Plants orient their growth in response to environmental cues called tropisms. Geotropism is growth in response to gravity.
Phototropism is growth as a result to exposure to light.
The tip of the plant is the area that senses these environmental cues. Watch the plants grow as they are exposed to different treatments. Fill 2 paper cups with potting soil and sow 3 bean seeds in each cup. Lay the cups on their side so that the seedlings are horizontal. Place a small cap made of black paper over the tip of the seedlings in one cup.