44 TRANSPIRATION AND ASCENT OF SAP ch. 



its elaborator and defender, made it survive an incredibly 

 long time, despite the accumulation of crushing evidence 

 from structure, and of clear inference from experiment. 



Osmotic hypothesis. Various writers have endea- 

 voured to explain the rise of the transpiration stream by 

 appealing to the action of osmotic phenomena. In 

 almost all the vital theories the hypothetical pumping 

 actions of the cells are supposed to be effected by osmosis. 

 Consequently, the osmotic hypothesis, so far as the osmotic 

 actions in question are supposed to be manifested in con- 

 nection with the cells of the wood, has been discussed 

 with those theories. It is, in fact, hard to see how osmotic 

 pressures can be generated in the conducting tracts apart 

 from these cells. In the cells only are to be found semi- 

 permeable or approximately semipermeable membranes. 

 Larmor's suggestion that an upward movement is deter- 

 mined by a gradient of concentration in the solutions 

 contained in the tracheas from below upwards, could only 

 apply to an ideal state of things, and is negatived by 

 the facts : (1) The membranes of the tracheae are freely 

 permeable to dissolved substances ; (2) no such differences 

 in concentration are found ; (3) the resistance to flow 

 upwards and downwards is the same. 



With regard to the second objection enumerated above, 

 some recent experiments on the concentration of the sap 

 in the roots and in the stem at various levels are of par- 

 ticular interest. 



It has been found possible to extract the sap from the 

 wood of roots and stems of transpiring trees by centri- 

 fuging short lengths cut from these organs. Considerable 

 quantities of the sap, quite unaltered, may be obtained in 

 this manner, and the molecular concentration of the solutes 

 in them accurately measured by means of cryoscopic and 

 electrical conductivity determinations. In each case it has 

 been found that the concentration of the sap is sensibly less 



