
Root penetration towards irrigation sources.
The official key marks Option A as the incorrect statement because phototropism is the response to light, not water.
Option B, C, and D are factually correct definitions of their respective tropisms.
A is correct — Phototropism is defined as growth in response to light, while movement towards water is termed Hydrotropism.
Always distinguish between 'nastic' movements (non-directional) and 'tropic' movements (directional) to avoid confusion in exam questions.
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