igiQ] HOAGLAND—CELL SAP OF BARLEY 301 



It is evident that all samples of tops have almost the same 

 Pj^ value, although the electrical resistances and osmotic pres- 

 sures may vary widely. It would seem that the reaction is 

 governed by a definite buffer system. In this connection it will 

 be of interest to state that other experiments reported elsewhere 

 have shown that the plant possesses a marked regulatory influence 

 in the selective absorption from the various phosphoric acid anions ; 

 that is, either an alkaline or acid nutrient solution has its reaction 

 quickly changed to approximate neutrality.^ 



The measurements of H ion concentrations on the sap from 

 the tops were very definite and constant, but the determinations 

 on the root sap were less satisfactory. An increase in alkalinity 

 was noted during the measurement, possibly due to the reduction 

 of NO3 and the absence of a sufficient buffer effect. Apparently, 

 however, the sap expressed from the roots has a nearly neutral 

 reaction in several cultures. 



Chemical analyses of plant sap 



The analyses presented in table II were made on the expressed 

 sap from plants grown in 6 soils of different origin and produc- 

 tivity. The soils were kept at optimum moisture content and 

 under strictly controlled conditions, and 6 or 7 weeks after plant- 

 ing one or two tillers were separated from each of about 20 plants 

 for each soil examined. The sap was obtained by the procedure 

 already described, and then diluted and filtered through a porce- 

 lain candle to separate out any suspended material. The analyses 

 were made by the methods described by Stewart (6). 



The content of the individual ions substantiates the high 

 conductivity measurements on the sap. All ions are present in 

 relatively great concentration, including the NO3 ion. Some idea 

 of the relation between the composition of the cell sap and soil 

 solution may be gained by comparison with the data for the 

 soil extracts made at about the same time. There is reason to 

 beheve that the relation of several important ions is somewhat 



'Later experiments have indicated that the HCO3 ion formed is of greatest im- 

 portance in regulating the reaction of complete nutrient solutions. A solution of 

 KH2 PO4 alone retains about the same P^ value. 



