SUCTIONAL RESPONSE AND ASCENT OF SAP 363 



correspondingly express itself in a diminished rate of move- 

 ment. The movement of sap is thus taken to be another 

 expression of that autonomous activity (multiple response) of 

 the plant, which we have already seen exhibited locally by 

 the motile tissue of Desmodium. The conclusive test of this 

 would lie in proving that all those agencies which acted 

 in a given way on the autonomous response of Desmodium, 

 act also in the same way in modifying the rate of movement 

 of sap. In other words, just as an exciting reagent will in 

 the one case induce a greater amplitude, frequency, or 

 both, of oscillation, above the normal, so in the other the 

 excitatory nature of a given reagent may be expected to 

 exhibit itself by an increase above the normal, in the rate of 

 flow. A depressing reagent, on the contrary, should produce 

 the opposite effect in both. That is to say, just as we may 

 study the multiple or rhythmic excitability of a tissue 

 through mechanical, electromotive, or electrotactile response, 

 so here, in hydraulic response, or the determination of changes 

 in the rate of flow of sap, we have an independent mode of 

 investigating the same phenomenon. 



The great difficulty of this investigation lies in the 

 absence of a method by which these changes can be imme- 

 diately recorded. In other words, we require some simple 

 means of making a direct record, which will show, in a 

 continuous manner, the changes produced by the various 

 agencies, enabling us to distinguish their immediate effects, 

 after effects, time-relations, and so on. 



We have seen that the propulsion of water forward by the 

 tissue is attended by a suction behind.^ The quantity of 

 water sucked up in a definite period will therefore give us an 

 indication of the rate of movement of sap in the plant. Thus 

 from the readings afforded by the water-index of a potometer, 

 and the times at which such readings are taken, we may derive 



' The forward movement of water, and the suction exerted, in the same tissue, 

 are not necessarily equal in all cases. Part of the water sucked up may be 

 deviated to increase the turgidity of the cells themselves. Suction may neverthe- 

 less be taken as a measure, other things being equal, of rhythmic activity. 



