Video Transcript
Fill in the blank. Phototropism is caused by an
unequal distribution of auxins. In stem cells, there are more
auxins on the blank side. The cells will thus grow more on
that side, causing the curvature. (A) Shady, (B) sunny, (C) windy, or
(D) rainy.
To answer this question, first,
let’s define some of the key terms that have been used. Phototropism is the directional
growth of an organism, like a plant, in response to light. Light is essential for
photosynthesis, which is the process by which many plants are able to make their own
sugars that can be metabolized to release energy. Often, the photosynthesizing parts
of a plant, like its leaves and stem which is sometimes referred to as its shoot,
will grow toward light source like the Sun to absorb more light for
photosynthesis. This behavior is coordinated by
auxins, a group of hormones produced mainly in the tip of the stem that control
directional plant growth and cell elongation among their many other functions.
When a light source is approaching
the plant stem from one side, auxins produced by the cells in the tip of the stem
accumulate in the cells on the shaded side of the stem. This causes those cells on the
shaded side to elongate comparatively more than the cells on the illuminated side of
the stem. We can see this has occurred in the
image provided by the question, as the cells on the illuminated portion of the stem
are shorter and those on the shaded side are more elongated. This asymmetrical cell elongation
causes the stem to curve and bend toward the light source so that more light can be
absorbed by the stem and leaves for photosynthesis.
We now have enough information to
fill in the blank in the statement and therefore answer this question correctly. In stem cells, there are more
auxins on the (A) shady side.
