ALTERNATING AND INTERMITTENT STIMULATION 79 



acquires the power of curvature, then set up a reaction to direct 

 the tip vertically. 1 



119. Alternating and Intermittent Stimulation. It is found 

 that an intensity or duration of the geotropic stimulus not suffi- 

 cient to call out a reaction alone, may do so, if repeated at regu- 

 lar intervals for a sufficient number of times. One method of 

 accomplishing this demonstration consists in placing the stem of 

 a grass in a moist chamber and revolving it on its axis in a hori- 

 zontal position. The clinostat is so arranged however that the 

 stem remains motionless for a regular length of time and is then 

 revolved an equal period, then is held motionless, etc. After a 



Fig. 35. Showing method of cutting off shoots under water. 



time curvature is produced as a result of the summation of the 

 effects. 



Most interesting results have been obtained by F. Darwin and 

 D. Pertz by giving a stem two sets of diametrically opposed 

 stimuli. The clinostat was arranged to make half revolutions 

 with periods of rest so that the stem bent first in one direction 

 then in another. As the result of this alternating stimulation the 

 stem acquired a regular rhythm of movement which continued 



1 Czapek, F. Ueber den Nachweis der geotropischen Sensibili tat der Wurzelspitze. 

 Jahrb. Wiss. Bot. 35 : 313. 1900. 



