Charles Darwin and his son Francis conducted some of

the earliest experiments on phototropism in the late 1800s
(Figure 39.5). They observed that a grass seedling ensheathed
in its coleoptile (see Figure 38.9b) could bend toward
light only if the tip of the coleoptile was present. If
the tip was removed, the coleoptile did not curve. The
seedling also failed to grow toward light if the tip was covered
with an opaque cap; but neither a transparent cap over
the tip nor an opaque shield placed below the coleoptile tip
prevented the phototropic response. It was the tip of the
coleoptile, the Darwins concluded, that was responsible for
sensing light. However, they noted that the differential
growth response that led to curvature of the coleoptile occurred
some distance below the tip. The Darwins postulated
that some signal was transmitted downward from the tip to
the elongating region of the coleoptile. A few decades later,
the Danish scientist Peter Boysen-Jensen demonstrated that
the signal was a mobile chemical substance. He separated
the tip from the remainder of the coleoptile by a cube of
gelatin, which prevented cellular contact but allowed chemicals
to pass through. These seedlings responded normally,
bending toward light. However, if the tip was experimentally
separated from the lower coleoptile by an impermeable
barrier, such as the mineral mica, no phototropic response
occurred.
explain briefly

1 answer

In the late 1800s, Charles Darwin and his son Francis investigated phototropism, the growth of plants toward light. They discovered that a grass seedling, protected by a sheath called a coleoptile, could only bend toward light if the tip was intact. If the tip was removed or covered with an opaque cap, the coleoptile failed to bend. This led them to conclude that the tip of the coleoptile was responsible for sensing light and transmitting a signal down to the area below, where growth occurs.

Later, Danish scientist Peter Boysen-Jensen conducted further experiments to explore the nature of this signal. He used gelatin to separate the coleoptile tip from the rest of the plant, allowing chemical signaling but preventing direct contact. The seedlings still bent toward light, indicating that a chemical substance was responsible for the signaling. However, when he used an impermeable barrier (mica) to separate the tip, the phototropic response did not occur, reinforcing the idea that the signal was a mobile chemical that needed to reach the elongating region below the tip to initiate growth toward light.