LATENT PERIOD, REACTION TIME 



73 



111. Latent Period, Reaction Time, Presentation Period. It is 



a familiar observation that a stem of almost any ordinary plant 

 which grows upright habitually, will show a curvature of the tip 

 if the plant is laid on its side for an hour. The period elapsing 

 between the prostration of the stem, which allows the geotropical 

 stimulus to act, and the beginning of the upward curvature, is the 

 reaction time, or latent period. Not all of this period is necessary 

 to secure the curvature however. If the plant is laid on its side 

 for i 5 to 20 minutes and then set upright, or revolved on its own 



loo 



80 

 60 

 40 

 20 



20 40 60 80 100 120 140 160 180 



FIG. 31. Curve of geotropic reaction showing relation of exposition time and re- 

 action time. The figures along the vertical line indicate exposition times, and those 

 along the horizontal line reaction times. After Czapek. 



axis in a horizontal position, the curvature will take place. The 

 conclusion is justifiable therefore, that a portion of the original 

 period was necessary for the reception of the stimulus, and that 

 the remainder was necessary for the action of the curving mechan- 

 ism. The first period is termed the presentation time and is in- 

 cluded in the reaction time. The action of gravity during the 

 presentation time may be likened to a summation of numerous 

 stimuli, the effect of which cumulates with their repetition. Thus 

 most hypocotyls and roots show curvature after being placed in a 



Nemec, B. Ueber die Art der Wahrnehmung des Schwerkraftreizes bei den Pflan- 

 zen. Ber. Deut. Bot. Ges. 18 : 241. 1900. 



Haberlandt, G. Ueber die Perception des geotropischen Reizes. Ber. Deut. 

 Bot. Ges. 18: 261. 1900. 



