THE RISE OF THE TRANSPIRATION STREAM: AN 



HISTORICAL AND CRITICAL DISCUSSION, 



CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY, 



XXXVIIL 



Edwin Bingham Copeland. 



[^Concluded fro?n page /9J.] 



VIII. The theories ascribing the rise of sap to the atmospheric 

 pressure and to differences in the tension of gas within the wood 

 are not fundamentally distinct. It istrue that Bohm felt called 

 upon to drop the former when he accepted the latter; and R. 

 Hartig advocated the latter while altogether discrediting the 

 other. But if the living cortex of the root be regarded as inactive, 

 it is clear that an excess of pressure outside will force water into 

 the root, just as surely as it will make it move in the wood toward 

 the point where the pressure is least. This is realized in dead 

 roots, which, as Is well known (Saussure ; Hansen ; Janse ; Bohm, 

 1889), can temporarily at least supply the demands of transpira- 

 tion. In the living root the influence of the atmospheric pressure 

 must be the same; it may be opposed by friction or aided by 

 osmosis ; but in itself, so long as there is water outside the root 

 and the pressure within is less than an atmosphere, it will push 

 the water inward. Pressing water into the root must directly or 

 indirectly press other water up the stem. The cortex of the root . 

 is easily permeable to water; and water outside but in contact 

 with it will be subject to and will transmit the full pressure of 

 the atmosphere. There is, therefore, no foundation for the idea 

 (Strasburger, 1893: 55 ; Goppelsroeder, 1901 : 211) thatthe atmos- 

 pheric pressure cannot operate to raise the transpiration stream 

 merely because the walls obstruct the passage of the air itself. 

 When root pressure is not active the atmospheric pressure is 

 probably the chief cause of the passage of water into the root; 

 at least this conclusion is indicated by the excellent work of 



260 [OCTOBER 



