Video Transcript
The diagram provided shows a simple
outline of an experiment carried out by Boysen-Jensen when investigating hormones in
plants. When there is a block of gelatin in
between the tip and the rest of the coleoptile, the coleoptile still bends toward a
light source. Why? (A) Auxins are unable to diffuse
through the gelatin from the tip to the rest of the coleoptile. Or (B) auxins are able to diffuse
through the gelatin from the tip to the rest of the coleoptile.
To answer this question, let’s
first look at what auxins are. Auxins are a group of chemical
messengers called hormones that are produced in the cells at the tip of plant shoots
and roots. For example, auxins are produced in
the cells of the coleoptile, which is the sheath surrounding the tip of certain
plant shoots. Auxins are involved in several
functions including controlling cell elongation and play an important role in
certain plant tropisms, which are growth movement responses to specific stimuli.
Phototropism is an example of a
tropism which involves the photosynthetic parts of a plant, like the leaves and
shoots, responding to light by growing toward the light stimulus or the
nonphotosynthetic parts, like the roots, growing away from light. The shoot of a plant and any leaves
attached to it will grow towards the light to help the photosynthesizing cells they
contain capture more light for photosynthesis to make sugars more efficiently.
The diagram provided by the
question displays two different young plant shoots, in which the tip of the
coleoptile and the rest of the coleoptile have been separated with two different
substances, mica and gelatin, respectively. This experiment carried out by
Boysen-Jensen was built upon investigations by Darwin that showed that the tip of
the coleoptile is responsible for controlling growth when exposed to sunlight. Boysen-Jensen wanted to find out if
this response was due to the effects of a water-soluble chemical, which was later
named auxin, or perhaps something else.
To test this, Boysen-Jensen cut the
tip of a shoot and put a thin layer of gelatin on the cut shoot before replacing the
tip. Gelatin allows water-soluble
substances to diffuse through it. Theoretically, auxins produced in
the shoot tip would still be able to diffuse through the gelatin layer, resulting in
directional growth of the cells in the rest of the coleoptile and therefore the
shoot still bending toward the light source. In a separate plant, Boysen-Jensen
used the sheet of mica, which is impermeable to water-soluble chemicals, instead of
gelatin, which would prevent diffusion. If no growth occurred in this
plant, it would show that it was indeed the auxins that were responsible for this
directional growth.
Boysen-Jensen observed that the
plant with the gelatin block between the tip and the rest of the coleoptile still
grew toward the light as if there were no separation present. This response was not observed in
the plant with a mica separation between the tip and the rest of the coleoptile. This means that we have found the
correct answer to this question. The reason why the coleoptile still
bends toward light source when there is a block of gelatin in between the tip and
the rest of the coleoptile is (B). Auxins are able to diffuse through
the gelatin from the tip to the rest of the coleoptile.
