ABSORPTION AND MOVEMENT OF WATER 113 



the cell than in it. When there is no difference in propor- 

 tion, there will be no absorption or no further absorption. 

 When, by the vital needs and activities of the cell or of the 

 plant, a difference is maintained, there will always be ab- 

 sorption, proportioned in rate to the difference, propor- 

 tioned in amount to the duration of the difference. This 

 accounts for the much higher percentage of potassium than 

 of sodium in the ash of marine algae. The amount of 

 sodium salts absorbed is only such as to attain the osmotic 

 balance of sodium salts in the cell-sap and in sea water, 

 wherea the amount of potassium salts is such as to satisfy 

 the need of the plant for potassium and for the elements 

 associated with it in these salts. The accumulation of io- 

 dine in marine algae is due, not to the demand of these 

 plants for iodine, but rather for the element or elements 

 with which the iodine is combined in sea water : the iodine 

 is, therefore, removed from solution in cell-sap and accumu- 

 lates in insoluble form in the cell. 



THE MEANS OP ABSORBING NUTRIENT SOLUTIONS 



From the foregoing discussion of the physical principles 

 underlying the absorption of nutrient solutions, we can now 

 understand how an alga supplies itself with adequate 

 amounts of food-materials. A land plant, however, in addi- 

 tion to its demand for other food-materials, must regulate 

 its absorption according to its demand for water to make 

 good that lost by evaporation. We must now consider how 

 the land plant adapts itself to the conditions prevailing on 

 land and successfully employs the physical means of absorb- 

 ing the nutrient salts in the soil. 



The root is generally regarded as especially the absorbing- 

 organ of higher plants. The cells on the surface of the root, 

 being the only ones which are directly in contact with the 

 soil particles and with the soil water, are the only ones 

 which can absorb solutions from the soil. Only the young- 

 est of these cells have walls of such composition and thin- 

 ness that osmosis can take place rapidly. Furthermore, 

 owing to the granular nature of soils, and owing to the 

 fact that the water, at times when it is most needed, is held 



