In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the shoot to grow towards light (phototropism) and the roots to grow downward (gravitropism).In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone.Although the model has been criticized and continues to be refined, it has largely stood the test of time.