112 PLANT PHYSIOLOGY 



salt molecules will continue, there will be no accumulation 

 of these either within or without the cell. Common salt will 

 be absorbed hy the cell, therefore, as a purely physical 

 necessity, regardless of the presence of the other salts dis- 

 solved in the cell-sap, and regardless of the fact that it is 

 needed and used by the cell only in the minutest quantity 

 if at all. On the other hand, if the common salt were used 

 in quantity by the cell, or in any other way removed from 

 the cell-sap ( by decomposition, precipitation, or otherwise ) , 

 the proportion of common salt to water in the cell-sap 

 would always be lower than outside the cell, there would 

 always be osmotic pressure inward, the molecules of salt 

 would constantly force their way into the cell in the attempt 

 to attain osmotic equality or balance, there would be con- 

 tinued absorption of common salt, and the rate of absorp- 

 tion would vary with the osmotic pressure. Such is the case 

 with salts used by the cell or otherwise removed from their 

 solution in the cell-sap. For example, the nitrates are, as 

 we have seen, the best form in which nitrogen is taken in by 

 most plants, and of these potassium nitrate is perhaps the 

 most common. Potassium nitrate will be absorbed by the 

 plant with an avidity proportioned to the need of nitrogen, 

 or — to state this in physical instead of physiological terms — 

 at a rate depending upon the difference in the proportions 

 of potassium nitrate within and without the cell. If the 

 nitrate be decomposed and the nitrogen used by the plant 

 as fast as it is absorbed, the osmotic pressure will remain 

 as great, the absorption will continue as rapid, as at the 

 beginning. Upon the amounts needed, used, or otherwise 

 taken out of the cell-sap by the living protoplasm will de- 

 pend the amounts of different salts absorbed by plants be- 

 yond those amounts necessary to secure uniformity of 

 composition in cell-sap and outside water if no salts were 

 consumed. In this consists the so-called "selective power 

 of roots" and of plant-parts in general. 



We see, then, that the absorption by the cell of those sub- 

 stances which can pass through cell-wall, cytoplasmic mem- 

 branes, and protoplasm is a physical necessity whenever 

 there is any higher proportion of these substances outside 



