GEOTROPISM. 195 



geotropism.* He placed germinating seeds upon wheels, 

 which were made to rotate rapidly, in one series of experi- 

 ments in a vertical, and in the other in a horizontal direction. 

 In the first case he found that the roots grew directly away 

 from the centre of the wheel, and the stems toward it — that 

 is, having in his experiment substituted centrifugal force for 

 gravitation, leaving all other conditions unchanged, he found 

 that the root grew in the direction of that force, and the 

 stem opposite to it. In the second series of experiments, in 

 which gravitation and centrifugal force were made to act at 

 right angles to each other upon the growing plantlets, the 

 direction of growth coincided with that of the diagonal of 

 the two forces, the roots growing diagonally outward and 

 dovmward, the stems inward and upward. Dutrochet after- 

 ward showed, by similar experiments, that many leaves are 

 geotropic, turning their under surfaces toward the circum- 

 ference, and their upper toward the centre of the wheel, f 



256. — If positively and negatively geotropic organs are 

 placed in what may be termed their normal positions, they 

 grow on the one hand downward and on the other upward, 

 without any curvature, and in such case the cells in all parts 

 of any section of either the ascending or descending portions 

 show a symmetrical development. But if such symmetrically 

 developed positively and negatively geotropic organs are af- 

 terward placed in a reversed or horizontal position, they 

 will become considerably curved in order to assume their 

 normal positions. Thus the first roots of most young plants, 

 if placed horizontally, soon become curved downward near 

 their tips ; this takes place even when there is considerable 

 resistance to the curvature, as is shown by the penetration 

 of roots into mercury. A similar curvature in, an upward 

 direction, however, takes place in most stems when placed 

 horizontally ; in grasses the curvature is almost entirely con- 

 fined to the nodes. In such curved parts of roots and stems 

 the cells are more elongated upon the convex than upon 



* 



"On the Direction of the Ratlicle and Plumule during the "Vegeta- 

 tion of Seeds." Philosophical Transactions, 1806. 

 \ " Memoires," Paris, 1837. 



