Positive Phototropism
Objective

Students will demonstrate the direction of plant growth towards a strong light source, e.g.
phototropism.
Explore
For this experiment, use two bean plants of identical height, both relatively vertical in growth.
Construct two covers from sneaker boxes, by removing a square window of approximately 2
inches each side from one end. Be sure to keep the openings in both boxes as identical as
possible. Cut a circular shaped opening in the lid (or base) of the sneaker box, so that it can fit
over the bean plant, and its pot or container, allowing a couple of inches for growth. Place the
pots with both plants on the same window sill, but orient one opening left, and one right. Have
students predict what they think will happen, and plot results over time. (In a few weeks, one
bean plant should be growing noticeably left, and one noticeably right.)
Explain
Chemicals (hormones) in the plant causes cells on the shaded side to grow longer than on the lit
side. This results in the plant growing towards the light (a response called positive phototropism).
In the rainforest, when a tree-fall suddenly creates new open areas, plants vigorously compete for
their places in the sun. But not all plants seek light all the time. And some desert plants actually
grow away from the sun to keep from over-heating.
© PASSPORT TO KNOWLEDGE