How biologically-important chemicals often got discovered (auxin and serotonin, for example).

Charles Darwin experimented on elongation and bending of plant seedlings; & hypothesized that some chemical gets produced at shoot tips, gets pumped downward, and stimulates elongation of cells in plant stems.

In the 1930s, a Dutchman named Went developed methods for comparing amounts of Darwin's hypothetical chemical, one of which is shown in the following diagram. This is a bioassay.

Extracts of plant tissues were fractionated, and compared using this assay. Eventually, pure chemicals were isolated (by Kenneth Thimann) that produce strong elongation effects.

Also in the 1930s & 40s, there was evidence that some chemical in blood serum stimulates smooth muscle contraction.

Please invent a bioassay by which this hypothetical chemical could have been identified. (Guess why it was named "serotonin"?)

Because it is in serum and stimulates muscle "tone" (=long term contraction)

Serotonin also turns out to be an important neurotransmitter.
Prozac selectively inhibits re-uptake of serotonin. LSD acts on serotonin receptors.

Notice the analogy between 2, 4-D versus LSD and Prozac.

Spemann's hypothetical inducing substance was sought using bioassays.
Guess why that approach failed.

Because salami can induce second embryos; The effect isn't specific enough.

Please invent a bioassay for auxin based on its stimulation of root formation.

THE POWER OF MOVEMENT IN PLANTS.

BY CHARLES DARWIN, LL.D., F.R.S.

ASSISTED BY FRANCIS DARWIN.

CONTENTS

INTRODUCTION...Page 1-9. CHAPTER I.

THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS