Video Transcript
During growth, Helianthus,
or sunflowers, are well known for tilting during the day to face the Sun. What tropism is taking place
here? (A) Hydrotropism, (B) chemotropism,
(C) phototropism or heliotropism, (D) thigmotropism or haptotropism, or (E)
geotropism.
A tropism is a directional growth
or movement response of an organism, in this case a plant, either toward or away
from a stimulus. In this example, the stimulus is
sunlight and the response is for the sunflower to grow toward the light.
Whenever we are referring to light,
we use the prefix photo-. So this type of response is known
as phototropism. And because the sunflower grows
toward the light, it is exhibiting positive phototropism. If it were to grow away from the
light, it would be negative phototropism.
In this example, the sunflower is
specifically receiving light from the Sun. So its response can also be termed
heliotropism, because helio- means Sun. It’s not a coincidence that this
same prefix can be found in the Latin name for sunflowers, Helianthus. They were named for their tendency
to respond in this way.
We have therefore determined that
the correct answer is (C). The tropism that is taking place
when sunflowers tilt during the day to face the Sun is phototropism or
heliotropism.
