366 COMPARATIVE ELECTRO-PHYSIOLOGY 



in a highly dilute solution of silver nitrate. A portion of the 

 tissue higher up is now excited by contact with a hot wire. 

 The excitation thus induced is then found to travel through 

 the intervening distance, with a velocity characteristic of the 

 conducting power of the tissue. The arrival of the excita- 

 tory wave at the lower end is attended by an expulsion of 

 the cell-sap containing the salt solution previously absorbed. 

 This expulsion is instantly made visible by the formation of 

 a dense white precipitate of silver chloride. From this 

 experiment it is seen that the passage of excitation is at- 

 tended by a forward movement of water in the direction of 

 propagation. 



The next point to be realised is that a strong or a long- 

 continued stimulus will give rise in the tissue, not to one, 

 but to a multiple series of propagated waves. If the tip of 

 a leaf of Biophytum be strongly excited, we see successive 

 waves of excitation, marked by the serial fall of the motile 

 leaflets, proceeding again and again in the centripetal 

 direction, from the terminal excited point. If, similarly, the 

 end of the root be excited, by any means, an excitatory move- 

 ment of water will be induced, proceeding away from this 

 end, in an upward direction. It must be remembered, how- 

 ever, that excitation proceeds in all directions from the 

 excited point. If then the point of excitation be terminal, it 

 is evident that the direction of propagation, being away from 

 this, will be upwards. But if the tip of the root be highly ex- 

 citable, then, owing to local excitation, there will also be a 

 certain amount of secretion into the soil. Even highly 

 excitable tissues, however, after continuous stimulation, show 

 a tendency, as we have seen, to the reversal of their character 

 istic response. This secretion at the terminal point of the 

 root will tend to become changed into absorption. Again, 

 looking at the succession of excitatory waves propelling 

 water upwards, we can see that these will leave a deficit of 

 cell-sap behind, which will further act rather for the absorp- 

 tion than for the secretion of fluid. And in addition to 

 these, if there be any other directive influences, such as 



