CHAPTER VI 



ESTIMATE OF THE TENSION REQUIRED TO RAISE THE SAP 



In the previous chapter it has been convenient to quote 

 the various estimates hitherto obtained as to the tensile 

 strength of water. We have seen that, contrary to what 

 our everyday experience seems to teach us, water has a 

 very considerable tenacity, amounting at least to 150 atmo- 

 spheres, about 15 kilos, per sq. cm. Sap extracted from 

 the conducting tracts is, in this respect, not inferior to 

 pure water. Moreover, the presence of dissolved air in it 

 does not diminish its cohesion, which, in this condition, 

 has been demonstrated up to 200 atm., or 20 kilos, per 

 sq. cm. 



It now remains to determine how far this tenacity will 

 be taxed, and what forces are required for moving the 

 tensile sap through the water tracts of the plant. 



Ewart's high estimate of the resistance. 

 In a paper published in 1905, Ewart investigates the 

 question as to what force is required to move water 

 through the waterways of plants at the same velocity as 

 the transpiration current. His general conclusion is that 

 the resistance is so great that neither are sufficient forces 

 generated in the leaves to raise the water at the required 

 velocity, nor is the tensile strength of water adequate to 

 transmit these forces downwards, if such existed. In a 

 subsequent paper he seems to have modified this latter 



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