146 



PLANT PHYSIOLOGY 



exposure to gravity, its effect being neutralized. The direction 

 of the force of gravity being continuously changed, the shoots 

 grew in any direction that the investigator chose to give them. 

 This apparatus, called ''Knight's wheel," is rotated at rather 

 high speed. This not only helps to prevent the unilateral effect 

 of the force of gravity but also creates a somewhat complicated 

 situation by introducing the effect of centrifugal force, which 

 acts in the same way as gravity. To be able to study better the 

 consequences resulting from the removal of the effect of gravity, 

 Sachs modified somewhat Knight's method by substituting for 

 the water wheel a clock mechanism called a ''clinostat." By 



Fig. 39. — A clinostat of Pfeffer's construction (after Benecke-Jost). 



means of this device, a rather slow but very regular rotation is 

 attained, without the development of any appreciable centrifugal 

 force (Fig. 39). The essential part of the apparatus consists of 

 a powerful clock mechanism, which is attached to an axial rod. 

 This axis may be adjusted in any desired direction and set in 

 motion at any required speed, most frequently 1 to 10 rotations 

 per hour. This speed has been found to be sufficient to remove 

 the unilateral effect of the force of gravity, so that it has insuffi- 

 cient time to produce the reaction of curvature in a definite 

 direction. 



If a seedling is placed in a horizontal position and is rotated 

 slowly on the clinostat, no curvatures will be produced. But 

 if before the clinostat is set in motion, the seedling is left for 

 several minutes in a horizontal position, not so long, however, 



