Plant hormones — Jensen (1913)

• B.
• Jensen explained phototropism (bending of the growing tip towards light) by auxins.
• He showed that the link between the receptor (the tip that perceives the light) and the region of curvature (the growing region below it) is a chemical substance — an auxin such as indole acetic acid (IAA).
• (Jensen showed that a tip cut and replaced on a decapitated coleoptile, with a thin separating block of gelatin that lets chemicals pass but not cells, still allowed bending.)

Plants have no special glands for hormones; auxins are made by the tips of coleoptiles and by buds and affect other parts of the plant.

Functions of auxins:

1. They organise the development and differentiation of tissues; they cause cell elongation (growth).
2. They control flowering, fruit formation and ripening (e.g. the ovary produces auxin that makes it develop into a fruit — see Chapter 3 on parthenocarpy).
3. They control tropisms: in a shoot the auxin moves to the shaded side, where it speeds up cell elongation, so the shoot bends toward the light.