194 TRANSPIRATION AND ASCENT OF SAP ch. 



the osmotic pressure of the leaf cells (ranging as it does 

 from 5 to 27 atm.) was such that it was well able to sustain 

 the tension necessary to lift the water current in the plant 

 and to keep the cells turgid during normal transpiration. 

 Renner's figures obtained by a different method are in 

 accordance with this result. He compared the flow due to 

 the tension set up by the leaves, with that caused by a 

 known difference of pressure, in branches in which the 

 resistance had been artificially increased. The tension 

 thus indicated varied between 10 and 20 atmospheres. 



Variations in osmotic pressure. With regard to 

 the causes of the variations in pressure much still has to 

 be found out, but the results obtained up to the present 

 may be of interest. 



In the first place, a series of experiments was designed 

 to test the possibility that the osmotic pressure of the sap 

 of the leaves on any region of the branches is defined by 

 the resistance which has to be overcome in drawing the 

 transpiration current from the roots to that part. Ewart 

 had previously looked for such a difference by means of 

 the plasmolytic method, but he seems to have encountered 

 difficulties and left the question undecided. In the following 

 experiments sap was pressed from leaves taken at a con- 

 siderable height above the ground ; and its freezing-point 

 was compared with that of leaves from near the ground - 

 level. The results of pairs of experiments bearing on 

 this question are shown in Table 31 (p. 195). 



It there appears that, on the whole, taking the experi- 

 ments in pairs, the leaves at the lower level contained sap 

 with a lower (sometimes considerably lower) osmotic 

 pressure than that of higher leaves. But experiments 

 are far from satisfactorily bearing out this view ; for it 

 has been noted that the osmotic pressures of the sap from 

 leaves at the same level, but at different times and under 

 different conditions, by no means correspond in each case, 

 although they are often higher than those of leaves at a lower 



