2. Gravitropism Gravitropism is growth in response to gravity. It is a common observation that roots tend to grow downward and stems tend to grow upward. Lateral roots are not as strongly gravitropic as the main root. The control over the direction of lateral root growth is not understood, although the main root seems to exert influence over the orientation of lateral roots. A similar phenomenon can be observed in both the phototropic and gravitropic responses of shoot systems, where the lateral stems are less directly oriented with respect to light and gravity than the main stem. Branches must orient themselves for light reception without competing with or shading other branches on the same plant. The mechanisms by which plants detect gravity are not well understood, although it is an area of active research. The hormone involved is auxin. Under normal conditions, auxin is transported toward the tip of the root, and then flows toward the root epidermis and back up the root, and away from the tip. When a root is tipped on its side, more auxin is distributed on the downward side of the root than on the upper side. The response to this unequal distribution of auxin is elongation of cells on the upper side of the root, resulting in curvature, much like that seen in the phototrophic response. Stem Gravitropism Do both stems and roots respond to gravity as a stimulus? If they do, do they grow toward it (positive gravitropism) or away from it (negative gravitropism)? 1. At the side table , a potted plant has been placed on its side in a darkened box. 1. Before you look in the box, hypothesize as to what direction the stem will have grown, and why you think so. 2. Why was the stem in a darkened box? If the stem were on its side in the light, how would this affect your conclusions? The stem was in a darkened box to show.

1. 2. Gravitropism Gravitropism is growth in response to gravity. It is a common observation that
roots tend to grow downward and stems tend to grow upward. Lateral roots are not as strongly
gravitropic as the main root. The control over the direction of lateral root growth is not
understood, although the main root seems to exert influence over the orientation of lateral roots.
A similar phenomenon can be observed in both the phototropic and gravitropic responses of
shoot systems, where the lateral stems are less directly oriented with respect to light and gravity
than the main stem. Branches must orient themselves for light reception without competing with
or shading other branches on the same plant. The mechanisms by which plants detect gravity are
not well understood, although it is an area of active research. The hormone involved is auxin.
Under normal conditions, auxin is transported toward the tip of the root, and then flows toward
the root epidermis and back up the root, and away from the tip. When a root is tipped on its side,
more auxin is distributed on the downward side of the root than on the upper side. The response
to this unequal distribution of auxin is elongation of cells on the upper side of the root, resulting
in curvature, much like that seen in the phototrophic response. Stem Gravitropism
Do both stems and roots respond to gravity as a stimulus? If they do, do they grow toward it
(positive gravitropism) or away from it (negative gravitropism)?
1. At the side table , a potted plant has been placed on its side in a darkened box. 1. Before you
look in the box, hypothesize as to what direction the stem will have grown, and why you think
so.
2. Why was the stem in a darkened box? If the stem were on its side in the light, how would this
affect your conclusions? The stem was in a darkened box to show