PROPULSION OF SAP AND ITS VARIOUS EFFECTS 393 



hitherto suggested has been held to account for this. As, 

 however, we now find that the ascent of sap is due to the 

 propulsive energy of vigorous excitatory contraction pro- 

 ceeding from cell to cell, the rapidity of the movement is 

 easily understood. 



We thus see how by the action of this cellular machinery, 

 set in motion by stimulus, an upward movement of water 

 takes place. We have in fact an active chain of pumps, 

 working throughout the length of the plant, partly carrying 

 water themselves, and partly pumping it into the better con- 

 ducting vessels of the xylem ; and there is no limit to the 

 height to which it may, by such means, be lifted. 



Positive and negative pressures due to one cause. — Let 

 us suppose an india-rubber pipe,-open at its upper end, and 

 provided throughout its length with a series of pumps, one 

 above the other, each of these being independently engaged 

 in raising water upwards. The individual activity of these 

 several pumps may or may not be uniform, but, provided 

 that they are sufficiently numerous, when the pipe is placed 

 in connection with a supply of water, the result of their com- 

 bined action will be that water will be sucked in at the lower 

 end, and ejected at the upper, in a uniform stream. 



If now we confined our attention to the lowermost pump, 

 it would appear to us to \iQ forcing water up ; if, on the other 

 hand, we observed the uppermost pump alone, it would 

 appear to be sucking water up ; and if, finally, we selected 

 some intermediate point for scrutiny, we should discover that 

 the pumps above were sucking, and those below pressing 

 water upwards. Thus, one single effect, namely, the rhythmic 

 activity of pumps, is made to appear various, by simply 

 changing the point of view. Again, certain peculiarities of 

 variation of pressure may appear in the pipe as a whole or 

 in particular parts of it, depending on the rates of supply and 

 removal of water. 



(i) Positive pressure. — We may now suppose the 

 aperture of escape at the upper end to be narrowed. The 

 water pumped into the flexible pipe being thus in a state of 



