THE MOVEMENTS AND SENSITIVITY OF PLANTS 323 



objects of his study from the effect of gravity, Knight nevertheless 

 succeeded in discovering a simple method of avoiding its unilateral 

 effect. He placed the seedlings of various plants on a small wheel 

 which was rotated in a horizontal plane. This wheel was kept in 

 motion by means of a stream of water obtained from a brook, while 

 the seedlings, being dipped into the water at every rotation of the 

 wheel, were prevented from drying out. With each turn of the 

 wheel the shoots went through all the positions, from the normal 

 vertical to the horizontal position, then to the inverted vertical, 

 then back to the horizontal, but with a reversed situation of the 

 top and root ends, and finally returned to their original position. 

 This was repeated over and over again. As a result of such con- 

 tinuous motion, these plants no longer had any upper or lower 

 parts. The direction of the force of gravity being continuously 

 changed, the shoots grew in any direction which the investigator 

 chose to give them. 



Knight's wheel, so called by the name of its constructor, is 

 rotated at rather high speed. This not only helps to prevent the 

 unilateral effect of the force of gravity, but also creates a some- 

 what complicated situation, by introducing the effect of centrifugal 

 force. Hence in order to be able better to study the consequences 

 resulting from the removal of the effect of gravity, Sachs modified 

 somewhat Knight's method of procedure by substituting the wheel 

 by a klinostat. By means of this device a rather slow, but very 

 regular rotation is attained, without the development of any 

 appreciable centrifugal force (Fig. 131). 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 to motion at the required speed — most frequently from one 

 to ten rotations per hour. This speed has been found to be suf- 

 ficient to remove the unilateral effect of the force of gravity, so that 

 it has no time to become fixed and to produce the reaction of curva- 

 ture in a definite direction. 



If a seedling, for instance, of pea or lupin, is placed horizontally, 

 it may be observed that after a certain period, usually a few hours, 

 its stem will turn upwards and its root downwards. If these 

 organs are previously marked at certain intervals with India 

 ink, as is done for the purpose of studying the rate of growth 

 (Art. 85), then it will be noted that the greatest curvature occurs 

 in the region of the most rapid elongation. The fully grown parts 



