A student investigates gravitropism in the radicles (young roots) of germinating broad bean seeds. A group of seeds with straight, intact radicles is pinned horizontally to a cork board inside a completely dark, humid chamber. After 36 hours, the radicles have grown and curved significantly downwards.
Suggest how the student can accurately measure the length of these curved radicles at the end of the experiment using a standard flat millimetre ruler.
Explain how the lateral distribution of the plant hormone indoleacetic acid (IAA) in the root tip is affected by gravity, and explain how this distribution causes the radicle to bend downwards.