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 : — 
(1) 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. 
