Cambridge O Level Biology · Syllabus 5090 · Coordination and Response in Plants
Phototropism
What is Phototropism?
A response in which parts of a plant grow toward or away from light. Shoots are usually positively phototropic, growing toward light, which positions the leaves where light for photosynthesis is available; roots may be negatively phototropic, growing away from light.
This definition is part of the Coordination and Response in Plants chapter in Cambridge O Level Biology.
Questions students ask about Phototropism
What is phototropism?
Phototropism is a response in which parts of a plant grow toward or away from light. Shoots are usually positively phototropic, growing toward the light so that the leaves are positioned where light for photosynthesis is available.
Why does a shoot bend toward the light?
Light arriving from one side causes auxin to become unequally distributed, with more auxin on the shaded side. Auxin stimulates cell elongation, so cells on the shaded side elongate more and that side becomes longer than the illuminated side. The unequal growth curves the shoot so that its tip points toward the light. This is positive phototropism.
How do you investigate phototropism in shoots?
Grow similar seedlings in light-proof boxes, with a hole in one side of the treatment box so light enters from one direction only, and a control lit evenly from above or rotated regularly. Control species, age, initial shoot length, water, growth medium, temperature, lamp distance, duration and starting orientation. After a fixed time, measure the bending angle in degrees from the marked starting orientation for several seedlings and compare the means.