in ASCENT OF SAP IN STEMS 49 



maier's by stating that lie holds the wood parenchyma 

 cells to correspond to the piston, and the tracheae to the 

 tube of a pump, while the latter holds that the cells act 

 as tube and piston, and the tracheae are the reservoirs. 

 Furthermore, Westermaier assumes that the water is held 

 by capillarity in the tracheae in the form of short columns 

 of water alternating with columns of air. 



It is evident that Godlewski and Westermaier saw the 

 difficulty of assuming that the water, once set free from 

 the cells, would move upwards rather than downwards. 

 Godlewski supposed that the difference in air pressure 

 above and below decided its movement. Westermaier 

 believed it is held by capillary forces in the tracheae at 

 the level to which it is raised by the cells. 



According to the first assumption, however, we are 

 reduced to the theory that the whole upward motion is 

 due to a difference of air pressure above and below. Inas- 

 much as this difference is never so great as one atmosphere, 

 a rise of more than about 10 metres cannot be accounted for. 



Neither could Westermaier's hypothesis be followed, in- 

 asmuch as it was known that the major part of the motion 

 of water upwards takes place in the tracheae and not in 

 the living cells, which, as a matter of fact, offer a great 

 resistance to the passage of water. It was also rendered 

 invalid owing to the fact that water is free to stream past 

 the bubbles, and that they do not offer support for the 

 water in the wood, but only increased resistance to its 

 downward motion. This was made evident by the " drop 

 experiment " ascribed to Sachs and Hartig, and also by 

 Schwendener's observation, that the bleeding sap does 

 not carry the bubbles with it from the tracheae. Wester- 

 maier's hypothesis also, as has been often pointed out, is 

 quite inapplicable to Conifers, for in their wood there is 

 no vertical connection of cells placing the medullary rays 

 of various levels in communication with one another. 



Boucherie's experiment. One of the most 



E 



