The Plant as a Metabolic Unit 233 



temperature to mineral nutrition has received but scant general 

 attention, and adequate data on temperature coefficients for 

 plants of diverse types do not exist. It may be reasonably assumed 

 that temperature limitation of mineral solute absorption some- 

 times constitutes a limiting factor for plant growth. And yet, 

 high temperatures hasten depletion of carbohydrate reserves 

 through accelerated respiration. When plant growth is being 

 studied over an extended period, rate of photosynthesis and of 

 translocation of carbohydrate to the root system, as well as rate 

 of translocation of salts from root to shoot, should be considered 

 in the evaluation of soil temperature effects. 



Hydrogen Ion Effects 



Few factors in the plant's environment have received so much 

 discussion as hydrogen ion concentration, but with each addition 

 to our knowledge it becomes clearer that hydrogen ion concen- 

 tration cannot profitably be interpreted in terms of an isolated 

 variable. Leaving aside the well-known effects of hydrogen ion 

 concentration on the solution of certain physiologically impor- 

 tant chemical elements of the soil, there remains a highly compli- 

 cated interrelationship between hydrogen ion concentration of 

 the soil solution, buffering system of the soil, metabolism of the 

 plant, and accumulation of mineral solutes. 



Some of the recent studies of this and other laboratories lead 

 to doubt whether, within a wide range of values, external hydro- 

 gen ion concentration per se is as important a factor as many 

 investigators have beUeved. It is evident from data already avail- 

 able that the effects of hydrogen ion concentration must be re- 

 examined with reference to root temperature, oxygen supply of 

 the culture medium, and other factors controlling rates of root 

 metabolism. The view now suggested is very different from that 



