Tropisms are often caused by hormones.
Slide 15
In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light.
They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present.
They postulated that a signal was transmitted from the tip to the elongating region.
Slide 16
Signaling and Phototropism
RESULTS
Control
Light
Light
Phototropic response only when tip is illuminated
Illuminated
side of
coleoptile
Shaded
side of
coleoptile
Tip
removed
Light
Tip covered
by opaque
cap
Tip
covered
by trans-
parent
cap
Site of
curvature
covered by
opaque
shield
Phototropic response when tip separated by permeable barrier,
but not with impermeable barrier
Tip separated
by gelatin
(permeable)
Tip separated
by mica
(impermeable)
Slide 17
What part of a grass coleoptile senses light, and how is the signal transmitted?
RESULTS
Control
Light
Illuminated
side of
coleoptile
Shaded
side of
coleoptile
Slide 18
RESULTS
Light
Tip
removed
Phototropic response only when tip is illuminated
Tip covered
by opaque
cap
Tip
covered
by trans-
parent
cap
Site of
curvature
covered by
opaque
shield
Slide 19
In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance.
In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments.
Slide 20
RESULTS
Light
Boysen-Jensen: phototropic response when tip is separated
by permeable barrier, but not with impermeable barrier
Tip separated
by gelatin
(permeable)
Tip separated
by mica
(impermeable)
Slide 21
Question: Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?
Excised tip placed
on agar cube
RESULTS
Growth-promoting
chemical diffuses
into agar cube
Agar cube
with chemical
stimulates growth
Offset cubes
cause curvature
Control
(agar cube
lacking
chemical)
has no
effect
Control
Slide 22