Photoperiodic curvatures of the hypocotyl may be induced in young but fully green seedlings of Cucumis sativus growing in natural, alternate light and darkness if one cotyledon is kept in the dark and the other in light or is cut off. An equally strong photoperiodic curvature of the hypocotyl does not occur if one cotyledon is cut off and the seedling is then grown in a daily alternation of light and darkness or is placed in a dark chamber. The periodicity of movement is most clearly shown if part of the plant is subjected to a daily alternation of light and dark while the remaining part is not thus treated. The flow of growth substance subject to daily rhythm from the leaves to the stem is assumed to be the explanation of the results and an important function for all photoperiodic growth and developmental processes.