50 CHAP. VII. THE PHYSIOLOGICAL FACTOR 



energy of the system, while the negative or catabolic D 

 is associated with run-down of energy. Under external 

 variation there is thus produced either the A or the D effect, 

 or both A and D in different proportions, the resultant 

 being A— D. The different reactions under the incidence 

 of external stimulus may be likened to the effects of the 

 supply of external energy from a dynamo to a storage-cell, 

 and of the expenditure of that energy by a motor. Imagine 

 the storage-cell connected (i) with the dynamo, (2) only 

 with the motor, and (3) with both the dynamo and the motor. 

 In the first case a storage of energy would result (A effect). 

 In the second case there would be expenditure and loss of 

 energy (D effect). In the third case there would be both 

 storage and expenditure, the resultant effect being A— D. 

 As in a storage-cell so also in the plant, the energy-content 

 at any moment is A— D, the algebraical sum of the work 

 done on the plant in storage and the work done by the plant 

 with consequent expenditure and run-down of energy. 



The energy-condition of the storage-cell can be ascer- 

 tained by an electric indicator, the movement of which in 

 a plus or in a minus direction shows the accession or de- 

 pletion of energy. The electrical current, though invisible, 

 can be detected by its chemical effect, or by its deflecting 

 action on a suspended magnetic needle, these being but 

 different indications of an identical current. I have shown 

 elsewhere l that the invisible internal changes in the plant 

 can also be detected by various physiological indicators. 

 For example, the D effect induced by a stimulus is indicated 

 by a negative response of fall of the leaf of Mimosa, and 

 by a simultaneous galvanometric negativity of electric 

 response. The sign of the A effect is opposite, a positive 

 or erectile response of Mimosa, and an electrical response 

 of galvanometric positivity. The fundamental proto- 

 plasmic reactions A or D are not manifested at haphazard, 

 but are responsive changes associated with definite external 

 expression in different modes of response. 



1 Plant Response (1906) ; Irritability of Plants (191 3). 



