Plant Growth and Tropisms

Time frame: periodic observations over
several hours or days
Space needed: sunny window sill
Objectives: To demonstrate how plants orient their growth in response to environmental cues.
Materials needed:
  • green bean seeds
  • small paper cups
  • bag of potting soil
  • single edge razor blade
  • black paper
  • tape
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. It will take approximately 1 week for the seeds to grow to 2 inches in length; at this size the treatments can begin.

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.

Within a few hours the uncovered seedlings will be pointed upwards in response to gravity (geotropism). The seedlings with the caps on them will not have moved, proving that is is the tip of the plant that senses gravity.

Now arrange the cups so that sunlight only strikes one side of the seedlings.Again place the cap on the seedling tips of 1 cup. Within a few hours the uncovered seedling will be pointing their tips towards the sunlight(phototropism). The seedlings with the caps over the tips will be in the same position as before, proving that again it is the tip of the seedling that senses light.

At this point, explain to the participants that growth is controlled by hormones called auxins that move down from the tip and tell the plant to elongate. Auxins tend to fall to the lower side of a horizontal shoots, telling the lower side to elongate, which bends the stem upward. Auxins also tend to breakdown on the sunny side of the shoot, creating a situation where the shady side grows faster, bending the shoot tip towards the sun.