in ASCENT OF SAP IN STEMS 69 



Attempts to detect the action of vital forces. 

 In another form the vital hypothesis has been lately 

 supported by Ewart. Adopting Janse's method of deter- 

 mining the resistance opposed to the transpiration current, 

 Ewart obtained results which indicate that, in order to 

 move water in stems of plants at the velocity of the trans- 

 piration current, pressures equivalent to a head of six to 

 thirty-three times the height of the plant are required. 

 To overcome this resistance, which in the case of the highest 

 trees would amount to 50-100 atm., Ewart feels constrained 

 to fall back on the vital activities of the wood parenchyma. 

 He supposes them to lend a helping hand in some way, 

 and to overcome the resistance of the stem all along its 

 length. 



These hypothetical forces, however they are imagined 

 to be exerted by the living cells, Ewart believes to be but 

 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 re- 

 quired to raise water at the transpiration-velocity in an 

 elm tree 12 m. high would be equivalent to a head of 

 75*6 m., i.e., about 7 '5 atmospheres. Of this he admits 

 about 2 atmospheres might be supplied by the tension set 

 up by the transpiring leaf-cells, leaving about 5*5 atmo- 

 spheres to be made good by the lifting forces of the cells 

 in the 12 m. 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 four and 

 a half times the length of stem. 



A lifting force of this magnitude should be easily re- 

 vealed 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, 



