AFTER-EFFECTS OF STIMULATION 77 



114. Effect of Slow Revolution of the Clinostat. Secure a 

 seedling of a species which has been tested as above, and put it 

 in a cylindrical glass moist chamber and fasten it to a piece of 

 cork by means of a pin. Cover the chamber with a cloth to ex- 

 clude the light, or perform the test in a dark chamber. Fasten 

 the glass cylinder to the horizontal arm of the clinostat so that it 

 may revolve about its own axis. Adjust the instrument so that 

 it will turn the main shaft through one revolution in about twice 

 the reaction time of the seedling. Note an hour or two later. 

 Generally no curvature will be seen. As the root revolves about 

 its own axis it is stimulated successively upon all sides, but not 

 sufficiently to secure a perception or presentation of the stimulus. 

 Now lengthen the period of revolution of the clinostat to four or 

 five times that of the presentation period. The roots will be 

 seen to curve toward all sides of the chamber in succession, in 

 response to the several geotropic stimuli which it has received. 



115. Influence of External Conditions upon Geotropic Reactions. 

 Changes in temperatures between 15 and 30" C. cause only minor 

 variations in geotropic reactions. Above and below these figures 

 the presentation time is greatly lengthened. 



Anaesthetics even in minute proportions decrease the delicacy 

 of the sensory action, increasing both the presentation and reac- 

 tion times. Carbon dioxide, or any inert gas that excludes oxygen, 

 hinders geotropic reactions, and other chemical agents exercise 

 an effect due to the manner in which they promote or decrease 

 general metabolic activity. 



116. Delayed Reactions. If an organ is placed in a position to 

 receive a geotropic stimulus, and is held mechanically so that the 

 reaction movement may not take place, for a period of several 

 hours, the removal of the mechanical hindrance may result in a 

 curvature even though the zone of growth has moved forward 

 and the curvature is caused by a different lot of protoplasts from 

 that which was originally stimulated. 



117. After-Effects of Stimulation. The after-effects of a stim- 

 ulus which has been given an organ, and response prevented 



