MODIFICATION OF SUCTIONAL RESPONSE 379 



its sluggishness was now obviated, but that the lower end of 

 the specimen was actually rendered the more excitable of 

 the two, and we observe a second reversal of the direction 

 of current, which now flows upwards. 



(3) Method of Hydraulic Balance. — This Hydraulic 

 Method of Balance is much easier to carry out than the 

 arrest of movement by hydrostatic pressure. It does not, 

 moreover, in any way interfere with the normal movement of 

 water through the plant. For the balance is obtained and 

 the index rendered stationary, by the simple device already 

 explained, of allowing a subsidiary flow of water to enter the 

 plant-vessel, at a rate exactly sufficient to compensate for the 

 loss by ascent of sap. 



(a) Action of cold. — This experiment was performed on a 

 leafy branch of Croton. The balanced horizontal record was 

 first taken at 22° C, after which ice-cold water was passed 

 into the vessel. A record was made of the immediate or 

 preliminary excitatory effect of this cold water on the rate of 

 suction, and continued during the return of the water to the 

 temperature of the room. In this record, then, we shall find, 

 besides the immediate, the continued effect of cold, and 

 subsequently the effect of gradual restoration to an ordinary 

 temperature. In the present record (fig. 162) we see the 

 transient and permanent effects of cold exhibited as before. 

 According to the method of Hydraulic Balance, as has been 

 explained, an ascending line in the record means a positive 

 variation, or increase of the rate of suction over the normal. 

 A descending line, on the contrary, denotes a negative 

 variation, or diminution of the rate of suction below the 

 normal. And a horizontal line shows return to the original 

 rate. In the first, or upper, of the two curves in fig. 162, 

 the immediate effect of cold is seen in a very marked positive 

 variation. After the lapse of about five minutes, the effect 

 of continued cold is seen in the depression, which shows 

 itself by the reversal of the curve. This depression continues, 

 till the temperature of the vessel has returned to that of the 

 room, which takes place in the course of about forty minutes. 



