22 Scientific Proceedings, Roijal Dublin Society. 



however, are likely to commend themselves, and indeed are scarcely seriously 

 put forward by their autlior. 



Ewart believes these lifting forces to be feeble, and has not, as he himself 

 admits, in any case obtained unequivocal evidence for their existence. And 

 yet, according to his own figures, these forces should be easily demonstrable.' 

 According to him, the pressure required to raise water at the transpiration- 

 velocity in an elm tree 12 metres high would be equivalent to a head of 

 75'6 metres, i.e., about 7'5 atmospheres. Of this he admits^ about 2 atmo- 

 spheres might be supplied by the tension set up by the transpiring leaf-cells, 

 leaving about 5-5 atmospheres to be made good by the lifting forces of the 

 cells in the 12 metres of stem. Therefore the lifting force of the cells of this 

 stem must amount to 0*45 atmosphere per metre of stem, or to a head of 

 water equal to 4f times the length of stem. 



A lifting force of this magnitude should be easily revealed if the velocity 

 of flow through a branch in the normal direction for a given head were 

 compared with the flow in the reverse direction, or, again, if the amount 

 transmitted downwards in a living stem were compared with that transmitted 

 after death. As is well known, experiments have not been able to demonstrate 

 a sensible difference in either case. Furthermore, Ewart,^ working very 

 carefully by a different method, has failed to detect the existence of these 

 pumping actions in stems. Consequently it is quite impossible to admit 

 that any large amount of work falls on the cells of the stem in the raising 

 of the sap. 



While these considerations show that forces of any great magnitude are 

 not exerted by the cells in the wood on the transpiration-current, it seemed 

 desirable by some more careful method to test the matter, and see if some 

 much smaller force were not assisting the upward flow of water. 



In the ordinary methods of testing this question, uncertainties arise from 

 the fact that conditions are not the same before and after the reversal of 

 the current, or before and after the death of the branches. These differences 

 are principally due to changes in temperature, which, as Bwarf has pointed 

 out, entail large differences in viscosity, and to clogging in the experimental 

 stem. 



In order to eliminate these sources of error, and so be in a position to 

 detect the effect of even a very small force exerted by the stem-cells in 

 lifting water, I carried out some experiments in the following manner : — 



Two straight branches {A and B, fig. 1), about 80 cms. long, without 



' Roy. Soc. PhU. Trans., B, 1905, p. 56. - Roy. Soc. Phil. Trans., B, 1908, p. 379. 



' Roy. Soc. Phil. Trans., B, 1905, p. 77. * Roy. Soc. Phil. Trans., B, 1905. 



