78 TRANSPIRATION AND ASCENT OF SAP ch. 



After the pressure was applied, forcing distilled water through 

 the control, and picric acid through the experimental 

 branch, the rates rose to 0*502 g. and 0*508 g. respect- 

 ively for an average temperature of 120. That is, the 

 rate of transmission of the killed branch has increased by 

 8*3 per cent., while that of the living control has increased 

 by 11" 5 per cent. Probably the rise in both cases is due 

 chiefly to the flushing-out of the branch and the washing 

 away of mechanical obstructions by the stream under high 

 pressure, and partly to the small rise in temperature 

 form 11*8 to 120, which would perceptibly diminish the 

 viscosity of the water. Inasmuch as the observed rise is 

 as great in the case of the living branch as in that which is 

 killed during the observations, it follows that there were 

 no vital actions in either retarding the transmission. 



As was pointed out above, Ewart had recourse to the 

 vital theory, believing that the physical forces available 

 are insufficient to overcome the resistance to the trans- 

 piration current offered by the conducting tracts. It will 

 be shown later that this view was based on an erroneously 

 high estimate of both the resistance experienced and the 

 velocity attained by upward moving water. 



Summary. The structure of the conducting tissues to 

 which the Vitalists appeal as supporting their views not 

 only does not render this support, but the relation of the 

 living cells to the tubes renders it even impossible for them 

 to exert an elevating force upon the water contained in 

 the tubes. 



Experiments which had been quoted in support of the 

 vital hypothesis as demonstrating that when the vital 

 actions of the cells are removed the transpiration stream 

 is so slowed down, that the leaves above fade from lack 

 of water, have been shown rather to indicate that the 

 death of these cells is followed by a stoppage of the 

 trachea) and the more or less complete poisoning of the 

 leaves. 



