The transverse growth theory of plant torsions

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Abstract
Further evidence is offered in support of a transverse growth theory of the strophisms, or torsions made in response to stimulation, in growing dorsiventral organs, and also of the experimental auxin torsions of radial stems. The theory is a modification of a theory proposed by Rawitscher (1932). Seedlings of Phaseolus multiflorus were arranged with leaves horizontal and on edge, so that their petioles were stimulated transversely by gravity, but the petioles were prevented from twisting by being held in constraint for various periods up to 48 h. Then they were cut across at the distal end, and split in a longitudinal vertical plane right through to the cut end. The halves always twisted rapidly in opposite directions, so as to separate their lower edges. When the halves were bound closely together again, the whole reconstituted zone was found to be twisted in the direction determined by the stimulation in constraint. Also petioles were arranged similarly horizontal and on edge, but not in constraint, and when they had begun to twist, they were split in the previously vertical plane, but the distal ends were left intact to keep the halves together. The halves regularly gaped apart at once along their lower, but not their upper edges. An experiment similar to the last was performed on young stems also, that were laid horizontal and auxinated on one side so that they twisted in the direction to raise that side. The result was similar, but not conclusive, for a reason stated. Taken together the results support the theory, according to which in an organ stimulated as for a geostrophism or for a geo-auxin torsion, the lateral halves begin to curve upwards in the transverse direction. In doing so they develop opposite twisting forces, and the dorsal half, or the auxinated half, prevails and so makes the whole organ twist. The problem of working out the theory more exactly is discussed, and various other points also.

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