CHAPTER III 



ASCENT OF SAP IN STEMS. CRITICISM OF VITAL THEORIES 



With the overthrow of the imbibition hypothesis, and 

 the establishment of the fact that the major part of the 

 transpiration current moves in the lumen of the vessels 

 and tracheids, the pendulum of opinion among botanists 

 swung over to the vital theory of the ascent of sap. 



Godlewski. Thus in 1884 Godlewski formulated 

 a view which won many supporters, and, with modifica- 

 tions, still seems . to appeal to some. 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 pres- 

 sure of the cell and the resistance to filtration of the mem- 

 brane opposite to one tracheae 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 tracheae, Godlewski 



