Life Cycle - Plants (6A)

  • Comparing growth factors for plants.
  • Limiting and observing growth factors in plants.
  • auxin
  • hormone
  • phototropism
  • seedlings
  • gelatin (or other plant cap)
  • milk container
  • soil
  • exacto knife
  • rooted plants (geranium, ivy) 
  • worksheet

Students experiment with light and the growth of plants.


Charles Darwin in 1880 exposed seedlings to light and charted how they bent toward the light only if the tips were exposed to light. He exposed grass to light and noted that it curved toward the source. He covered the top of the seedlings with light proof caps, no bending occurred. He wasn't quite sure what was going on. Boysen-Jensen's study between 1910-1913 with beheaded seedlings, demonstrated that placing a bit of gelatin on the stump, replacing the top on the gelatin, caused curvature toward light to occur just as with intact seedlings. If the tip is not replaced, curvature does not result. Paal (1918) demonstrated that if a cut tip is replaced off center growth results and it will be curved.

The reason for the bending of plants toward light is that growth hormones called "auxins" on the non-illuminated side will cause the plant to grow more than on the illuminated side. Since the elongated cells on the non-illuminated side will grow faster than on the other side, the plant will bend. It looks as if the plant is moving toward light to make food, but no plant can seek anything. The higher concentration of auxins on the darker side stimulate growth. Growth responses resulting from external stimuli are termed tropisms and since this has to do with light (photo=light) this is called phototropism.

  1. In the first part of the lab, students will duplicate Darwin's and Boysen-Jensen's experiment. They will utilize three similar seedlings. They need to cut the tips off of two seedlings, cover the tip of one with a cap (have them experiment on how to do this), leave the other plant with the cut top exposed, and leave the last seedling as a control.
    Remind students how a "control" is used. A control represents what will happen if nothing is done at all. This is used to compare whether an action has an effect or if it is due to chance.
  2.  In the second part of the lab, students will set up an experiment that determines how long it takes different plants to "bend." They need a geranium, a seedling (tomato, or other vegetable), and an ivy (using different plants are acceptable).
  3. Students will plant the rooted geranium and ivy in new containers and need to place the plants in the same general area. Over the next week, they will observe how the pants grow in light. The experiment will proceed quicker if you have an area of intense light that the plants will turn towards. They will notice that some plants take longer to bend than others.

  [Back to Life Cycle Grid]  [Back to Plants (6)]