Which one of the following pairs is not correctly matched?

Correct — D, Hydrotropic : Flower. Hydrotropism is growth towards water, and it is a root response — a root growing in soil turns towards the moister side, which is why roots find their way to a leaking pipe. Flowers show nothing of the kind. What flowers do show is photoperiodism in their timing and, in some species, movements that follow the sun, but neither of those is hydrotropism. Every other pair in the list is a textbook match.

• (a)Positively phototropic : Shoot — A shoot bends towards light, which is positive phototropism. This pair is correct, so it cannot be the answer.
• (b)Positively geotropic : Root — A root grows downwards in the direction of gravity, which is positive geotropism. The shoot does the opposite and is negatively geotropic. This pair is correct.
• (c)Chemotropic : Pollen tube — A pollen tube grows down the style towards the ovule along a chemical gradient, which is chemotropism. It is the standard example of the phenomenon and the pair is correct.

A tropism is a directional growth movement of a plant part in response to an external stimulus, and it is named after the stimulus. Light gives phototropism, gravity geotropism, water hydrotropism, chemicals chemotropism and touch thigmotropism, as when a tendril coils round a support. The response is called positive when growth is towards the stimulus and negative when away from it, so the same organ can be positive to one stimulus and negative to another.

Matching items reward a candidate who checks each pair against the definition instead of hunting for the odd one out by feel. Here three pairs are the standard examples printed in every textbook, so the fourth stands out once the definitions are applied. It also helps to notice which organ each tropism belongs to: phototropism and geotropism are about the shoot and root, hydrotropism about the root, chemotropism about the pollen tube. Nothing on the standard list is about the flower.

• Tropisms are directional growth responses named after the stimulus that causes them.
• Shoots are positively phototropic and negatively geotropic; roots are the reverse.
• Hydrotropism is the growth of roots towards water.
• The growth of a pollen tube towards the ovule is chemotropism.
• Thigmotropism is the response to touch, seen when a tendril coils round a support.

• Assuming that because flowers move to follow the sun, they must show hydrotropism.
• Reversing positive and negative for the root and the shoot.
• Treating the folding of a touch-me-not leaf as thigmotropism — that is a nastic movement, not a growth response.

A matched-pair item on plant responses; NDA has asked the same content by describing the movement and requiring the name, which is the reverse direction.

• practice — not a real PYQ

  The growth of a root towards a source of moisture is

  • (a)phototropism
  • (b)hydrotropism
  • (c)geotropism
  • (d)thigmotropism

  Answer(b) hydrotropism — the stimulus is water and the responding organ is the root.

• practice — not a real PYQ

  The coiling of a tendril around a support is an example of

  • (a)chemotropism
  • (b)thigmotropism
  • (c)hydrotropism
  • (d)phototropism

  Answer(b) thigmotropism — a directional growth response to touch.