752 PART 17. — THE PHYSIOLOGY OF PLANTS. 



they are negatively geotropic; primary 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 cur- 

 vature. 



The principle of Knigbt's ma- 

 chine is to expose growing plants 

 to the action of centrifugal force, 



FIG. 482.-Geotropic curvature of a Pea-seedling ^'^^^^"^ ^^^"^ (Fig. 483 7i) or to- 

 placed horizontally. The thicker outline indicates gether with gravity (A). The 

 the original positions of the primary shoot and object of it is to demonstrate 

 root ; the shoot s has curved upwards in the that gravity is the directive force 

 course of its growth, the root a« has curved ^^^^^^ determines the relative 

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



is nutating. directions of growth of shoots 



and roots ; the line of reasoning 

 being, that if the direction of growth of roots and shoots can be affected by a 

 known physical force, such as the centrifugal force of a rapidly rotating wheel, in 

 a manner analogous to that observable in nature, it may be concluded that the 

 natural direction of growth of these members is the result of the action of a 

 physical force, and this force is doubtless that of gravity. In Fig. A, the seed- 

 lings have been grown on a rapidly rotating horizontal wheel, and exposed 

 consequently to the action both of gravity and of centrifugal force : the result 

 is, that the primary roots and shoots still grow in diametrically opposite direc- 

 tions, but the line of growth is not vertical, as in nature, but oblique ; hence it 

 follows that the direction of growth of both root and shoot has been affected by 

 the centrifugal force. In B, the seedlings have been grown on a rapidly rota- 

 ting vertical wheel : in consequence of the continuous change in position with 

 regard to the vertical, it is obvious that, in this experiment, 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, with the result that the primary roots have grown towards 

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

 the centrifugal force, whilst the primary shoots have grown outwards, away 

 from the centre of the wheel, in the same direction as the action of the centri- 

 fugal 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 of the natural phenomena is also a purely 

 physical force, and the force of gravity is the one which meets all the necessary 

 conditions. 



Another mode of experimental proof is by means of a machine termed a 

 clinostat. This machine differs from Knight's machine in that the rotation of 



