1919] HOAGLAND—CELL SAP OF BARLEY 303 



dynamic systems. The soil solution, as previous work in this labo- 

 ratory has definitely shown, is changing from day to day, and 

 gradually attains a very low concentration at the time when the 

 plant has completed its maximum absorption. Without consid- 

 eration of these phenomena it would be useless to hope for any 

 clear understanding of soil fertility problems as related to plant 

 requirements. 



That some relation must exist between the inorganic elements 

 in the plant and its nutrient solution is apparent from the data 

 obtained in the water culture experiments. Similarly, concentra- 

 tion and composition of the soil solution should affect the absorp- 

 tion by the plant, but here it is very difficult to establish the 

 relationship. For this purpose it would be necessary to appraise 

 not only the concentration of the soil solution at a given time, 

 but its potentiality for renewal. In the work previously referred 

 to an idea of this factor has been obtained by comparing cropped 

 and uncropped soils. The use of this method has made it possible 

 to show some rather definite relations between water extracts 

 of the soils and crop yield, but in the experiment now under con- 

 sideration the composition of the sap at the given period bears 

 no constant relation to the final yield of the crop, nor to the com- 

 position of the water extracts. An exception to this statement 

 may possibly be found in the case of phosphorus, where the 

 concentration of PO4 in the soil solution is not improbably reflected 

 in the sap. The K and Mg, and to a less extent the Ca, are of 

 approximately the same magnitudes in all samples. While the 

 soils in question varied considerably in their productivity, appar- 

 ently the average concentrations of the soil solutions were not 

 sufficiently different clearly to influence the concentration of the 

 cell sap. 



It does not follow from the foregoing that further study will 

 not indicate a connection between the soil solution and the ele- 

 ments absorbed by the plant. Before the question is decided it 

 will be necessary to examine not only the cell sap, but the total 

 composition and yield, with the strictest control of the soil and 

 study, not of the ripened plant, but the plant in the various stages 

 of active metaboHsm. 



