CHAPTER XXIX 



MODIFICATION OF SUCTIONAL RESPONSE 



Effect of lemperature on suction by three methods of inquiry: (i) Unbalanced 

 method of Shoshungraph : (a) Action of cold — (<'^) Action of moderate rise of 

 temperature — (2) Method of Hydrostatic Balance : {a) Action of cold — Re- 

 versal of normal direction of flow — (/;) Action of warm water — (3) Method of 

 Hydraulic Balance : (a) Action of cold — {i) Effect of warm water — Explana- 

 tion of suction when the root is killed by boiling water — Stimulation renews 

 suctional activity in plant whose suction has come to a standstill— Osmotic 

 I'ersiis excitatory action —.Vbolition of suction by poison — Suctional activity 

 continued until whole plant is killed by poison. 



I SHALL now proceed to prove that the movement of water 

 in plants is mainly due to rhythmic excitation of the tissue, 

 and that evaporation from the leaves, osmotic action, and 

 so on, are only co-operating factors, of subsidiary importance. 

 We have seen in Chapter XXI. that any part of a stem when 

 excited willjbecome the seat of rhythmic activity, and that 

 this pulsatory excitation causes movement of water. I shall 

 now demonstrate a similar phenomenon by means of suc- 

 tional response, eliminating from some of the typical ex- 

 periments all auxiliary factors, such as osmotic action and 

 evaporation from leaves. I have shown in the last chapter 

 how the effect of suctional activity may be continuously 

 recorded by means of the Shoshungraph. We saw also 

 that the effects of various agents, exciting or depressing, were 

 to be detected, under the unbalanced method, by appropriate 

 variation in the slope of the curve. By the balanced method, 

 whether hydrostatic or hydraulic, the derangement of the 

 balance upwards indicates an increase of activity, and its 

 derangement downwards a decrease. I shall first demon- 

 strate the fact that these observations, though obtained by 

 such various methods, are all reliable and mutually consis- 



