Hoagland — 134 — Plant Nutrition 



The general tendency is for the root cells to make 

 metabolic adjustments in the direction of maintenance 

 of hydrogen ion concentration in the sap within a 

 limited range of values. Evidence for this can be 

 gained only by observations on composite sap ex- 

 pressed from the tissues and this admittedly is a 

 crude method. It is true also that there are appre- 

 ciable variations of pH of the expressed sap under 

 different physiological conditions. Nevertheless, con- 

 sidering the impossibility of obtaining for study sap 

 in an unaltered state, and the great variety of con- 

 ditions to which the root tissues have been subjected, 

 the evidence for a certain stability of hydrogen ion 

 concentration, provided that injury to the cells is not 

 present, is noteworthy and must be regarded as of 

 physiological significance. 



In order to understand the effects on respiration 

 of selective ion absorption, it is necessary to determine 

 not only carbon dioxide produced, but also oxygen con- 

 sumed and to take into account the respiratory quo- 

 tient, CO2/O2. This has been done in the investigation 

 of root metabolism just mentioned. An apparatus 

 was employed through which air or other gas mix- 

 tures were circulated and in which oxygen could be 

 kept at a desired tension through the automatic gen- 

 eration of oxygen by electrolysis. As oxygen was ab- 

 sorbed by the tissues, compensatory volumes of oxygen 

 were produced. The carbon dioxide evolved was trapped 

 in a sodium hydroxide solution. The values as a whole 

 for respiratory quotients are in satisfactory accord 

 with the view suggested with regard to organic acid 

 metabolism and salt absorption. The entrance of basic 

 ions without equivalent entrance of anions (unless of 

 HCO3 or OH ions) results in less carbon dioxide given 

 off in respiration in relation to oxygen consumed — 

 sugar is oxidized in part to organic acids. The excess 

 absorption of anions accompanied by a decrease of 

 organic acids leads to an increase of the respiratory 



