ISOLATED cambium – phloem strips from Bryophyllum calycinum Salisb. show a curvature with the cambium at the concave surface, and this curvature is increased when the strips are placed in water. Plasmolysis causes the complete relaxation of the strip. Partial relaxation can be obtained in solutions of indolylacetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D) in concentrations of 1–50 parts per million, the action of indolylacetic acid being in some cases reversible. It can be shown that the average width of the cambium cells decreases immediately after placing the strip in the hormone solution. The reaction is strongly influenced by temperature; it occurs between 15° and 30° C., with an optimum near 30° C. At 29° C. a maximal curvature of the strip (or shrinkage of the cambium cells) is obtained within five minutes. In indolylacetic acid, 1–20 p.p.m., a reversal of this shrinkage occurs which may lead to an increase beyond the original size of the cell; this might properly be considered as growth. At 29° C. the reversal appears twenty minutes after the beginning of the experiment.