CH. V, 4] ABSORPTION BY ROOTS 229 



difference is vastly important in both plant and animal 

 physiology. 



It is perfectly clear that the water passes osmotically 

 into the root hair cells, and thence to the cortical cells, 

 which have solutions as strong as the hairs, or stronger. In 

 small simple plants, especially the Moss plants, the water 

 moves thus from cell to cell, throughout the plant. But 

 where ducts are present, as in all of the Flowering Plants and 

 Ferns, the water passes from the innermost cortical cells 

 into those ducts, and does so as pure water, and not as a 

 sugar solution. Why does water leave the cortical cells, when 

 it enters the similarly constructed hair cells ? In a physical 

 machine it would not do so ; the cortical cells would absorb 

 water from the ducts, instead of giving it out to them, 

 precisely as in case of the hairs and the soil. Herein we 

 face a still unsolved problem of plant physiology. Several 

 methods are imaginable, though none have been proven; 

 but there is little doubt that the explanation will be found 

 in some simple chemical or physical change controlled by 

 the living protoplasm. Presumably the method is dependent 

 on the relatively great thickness of the cortex in all ab- 

 sorbing roots ; and it may prove that each cortical cell 

 contributes a little towards breaking the osmotic hold 

 on the water, the cooperation of many being therefore 

 essential. 



In the experiment described a few pages earlier the solu- 

 tion was free to rise. What happens when the tube is 

 closed ? In this case pressure always develops, first stretch- 

 ing, and then bursting the cup, unless very strong ; and if a 

 suitable gauge be attached, the pressure can be measured. 

 The results are surprising, for with cells specially built for 

 great strength, and the use of strong sugar solutions, osmotic 

 pressures have actually been measured in excess of 24 atmos- 

 pheres, that is, 360 pounds to the square inch, which is more 

 than the pressure in most steam boilers ; and we know that 

 greater pressures occur. In cells of the higher plants the 



