ACTION OF GRAVITATION ON VEGETATION. 69 1 



similar phenomena occur, though not so strongly. Internal processes of vegetation, 

 the weight of the upper parts, or the influence of light, act in opposition to that of 

 gravitation, so that conditions of equilibrium arise which cause the organs to stand 

 horizontally or obliquely to the perpendicular. 



Thus the vertical direction of primary roots and stems, and the oblique direction 

 of their lateral branches, are determined solely by gravitation, or at any rate to 

 some extent, so long as these parts are still growing ; when they subsequently 

 become lignified or cease to grow, they maintain the position once acquired. If 

 therefore a growing plant rooting in the ground (inside a pot) is placed horizon- 

 tally, the mature parts remain in this position ; but the apex of the primary root 

 turns downwards, and the growing internodes of the end of the stem turn upwards, 

 the leaves, branches, and secondary roots also bend until they make about the same 

 angle with the horizon that they did before the change in their position. The parts 

 which were actually growing when the change was made are shown by the curvatures 

 caused by the influence of gravitation. 



Although we must defer till the fourth chapter the consideration of the internal 

 changes which accompany these curvatures, the proofs that they are really caused by 

 gravitation may be presented in the two following forms : — 



(i) Individuals of the same species have everywhere on the earth's surface the 

 same position with respect to the horizon, and therefore also with respect to the 

 earth's radius. Upright stems therefore, such as pines, grow in South America in 

 totally different directions from what they do with us ; if their axes of growth were 

 elongated downwards, they would intersect in the centre of the earth, and coincide 

 with its radii. It follows therefore that their direction of growth must be determined 

 by a force which stands in a perfectly definite relation to the position of the earth's 

 centre of gravitv. But there is only one such force, viz. gravitation or the attraction 

 of the mass of the earth. The same argument holds for horizontal or oblique 

 branches, leaves, and roots, since these form a constant angle with the primary 

 stem. 



(2) Gravitation differs from other forces in acting independently of the 

 chemical or other properties of the body, being regulated only by its mass ; but 

 the same property is also possessed by centrifugal force. If, as Knight^ first 

 showed, a growing seedling is made to rotate with a rapidity sufficient to bring 

 centrifugal force into play, this force acts on the different parts like gravitation; 

 t. e. the parts which would otherwise be influenced by gravitation (as the primary 

 root), now follow the direction of the centrifugal force and grow outwards from 

 the centre of rotation, while the stem, which would otherwise grow upwards con- 

 trary to the direction of gravitation, now assumes a direction towards the centre 

 of rotation, i. e. in a direction opposite to that of the acting force. This law is 

 strikingly illustrated when seedlings, the roots and stems of which had previously 

 grown^'in one straight hne, are fixed upon a rotating disc (protected from 

 evaporation by a bell-glass) in such a manner that the axis of growth has a 

 tangential direction. The mature parts maintain this direction during the rotation, 

 while those which are still growing bend so that the apices of the roots point 



Knight, Phil. Trans. 1806, part I, p. 99. 

 y \' 2 



