The Application of the Modified Sine Rule to Tension Wood Production and Eccentric Growth in the Stem of Crack Willow (Salix fragilis L.)

One-year-old shoots of S. fragilis were placed on specially prepared frames and grown at various angles from the upright position for six-and-a-half months. At the end of this period the stems were found to have made greater growth on the upper side than the lower, and the majority of the fibres of the upper xylem were of the gelatinous type. The degree of eccentricity and the amount of gelatinous fibres produced were clearly shown to reach a maximum when the stems were displaced by 120° from the upright position. This provides evidence that in stems, as in roots, geotropic responses are not proportional to the sine of the angular displacement from the vertical, but follow a modified sine relationship. These results are considered to provide support for the hypothesis that statolith graviperceptors must be present close to the vascular cambium along the length of young woody stems. There is no reason to suppose that the perception of gravitational stimulus in stems is in any material way different from that in roots.