ch. x OSMOTIC PRESSURES IN PLANTS 191 



exert ; ! for the fully distended cells of a leaf may be 

 considerably reduced in size before wilting takes place. 

 The variation in pressure which may be occasioned by 

 this change of volume can be judged by comparing the 

 osmotic pressure of the sap extracted from freshly plucked 

 turgid leaves with that of the sap from part of the same 

 sample when the first signs of wilting are apparent. An 

 experiment of this nature was made on a sample of leaves 

 of Fraxinus oxyphyUa. The sap from the fresh leaves 

 had an osmotic pressure of 15*15 atm., while that from the 

 same sample just beginning to wilt had a pressure of 

 24 - 09 atm. A similar experiment with the leaves of 

 Wistaria sinensis gave 6*61 atm. and 11 '38 atm. as the 

 pressure in the turgid and wilting leaves respectively. 



Again the osmotic pressures, calculated from the 

 freezing points of the saps, should, when applied to the 

 leaves at air temperatures be raised 10-15 per cent. 



We may conclude, then, that the tensions developed 

 on the upper ends of the water columns are usually at least 

 as great as the pressures recorded in the table ; but that 

 inasmuch as fresh leaves were always examined, the 

 pressures indicated in the table are by no means equal 

 to the maximum tensions which may be exerted on the 

 rising sap. 



Determinations. The majority of the determina- 

 tions have been made on sap extracted by the liquid-air 

 method, while some, which are marked in the table 

 ' untreated," have been made on sap simply pressed 

 from the tissues. It is highly probable that, if the 

 samples which yielded these measurements had been 

 treated with liquid-air before pressing, higher values 

 for the osmotic pressures would have been obtained. 

 A similar criticism is applicable in a less degree to those 

 marked ' crushed and diluted." In these an extract 



1 It may be pointed out that in low-growing plants high tensions are 

 probably only developed when the supply of water to the roots is restricted. 



