Grades 3–5 · Organisms
How does a young shoot grow toward light?
A young shoot growing beside a window can gradually curve toward the light. It does not swivel on a hinge. The growing stem changes shape because cells on its two sides lengthen by different amounts.
In the diagramA growing shoot bends through unequal growthLightGrowing-region sidesLeftRightShaded side grows moreGrowing tip bends left toward the lightCells enlarged; growth amount and time not measured.
Light-detecting proteins help the shoot respond when light arrives mainly from one side. This affects the distribution of a plant hormone called auxin, a chemical signal that influences growth. In this young shoot, more auxin on the shaded side encourages more cell elongation there. The light is not physically pulling the stem; it starts a response inside the growing tissue.
The two sides remain joined as they grow. The more strongly lengthening shaded side curves around the shorter lit side, bending the growing tip toward the light. Many small differences in cell length add up to a visible bend of the whole shoot. With even light from above, the illustrated sides elongate equally and the shoot has no sideways bend.
This growth response is called phototropism. It links a sensed condition to a change in where new growth goes. It applies here to a young growing shoot: roots and older woody stems do not all respond in the same way. The window-side curve records unequal growth over time, rather than a leaf or stem simply turning like a loose pointer.
About this explanation
Starting knowledge: Know that living plants grow and that stems contain cells.
Sources
- University of Minnesota — 4.2 Plant Hormones