22 Henry H. Dixon. 



like the equivalent to the pressure of a column of water equal to 

 6 times their height. 



To sum up: 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 slowed down, have been 

 shown rather to indicate that the death of these cells is followed by 

 a stoppage of the tracheae and the more or less complete poisoning 

 of the leaves. 



Experiment has failed to show directly the presence of vital sap- 

 lifting forces. 



Experiments have abundantly shown that when the vital activities 

 of the cells of the wood are removed, water under the action of purely 

 physical forces rises in the stems of high trees. 



Physical Theories. 



The vital Hypotheses of the Ascent of the Transpiration Current, 

 as we have seen, take no direct account of the in flow of energy at 

 the leaves. The entire sap-lifting force is applied in the stem. This 

 appears to hold good for all the vital hypotheses with the exception 

 of that of Ewart who admits that possibly some of the energy 

 needed to raise the water may be directly transmitted downwards 

 from the leaves to the stream in the stem. Of course ultimately 

 the energy assumed by the vital hypotheses to be expended in the 

 stem is derivable from the energetic substances formed in the leaves 

 during photosynthesis and afterwards distributed to the cells of 

 the stem. 



In the same way some hypotheses which invoke no vital acti- 

 vities, but which depend solely on physical forces, assume these 

 phj^sical forces to be exerted in the stem, and that any transference 

 of the energy from the energy-absorbing leaves to the rising water 

 is indirect, if such a transference occurs at all. 



The first of these which calls for mention is the gas-pressure 

 theory of Böhm and H a r t i g. 



