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.
- 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
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.
- 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).
- 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.