Osmotic Pressure in Plan'Ts. 



155 



If we compare the isotonic concentrations determined by 

 Fitting with those which are usual with ordinary plants in 

 humid regions (0.15-0.3 gram molecules of potassium nitrate 

 per liter), we find that the hi\jhest pressure developed by those 

 desert forms is more than thirteen times what we have hitherto 

 considered as usua/. They are perhaps three times as great 

 as the pressure observed in grass stems by Pfeffer. Here- 

 after the highest pressures observed for ordinary green plants 

 must be cited as at least over 100, perhaps as high as 130 atmos- 

 pheres, or even higher. By the use of other solutes — perhaps 

 cane sugar is the most promising — it should be possible to de- 

 termine these high pressures more accurately. 



It is to be noted especially that not all forms growing under 

 the most arid conditions are capable of exhibiting these enor- 

 mous pressures; Anagillis coerulea, for example, even in the 

 driest situations, fails to develop pressures above that of a half- 

 normal potassium nitrate solution. Tests by taste show that 

 manv, but not all, of the plants which exhibit great pressure 

 are highly charged with sodium chloride. Some have an acid 



