Question

Describe an experiment that would demonstrate that growth stimulating hormone is produced at the tip of coleoptile.

Answer 1

In 1881, Charles Darwin and Francis Darwin performed experiments on coleoptiles of canary grass and observed that they bend towards the light when exposed to light from a unidirectional source. They concluded that the tip of the coleoptile was responsible for sensing light, and proposed that a messenger is transmitted in a downward direction from the tip of the coleoptile, causing it to bend.

In 1913, Peter Boysen-Jensen concluded that the substance secreted at the tip is soluble in water (gelatin).

In 1926, F.W. Went published a report describing how he isolated a plant growth substance by placing agar blocks under coleoptile tips for a period of time then removing them and placing them on decapitated Avena stems. After placement of the agar, the stems resumed growth. In 1928, Went developed a method of quantifying this plant growth substance. His experiment showed that a chemical messenger diffuses from coleoptile tips and causes it to grow towards the light. He later proposed that the messenger substance is a growth-promoting hormone, which he named auxin. The Avena Coleoptile test is the most widely accepted bioassay for quantitative estimation of auxin concentration.