374 



PLANT RESPONSE 



external conditions is gauged by the uniform inclination to 

 the curve, and it may be well to mention here that through- 

 out the investigation every experiment was begun with this 

 test. 



The normal rate of suction, in the first experiment, in a 

 Croton at the temperature of the room (23 C), was eight 

 cubic mm. per minute. On now applying cold water at a 

 temperature of 4 C. to the root, by appropriate manipulation 

 of the stop-cock, the rate showed the preliminary excitatory 

 effect, due to s.udden variation of temperature, by an increased 



Hi 



Fig. 158. Effect of Cold on Suction 



The first part of the record shows the normal rate of suction at 23 C. 

 Asterisk marks moment of application of cold water at 4 C, which is 

 seen to produce a preliminary exaltation, followed by arrest of suction. 

 Abscissa represents time in minutes ; ordinate, the quantity of water 

 sucked up in milligrammes or cubic mm. 



rate during the first two minutes, of eighteen cubic mm. per 

 minute, or 2*25 times the normal value. But this temporary 

 exaltation gradually passed away, till there was an almost 

 complete arrest, ten minutes after the first application (fig. 

 158). This arrest by cold was not found to be permanent ; 

 for it disappeared on the return to a higher temperature, as 

 will be seen in the first part of the next figure (fig. 159), 

 which was taken after water at 23 C. had been substituted 

 for the cold water in the vessel. In this second curve, the 

 rate of suction is found to return almost to the normal 



