368 PLANT RESPONSE 



An agency which induces the positive variation will then 

 be excitatory, while that which induces the negative is 

 depressing. In order to carry out my investigations along 

 these lines, I have employed two different methods of balance. 

 In one — the Hydrostatic Method of Balance — the natural 

 suction of the plant is arrested by a counter-hydrostatic 

 pressure suitably applied. The effect of an external agent 

 is now studied by the direction, positive or negative, and 

 the extent to which it disturbs the static equilibrium thus 

 established. Experiments carried out on this method will 

 be described in the next chapter. 



The second — or Hydraulic Method of Balance — depends 

 upon an application of compensation, by means of which 

 under normal conditions the water-index is kept stationary, 

 though the suctional movement in the plant is in no way 

 disturbed. According to this hydraulic method, the normal 

 rate of suction, when balanced, gives rise to a neutral, or 

 horizontal, line in the record ; while an exciting agent 

 produces an inclination upwards ; and a depressing agent a 

 declination downwards. The balance, by which under normal 

 conditions the neutral line is secured, is obtained by allowing 

 water to enter the plant-vessel from the compensator C at a 

 rate exactly equal to that of its withdrawal by suction. 



In practice, this adjustment is roughly made by opening 

 the stop-cock A' in connection with c, to a greater or less 

 extent. Over-balance causes movement of the water-index 

 to the left ; under-balance to the right ; and when the adjust- 

 ment is perfect, the water-index becomes stationary. After 

 making the preliminary adjustment, the final balance may be 

 obtained, by very careful and gradual movement of the com- 

 pensating reservoir up or down. When the reservoir is 

 raised the flow is increased, owing to the greater difference 

 of level established, as between the reservoir and the plant- 

 vessel. The stand on which the vessel c is placed is pro- 

 vided with a rack and pinion, by means of which the necessary 

 adjustment of height is made. 



Introduction of changed conditions. — We have thus 



