1. It is shown that the rate of geotropic curvature of a piece of stem of Bryophyllum calycinum when suspended horizontally increases with the mass of an apical leaf attached to the stem.

2. It is shown that the dry weight of the stem increases with the mass of the leaf attached and also that the degree of curvature increases with this increase in the dry weight of the stem.

3. The conclusion is drawn that geotropic curvature is in this case a function of mass action of the material sent by the leaf into the basal part of the stem.

4. The material sent by a leaf into the apical part of a stem does not lead to the same geotropic curvature.

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