The purpose of this paper is to analyse and interpret, so far as is possible, the natural geostrophic torsions of growing organs, and also those caused by applied auxin, in the light of evidence presented previously (I942, I945, I946, I947) or to be presented here. The original purpose of these studies was to analyse only the process of movement; but after the discovery of the torsions which are caused in petioles, stems and roots by hetero-auxin applied to them on one flank when horizontal or inclined (I945, I947), it can hardly be doubted that the natural geostrophic torsions of petioles and dorsiventral stems are also introduced by changes in the auxin conditions within them: for the resemblances between these two kinds of torsions were pointed out previously (1945, I947) and will appear again in the present paper. So there has now emerged the new question, what are the changes in the internal auxin conditions which lead to these torsions? And it will be simpler to discuss these auxin changes before discussing the growth responses, since they are an earlier stage of the whole process. The term strophism will correspond with tropism, torsion with curvature, and strophic or geostrophic torsion with tropic or geotropic curvature. Epinastism will be used for the tendency to make an epinastic curvature or epinasty, in agreement with Munch (I938, p. 646). In discussing the auxin conditions, it will be accepted as being now a well-established theory that petioles and dorsiventral stems keep their normal inclination in relation to gravity through a balance between epinastism and negative geotropism. Evidence for this, both old and new, is well set out by Rawitscher (I932), and some further good evidence is given by Munch (1937, 1938). It is, however, a great handicap that the nature of epinastism is still not known with certainty. Experiments by Uyldert (I93I) on shoots of Tradescantia do indeed make it very probable that epinastism is based, as might be expected, on a tendency somehow to raise the concentration of auxin in the dorsal half above that of the ventral half. But unfortunately she was handicapped by being unable to extract auxin by diffusion from this species, though she could cause curvatures by applying auxin, so that the evidence is indirect. For the same reason it is still doubtful how this rise in the concentration of auxin is brought about. Uyldert's experiments 23, 24 and 25 make it seem that in an epinastic curvature down from the vertical the concentration of auxin is greater in the dorsal half because this half conducts auxin longitudinally towards the base whereas the ventral half fails to do so. But the geonegative curvatures up from the horizontal (Exp. 26) appear to be due to a transverse downward flow or redistribution of auxin, such as occurs in other plants, in the basal part of each internode. This is how Uyldert herself interprets her experiments, unless the writer