Cambridge O Level Biology · Syllabus 5090 · Coordination and Response in Plants
Tropism
What is Tropism?
A directional growth response of part of a plant, in which the organ grows toward or away from a stimulus that itself has a direction. The bending is produced by cells on one side of the organ elongating more than cells on the other side, so a tropism is a change in growth rather than a movement of the whole plant.
This definition is part of the Coordination and Response in Plants chapter in Cambridge O Level Biology.
Tropism in context
A tropism is a directional growth response: a plant organ grows unequally so that it ends up pointing toward or away from a stimulus that has a direction. Shoots are usually positively phototropic and negatively gravitropic; roots are usually positively gravitropic. In shoots the response is explained by auxin: it is produced in the shoot tip, it moves from the tip through the shoot, a one-sided stimulus makes its distribution unequal, and it stimulates greater cell elongation on the side where more of it collects. That side grows longer, so the shoot bends.
Common mistakes with Tropism
- 2. “Any movement of a plant is a tropism.” Why it is wrong: a tropism needs a directional stimulus and a growth response. Wilting and a flower closing at dusk are responses, but nothing has grown and no direction was involved. SayA tropism is a directional growth response. Other responses are not tropisms.
Questions students ask about Tropism
What is a tropism in biology?
A tropism is a directional growth response: part of a plant grows toward or away from a stimulus that has a direction. The organ bends because cells on one side elongate more than cells on the other, so a tropism is a change in growth rather than a movement of the plant.