REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 723 



transpiration from the leaves is seen from the fact that in a 

 saturated atmosphere it continues to take place. That it 

 is not, again, fundamentally due to the osmotic action of the 

 concentrated cell sap in the leaves is seen from the fact that 

 the ascent continues to take place on the removal of leaves. 

 It is seen again from the further fact that under favourable 

 circumstances, on the application of an osmotically strong 

 solution of sodium chloride to the root, the cell sap, instead of 

 being withdrawn by osmotic action, is made, by the excita- 

 tory effect of the salt, to ascend more vigorously. 



The ascent of sap is thus an excitatory phenomenon, and 

 its uni-directioned flow is due to the graduated passage from 

 point to point of the co-ordinated excitatory reaction, 

 propelling water forward. This rhythmic excitation is ini- 

 tiated in the intact plant at its root, by the stimulus of 

 contact with soil, the friction of the growing organ against 

 rough surfaces, the excessive turgidity caused by the absorp- 

 tion of water, and possibly by the chemical stimulus of 

 substances present in the soil. In the case of cut branches 

 placed in water, the excessive turgidity at the cut end 

 initiates rhythmic activity, which drives the water upwards ; 

 but if such a branch be placed upside down, with its foliage 

 in water, the now turgid anatomically upper end becomes 

 the seat of excitation, and the direction of the flow of sap 

 is reversed. 



The connection between the conduction of stimulus and 

 conduction of water is seen from the fact that the movement 

 of water takes place preferentially along those channels 

 which are also good conductors of excitation. Hence it is 

 transported more easily along the plant than across it ; and 

 while the movement is possible either upwards or down- 

 wards, yet it is quicker in the upward direction, which is 

 also preferentially the direction of conduction of stimulus. 



The same movement of water which is produced by the 

 co-ordinated rhythmic activity of cells throughout the plant 

 appears either as suctional or as pressure movement, accord- 

 ing to the point of view which we adopt. When the removal 



3 A 2 



