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SAI International School

Std -X, Subject- BIOLOGY
Chapter- Control and Coordination
Subtopic- Coordination in Plants- II
Module- 17
Lesson Notes

Response of plants to light: Phototropism

 This happens because of the action of auxin hormone.
 When the sunlight is coming from top, then the auxin hormone present at the
tip of the stem spreads uniformly down the stem making it grow straight up.
 When the sunlight falls only on right side then the auxin hormone prefers to
stay in shade and collects on left side.
 Since there is more auxin hormone on the left side of the stem, it makes left
side grow faster than the right side. Thus, the stem bends towards right.
 Auxin hormone has opposite effect on the roots of a plant. That is why when
sunlight falls on the right side of the roots, auxin hormone collects on the left
side and hampers their growth from that side. That is why, roots bend towards
left.

Response of plants to gravity: Geotropism