138 TRANSPIRATION AND ASCENT OF SAP ch. vi 



after that point is to be attributed to the clogging, which 

 is practically unavoidable when the flow is rapid. 



Summary. It appears that water may be moved 

 through a stem in a horizontal position with the velocity of 

 the transpiration current if urged by a head equal to the 

 length of the stem. To raise water in a vertical stem at the 

 same velocity, evidently twice the head will be required. 

 Consequently when the force is applied as tension at the 

 upper end, the greatest stress the water need be subjected 

 to is double the weight of the moving column. Even in 

 the highest trees this is vanish ingly small compared to the 

 tensile strength of water. 



Literature. 



Darwin, F., " Observations on Stoinata/' Phil. Trans. Roy. Soe. London, 

 1898, vol. 190 B, p. 539. 



Dixon, H. H., "On the Transpiration Current in Plants,'' Proc. Roy. Sue. 

 London, 1907, vol. 79 B, p. 41. 



Ewart, A. J., "Ascent of Water in Trees," Phil. Trans. Roy. Soc. London, 

 1905, vol. 198 B, p. 41. 



Id. " Resistance to Flow in Wood Vessels," A tin. of Botany, 1905, vol. 19, 

 p. 442. 



Strasburger, E., " Ueber den Bau und Verrichtungen der Leitungsbahnen 

 in den Pflanzen " (Jena, 1891.) 



