sect, ii PHYSIOLOGY 255 



B. Geotropism 



That the stems of trees and other plants should grow upwards and 

 their roots downwards, is such a familiar occurrence and so necessary 

 for the performance of their respective functions as to seem almost a 

 matter of course. Just as in the discovery of gravitation, it required 

 an especially keen spirit of inquiry to lead to the investigation of 

 this everyday phenomenon. The fact that everywhere on the 

 earth, even on the sides of the steepest mountains, stems take a 

 perpendicular direction ; and that, while buried in the earth, this same 

 direction is assumed with certainty by germinating seeds and growing 

 shoots ; and chiefly the fact also that a shoot, when forced out of its 

 upright position, curves energetically until it is again perpendicular, 

 led to the supposition that the cause of these phenomena must be in a 

 directive force proceeding from the earth itself. The correspondence 

 in the behaviour of a stem in always assuming a perpendicular position, 

 with the continued maintenance of the same direction by a plumb-line, 

 suggested at once the force of gravitation, and the English investigator 

 Knight, in 1809, demonstrated that the attraction of gravitation, in 

 fact, exerted an influence upon the direction of growth. As Knight 

 was not able to nullify the constantly operative influence of gravity 

 upon plants and so directly prove its influence, he submitted them to 

 the action of centrifugal force — an accelerative force operating like 

 gravity upon the masses of bodies, and which had, in addition, the 

 advantage that it could be increased or diminished at will. Knight 

 made use of rapidly rotating, vertical wheels, upon which he fastened 

 plants and germinating seeds in various positions. The result of his 

 experiments was that the stems all turned towards the centre of the 

 wheel and the roots directly away from it. On wheels rotating in a 

 horizontal plane, where, in addition to the centrifugal force, the one- 

 sided action of gravitation was also still operative, the shoots and 

 roots took a definite middle position ; the shoots and roots still grew 

 in. opposite directions, but their line of growth was inclined to the 

 plane of rotation, at an angle dependent upon the rotating velocity. 

 The position thus assumed was evidently the result of the combined 

 action of the centrifugal force and gravity, which was manifested in 

 the directions taken by the plants according to their comparative 

 strength and respective influence on growth. In this way it was posi- 

 tively ascertained that terrestrial gravitation determines the positions 

 of plant organs in respect to the earth. 



Later, it was also shown that not only the perpendicular direction 

 of stems and primary roots, but also the oblique or horizontal direction 

 taken by lateral branches, roots, and rhizomes, is due to a . peculiar 

 reaction towards the force of gravitation. 



The property of plants to assume a definite position with respect 

 to the direction of gravitation is termed GEOTROPISM. It is customary 



