MODIFICATION OF SUCTION AL KESPONSl'. 377 



pressure, exerted by suction from below, and the downward 

 pressure of the superincumbent water-cokimn. There are 

 reasons for thinking that there may also be an important 

 additional factor, of opposed rhythmic activities, balancing 

 each other. For when a cut branch is placed in water, the 

 lower end becomes over-turgid, and this, as we know, is 

 a condition for the initiation of rhythmic activity. The 

 upper end of the branch not being so turgid, activity will 

 be greater below than above. We therefore obtain a 

 movement of water from the more to the less active. But 

 if the leafy end of the branch be immersed in water, and 

 the cut end held in the air, it is known that the direction of 

 the flow becomes reversed. This is evidently due to the fact 

 that it is now the upper end which is over-turgid, and there- 

 fore relatively the more active. Similarly, in the case of the 

 balance described, the activity of the lower end, which deter- 

 mines the flow upwards, was opposed and balanced by the 

 increased activity of the upper half, induced by increased 

 hydrostatic pressure.^ The experiment which I am about 

 to describe, besides demonstrating the effects of cold and 

 warmth, also lends strong support to the view that the direc- 

 tion of the resultant movement of sap is determined by the 

 relative activities of the two ends of the stem. 



When the balance had been obtained, as already described, 

 with a pressure of water of 105 cm., the record on the drum 

 became horizontal, as has been explained. Cold water 

 was now applied to the specimen at its lower end. As 

 sudden cooling constitutes a stimulus, while its continued 

 Action produces depression, we should expect a transient 

 augmentation of the activity of the lower end of the specimen, 

 followed by its diminution and arrest. The record should 

 therefore show a preliminary movement of water upwards, 

 followed by the reversal of the current, which should now, as 

 the permanent effect of cold applied below, be from the 



' Rhythmic activity is, in general, increased by an increase in the hydrostatic 

 pressure. But there is a limit to this. Excessive pressure, above a certain critical 

 point, is found to depress rhythmic activity (p. 350). 



