Go to the light! That's what trees do; they grow towards light since they need it to produce energy. This phenomenon is called phototropism. Trees also grow in response to gravity, a phenomenon known as geotropism. In this video, the narrator briefly explains both. In a roundabout way, he relates geotropism to the force of gravity by accidentally calling it "gravitropism," but he never fully explains the concept. The trees that he displays somewhat exhibit the movements, but make sure to set up some in-class demonstrations for your scientists.