Now that we have our seeds germinated, they must grow. Early on, people noticed that seedlings grow toward the light. This response is called phototropism.
Early in the history of science, Darwin kept birds. To provide the birds with vitamins in those days, one needed to grow sprouts. A common species was Phalaris or canary grass!
Darwin noted that the first leaf (coleoptile) of canary grass was very sensitive and responsive to light. He sprouted the seeds in flats and fed the mature seedlings to the birds. But before they made green leaves, the white first leaf (coleoptile) appeared and grew toward the light coming from the nearby window. Darwin was very curious about this and did a few experiments. Later scientists added to this queue of projects to elucidate the mechanism of phototropism.
Darwin, and others, obviously found that the coleoptile's tip was the light-sensitive part. The growth response to light, however, was produced further down the coleoptile.
This observation led scientists to think about hormones again. A chemical substance made in the tip would be transported down the coleoptile and the cells some distance away from the tip would respond by growing.
Fritz Went, a Dutch scientist, figured out that, if the growth stimulus was a chemical, he could trap it in a block of agar (an aqueous gel). Then he could put the block of agar on the tip of a decapitated coleoptile, and it should respond as if the block took the place of the tip.
This did not work out as expected...the coleoptiles grew straight up even with light coming strongly from the side!
He did find some coleoptiles curving as they grew, but they did not all curve toward the light! He noticed that those curving were doing so because the agar blocks were off-center; and these were curving by additional growth on the side underneath the block. He found that this response could be generated by off-center blocks applied to coleoptiles kept in the dark.
Apparently when high levels of the hormone were on one side of the coleoptile, that side would grow faster and the coleoptile would curve accordingly. The opposite side had less of the hormone and therefore grew less rapidly, accounting for the curvature.
Went called the hormone auxin (auxein: to grow). It took 20 years before this auxin was identified chemically as indole-3-acetic acid.
Go back to the Course Schedule.