162 The Plant World 



reel effects of such high concentrations as Fitting has demon- 

 stiated need not be considered here. 



The very moderate lowering (about 10 %) of the 

 vapor tension in the presence of enough solute to yield an os- 

 motic pressure of 100 oi more atmospheres renders it improbable 

 that the concentration of the cell sap of roots should play a 

 direct role in determining the lower limit of available soil mois- 

 ture. When the roots are bathed in a solution ( as in the case 

 of the salt swamps'), and not merely wetted by imbibed or ad- 

 sorbed moisture (covered by a capillary film), then the internal 

 osmotic pressure should be directly opposed to the pressure 

 of the surrounding solution, and it seems quite possible that 

 high sap concentration may accelerate water entrance. At 

 any rate, such osmotic pressures in submerged roots must play 

 an important role in maintaining cell turgidity, which appears 

 to be very necessary for root absorption. Until more is known 

 of the quantitative relations between the energy of surface 

 tension, imbibition, etc., and that of vapor tension and osmotic 

 pressure, as these forms of energy are manifest in the plant 

 and its en\ironment, the whole problem here touched upon 

 must rest in abeyance. 



An interesting suggestion is made by Fitting (p. 257-8) 

 to the eftect that transpiration water in certain desert forms 

 may possibly, during the driest seasons of the year, be derived 

 from the decomposition of carbohydrates, etc., through the pro- 

 cess of respiration. I am not aware that this has ever been 

 suggested before, and the idea seems well worthy of an investi- 

 tion. It is thus rendered paradoxically possible that there 

 may sometimes occur a continuous loss of water from an organ 

 or tissue, which, upon being tested, exhibits almost none 

 of this liquid and which receives no moisture from without! 

 Our author supf oses that this consideraiton may apply to forms 

 which possess accumulated carbohydrates, as in tubers and 

 enlarged roots. 



Fitting's statement (p. 2L5) that "from a purely physical 

 standpoint, all the hinderances to water supply in a saline 

 soil seem to be overcome when the plants growing therein, by 

 an increased cell sap concentration, maintain a concentration 



