PROPULSION OF SAP AND ITS VARIOUS EFFECTS 395 



When the stem of an actively transpiring plant is cut across, 

 the stump of the plant does not always immediately show 

 bleeding, but often, on the contrary, will suck in water. This 

 is generally ascribed solely to the existence of negative pres- 

 sure in the stem. There is another element in the problem, 

 however, which is generally overlooked. By such stimulus 

 as that of amputation, excitation must be produced at the 

 cut end of the stem, which is propagated downwards, and 

 tends to induce a reversal of flow. But when the negative 

 tension and the excitatory effect, due to stimulus of cut, have 

 both subsided, exudation will begin to take place. 



(3) Irregular variations of pressure.- It is evident 

 from what has been said that the hydrostatic pressure at any 

 given zone of the tissue will depend on the relative activities 

 of cells below and cells above. Cells immediately above, by 

 their activity, produce suction or negative pressure ; and 

 those immediately below, an increase or positive pressure. 

 The resultant pressure at any individual zone depends, 

 therefore, on the algebraical summation of these. Now, 

 though all the cells throughout the length of the plant may 

 be active, yet there will be some difference in their activities. 

 Or the same cells again, at different periods of life and 

 different conditions, may undergo variations of their activity. 

 There are, in nature, many disturbing influences which 

 produce local variations of excitability ; hence, by the dis- 

 tribution of cells whose excitabilities are irregular, we may 

 obtain a variation of internal pressure from the top to the 

 bottom of the plant, which is not uniform but fluctuating. 



Direct conduction and conduction by relays. — I have 

 already said that the movement of water, being mainly 

 brought about by excitatory reactions, will take place 

 preferentially along conducting channels. We have seen that 

 every tissue possesses the power of conduction to a greater 

 or less degree ; in parenchymatous cells, however, owing to 

 the presence of numerous more or less complete septa, trans- 

 mission is enfeebled, whereas in prosenchymatous fibro- 

 vascular elements it is very much facilitated. Thus, for 



