Part a - experimental technique: manipulating movement of compounds from the coleoptile tip the hypothesis that a mobile compound produced in the coleoptile tip controlled its bending toward directional light was tested by p. boysen-jensen in 1893. his experimental setup is represented below. both dark and light treatment groups had intact coleoptiles as controls. the experimental groups included: coleoptiles in which the tips were removed coleoptiles that were cut with the tips separated by mica coleoptiles that were cut with the tips separated by agar boysen-jensen left one set of coleoptiles in darkness for 24 hours and one set in directional light for 24 hours and observed whether they elongated and/or bended toward the light.

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Explanation:

The objective of the experiment was to test whether there was a substance produced at the tip of the coleoptile that caused the stem below to grow or to bend towards the light (phototropism).

So, the experiment had two groups, one in the dark and the other one had directional light. Both had intact coleoptiles as controls, and coleoptiles that had their tip cut, others had a sheet of impermeable mica inserted between the coleoptile tip and the stem below so as to block the transport of any substance produced at the tip, in other plants the mica was placed in the dark half or in the lighted half. In yet another plant they inserted a sheet of permeable gelatin. The gelatin allowed the substance to pass through and the stems behaved in the same way as the uncut coleptiles. However, the mica showed interesting results. The mica that was placed in the half that was lighted showed normal phototropism, whereas the one that had the mica blocking the dark side did not. This showed that the substance migrated down on the dark side of the coleoptile only. The stem that had the mica covering the whole stem did not show phototropism. The conclusion was that the tip of the coleoptile is the place where the substance that regulates phototropism is produced and that its effect occurs on the dark side (cell elongation) of the stem below.

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