BOTANICAL GAZETE. 176 



trated by the cotyledons of the seedling radish. If illuminated from 

 above they take a horizontal jr-'sition, standing thus at right angles to 

 the light. If lighted obliquely from above one becomes depressed 

 and the other elevated, so as to assume again the position at right 

 angles to the incident light. 



The two theories which have been proposed to account for this 

 tendency bear the names of Frank (1870) and de Vries (1872), the 

 latter endorsed by Sachs, with modifications, in 1879. Frank sup- 

 poses a certain sensitiveness to light, which he calls "transverse-helio- 

 tropism," to be an inherent tendency in leaves and some other parts of 

 plants, which cau<^es them to assume a position at right angles to 

 light, just as heliotropism inclines them to parallelism with the rays. 

 A similar difference is shown in the tendency of aerial Siems to grow 

 upward, and of rhizomes to grow horizontally. 



De Vries considers that the ordinary forces of heliotro])ism and 

 geotropism may so balance each other as to keep horizontal the leaves 

 of a plant lighted from above. Modes of growth may also assist in 

 maintaining this equilil)rium. For instance epinasty may be opposed 

 by heliotropism and hyponasty by geotropism. 



Darwin's experiments were made with a view to test these two 

 theories. In order to be rid of the di';turbing element, the force of 

 gravitation, he used the klinostat, an instrument in the use of which 

 the jjlant is fastened to a horizontal spindle and illuminated from the 

 direction of the axis. By clockwork the spindle is kept in steady but 

 slow rotation. It a plant which has been previously illuminated from 

 above be fastened with its axis parallel to the axis of the klinostat, 

 and the direction of the incident light, according to Frank's theory 

 the leaves ought to remain stationary, but according to de Vries and 

 Sachs they ought not to be able to retain the rectangular position. The 

 results with Ranuncuhts ficaria were decidedly in favor of Frank's 

 theory. The leaves of this plant, which are sometimes extremely 

 epinastic. moved forward until a])proximately at right angles to the 

 light and then came to rest. Again, when the plant had been placed 

 in the dark so as to compel the leaves to jjoint upwards, and then 

 placed on the klinostat, the leaves turned backward till again at right 

 angles. 



A series of experiments with seedling cherries seemed to lead to 

 a somewhat different result. When placed on the klinostat the leaves 

 were unable to keep their horizontal position, but became parallel to 

 the stem of the plant. 



He concludes that transverse-heliotropism is really the important 

 influence at woik, and that this sensitiveness to light is sufficiently 

 strong in the case of the Ranunculus to determine the position of the 

 leaves, notwithstanding the annihilation of heliotropism. The cherry, 

 when growing normally, he believes to trust to the approximate equi- 

 librium between ej^inasty and helioiropism, which is made complete 

 by the influence of light. "But when the balance is disturbed by 

 placing the plant on the klinostat, the light stimulus is not strong 

 enough to produce a condition of equilibrium." 



