EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 367 



evaporation from the leaves, they will tend to help the 

 particular uni-directioned flow. We have seen that these 

 conclusions are confirmed by the results obtained in the 

 electrical response of the roots. We there saw that very 

 young roots give at first negative responses, which are after- 

 wards reversed to positive, under continuous stimulation. 

 Older roots, as we also saw, give response by positivity. We 

 saw, further, that there was much reason for regarding 

 negative response as associated with the secretion, and 

 positive with the absorption of fluid. 



Thus the serial propagation of excitation from cell to 

 cell, with the concomitant movement of water, will normally 

 be upwards. In this connection it is very significant that the 

 younger portion of the fibro-vascular bundle is the preferential 

 channel for the conduction, at once of water and of excitation. 

 It is thus seen that a one-directioned movement of water 

 may be produced by the multiple excitatory activity 

 of the tissue. And just as the multiple activity of certain 

 tissues, say, for instance, the leaflets of Desmodium, may 

 be gauged by their multiple mechanical movements, so 

 in the rate of the water-movement we have a means of 

 measuring the intensity of the multiple rhythmic activity of 

 those which are concerned in the ascent of sap. This would 

 be analogous to the measurement of the rhythmic activity of 

 the heart, by a determination of the rate of flow of the 

 circulating blood. In the case of the plant, however, this rate 

 of movement might be measured, either by means of the pro- 

 pulsion of water forwards, or by the suction exerted behind. 



In order to demonstrate the fact that the water movement 

 in the ascent of sap is mainly dependent on excitatory 

 reactions, it is necessary to have at our disposal some means 

 of rapidly observing and recording the variations induced by 

 physiological changes in the rate of ascent. For this pur- 

 pose I was successful in devising the Shoshungraph or 

 suction-recorder, described in detail in my book on c Plant 

 Response.' L 



1 Bose, Plant Response, pp. 364-371, 



