4 8 TRANSPIRATION AND ASCENT OF SAP ch. 



assumed it to move upwards until it was drawn into a 

 medullary ray cell lying at a higher level in the stem. 

 The reason given for the motion upwards in the tracheae 

 rather than downwards in obedience to the gravitational 

 force, is because the air pressure in the tracheae above is 

 less than in those at lower levels. For this difference of 

 pressure in the air bubbles in the stem Godlewski relies 

 on Hartig's results. Hartig had, indeed, previously shown 

 that the percentage of air in the higher parts of stems is 

 less than that in the lower, but this does not necessarily in- 

 volve a lower pressure. Godlewski claimed for his hypothesis 

 that it explained the relations of the tracheae to the paren- 

 chymatous tissues, the radial position of the bordered pits, 

 which facilitates a staircase motion of the water upwards 

 in the stem, and the radial intercellular spaces along the 

 medullary rays, which afford the aeration necessary for 

 the respiratory liberation of energy in these cells. 



Janse supported Godlewski in a general way, and was 

 one of the first to point out that, if the lower part of a 

 branch be killed, the leaves above fade, as a rule, within 

 a few days. This result is obtained even when the branch 

 remains attached to its supporting stem. Janse, at first, 

 assumed that this wilting is due to the interference with 

 the vital sap-raising functions of the wood parenchyma, 

 and the consequent failure in the water supply. He also 

 conceived that the pumping action of the medullary rays 

 is polarised, water being regularly absorbed on one side 

 and expelled on the other. 



Westermaier, who stated his views about the same time 

 as Godlewski, also maintained the water-raising function 

 of the cells in the wood. But, while Godlewski believed 

 the major part of the motion to take place in the tracheal 

 tubes, Westermaier considered the upward passage to be 

 effected in the wood parenchyma, while the vessels and 

 tracheids acted as water reservoirs rather than as con- 

 ducting pipes. Godlewski contrasts his view and Wester- 



