218 PART III. PHYSIOLOGY. [ 48 



just as they do to the direction of the incident rays of light. They 

 are diageotropic, just as they are diaheliotropic. 



It is a familiar fact that at all points of the earth's surface typi- 

 cal radial members, such as primary shoots and roots, grow with 

 their long axes vertical, but with this difference, that the direction 

 of growth of the primary shoots is away from the centre of the 

 earth, whereas that of the primary roots is towards the centre of 

 the earth. It can be readily demonstrated (by Knight's machine) 

 that this vertical direction of growth is due to the force of gravity, 

 that it is, in fact, a phenomenon of geotropism. But the effect 

 produced is precisely opposite in the two cases ; primary shoots 

 grow in a direction opposed to that of the action of gravity, they 

 are negatively geotropic ; primaiy roots grow in the same direc- 

 tion as that of the action of gravity, they are positively geotropic. 

 If these members be moved out of their normal position, they will 

 return to it by performing geotropic curvature. 



The principle of Knight's 

 machine is to expose growing 

 plants to the action of centri- 

 fugal force, either alone or 

 together with gravity. The 

 object of it is to demonstrate 

 that gravity is the directive 

 force which determines the re- 



_ . lative directions of growth of 



Fie. 132. Geotropic curvature of a Pea-seedling e 



placed horizontally. The thicker outline indicates shoots and roots. When seed 

 the original positions of the primary shoot and lings are grown on a rapidly 

 root ; the shoot s has curbed upwards in the rotating vertical wheel, in 

 course of its growth, the root v> has curved con sequence of the continuous 

 downwards. The bud at the apex of the shoot . . . . , 



is nutating. change in position with re- 



gard to the vertical, it is 



obvious that the directive action of gravity is eliminated, for all parts 

 of the seedlings are acted upon by gravity for successive equal times 

 in opposite directions: the only force in action is the centrifugal force. 

 The result is that the primary roots grow towards the centre of the 

 wheel, in a direction contrary to that of the line of action of the cen- 

 trifugal force, whilst the primary shoots grow outwards, away from 

 the centre of the wheel, in the same direction as the action of the cen- 

 trifugal force. It is clear from these facts (1) that a purely physical 

 force can determine the direction of growth of roots and shoots : (2) that 

 the physical force employed (centrifugal force) affects primary roots and 

 shoots in a precisely contrary manner : and it may be concluded that 

 since the phenomena produced by the action of centrifugal force in these 

 experiments are quite analogous to those observable in nature, the cause 



