772 



GENERAL CONDITIONS OF PLANT-LIFE. 



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 those of 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 gravity. 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 ; 

 z. 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 line, 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 

 outwards and the apices of the stem inwards (towards the centre of rotation). If 

 the rotation takes place in a horizontal plane, gravitation acts, in addition to 

 centrifugal force, on the growing parts, and the direction of the stem and root 

 becomes oblique. But when the rotation is very rapid, it is possible to increase 

 the centrifugal force to such an extent that the axis of growth remains nearly 

 horizontal. If, on the contrary, the seedlings are fixed to a disc rotating in a ver- 

 tical plane, each side of the growing part is in turn directed for a short time 

 upwards, downwards, to the right, and to the left. The action of gravitation there- 

 fore affects all sides equally ; i. e. the growth of the organ is practically inde- 

 pendent of gravitation. Centrifugal force is therefore the only force that acts on 

 the growing parts ; and the root takes an outward radial direction even when the 

 disc is not rapidly turned, the stem an inward radial direction. If however the disc 

 is made to turn very slowly in a vertical plane (round a horizontal axis), so that 



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



