EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 371 



were obtained with a tube having a bore I square mm. in 

 section, and employing an ordinary magic lantern objective, 

 which casts an image without any magnification. The 

 Method of Balance again, as we have seen, affords us another 

 opportunity of arriving at an experimental adjustment of 

 great sensitiveness. 



The experimental delicacy obtained by high magnifica- 

 tion can only be used to the greatest advantage when 

 coupled with the Method of Balance, if we have means at 

 our disposal for securing the utmost possible perfection of 

 the balance. We have seen that the balance is adjusted 

 when the rate at which water is removed from the plant- 

 vessel by suction is exactly equal to the rate of its inflow 

 from the compensating vessel, C. This compensation is 

 roughly effected by the rack and pinion, s, which serves to 

 regulate the flow by raising or lowering the compensating 

 vessel (fig. 218). For the purpose of the final adjustment 

 the narrow bore of the thick india-rubber tubing, which con- 

 nects the compensator with the plant-vessel, is capable of 

 gradual constriction. This must be accomplished by equal 

 compression on all sides, as bilateral compression alone 

 would act to induce a discontinuous closure and sudden 

 arrest of flow. If the compressing arrangement be something 

 after the model of the iris-diaphragm, then the bore, which 

 regulates the flow, may be constricted gradually and con- 

 tinuously. With such an arrangement it is easy to arrive at 

 a balance so perfect that the index appears to be quite 

 stationary. Under-balance would now make it move, say to 

 the right ; and over-balance to the left. 



We shall now proceed to show that those agents which 

 exalt physiological activity also act to enhance suction ; and 

 that those which induce physiological depression will also 

 depress suction. One of those which enhance the multiple 

 activity of the tissue is, as we know, the rise of temperature ; 

 whereas cooling, or lowering of temperature, tends to depress 

 it, even to the extent of abolition. Thus an automatically 

 vibrating leaflet of Desmodium has its vibration-frequency 



