A student investigates gravitropism in oat (Avena sativa) seedlings. A group of germinated seedlings is secured horizontally in a completely dark chamber. After 36 hours, the coleoptiles (shoots) have grown and curved significantly upwards, away from the pull of gravity.
Suggest how the student can accurately measure the actual length of these curved shoots at the end of the experiment using a standard flat millimeter ruler.
Explain how the distribution of the plant hormone auxin in the horizontally oriented shoot tip is affected by gravity, and explain how this distribution causes the shoot to curve upwards.