THE ASCENT OF SAP 191 



All the mechanical theories of ascent having broken 

 down under the test of experiment, it was left for 

 Godlewski, in 1884, to suggest an explanation based on 

 the osmotic activities of Hving cells. " He assumed 

 a periodic change in the permeability of the osmotic 

 membranes of the parenchymatous cells contained 

 within the wood in order to bring about a pumping 

 action which would account for the raising of water in 

 the tracheae of the stem. Thus, supposing a cell of a 

 medullary ray in contact with eight tracheae, four on 

 each side, to draw water into itself and to increase its 

 turgor so that its protoplasmic membrane is considerably 

 stretched, and assuming the osmotic pressure of the cell 

 and the resistance to filtration of the membrane opposite 

 to one trachea to be periodically and suddenly diminished 

 owing to a chemical change, then it is evident that the 

 contractility of the protoplasm will cause water to escape 

 through the most permeable spot of the membrane, viz. 

 into the trachea opposite to which filtration is most easy. 

 Once in the trachea, Godlewski 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 trachea rather than down- 

 wards in obedience to the gravitational force, is because 

 the air pressure in the tracheae above is less than in 

 those at lower levels. . . . Godlewski claimed for his 

 hypothesis that it explained the relation of the tracheae 

 to the parenchymatous tissues, the radial position of the 

 bordered pits, which faciUtates 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 Uberation of energy in these 

 cells" (Dixon). 



In the same year Westermaier put forward the same 

 hypothesis, though in a somewhat altered form, the 

 parenchyma being regarded as the chief pathway of ascent 

 and the vessels being rather water reservoirs than conduits. 



