The Plant as a Metabolic Unit 223 



cance to the student of plant physiology whether his interest lies 

 in ecology, or agriculture, or in specific plant functions, such as 

 transpiration, translocation, photosynthesis, respiration, or ab- 

 sorption of nutrients. Suitable control of environment and quan- 

 titative measurement of biochemical changes in plants subjected 

 to this control offer a promise that plant physiology may develop 

 in due course some of the attributes of an exact science, at least 

 so far as the methods of exact science are at present applicable 

 to biological systems of such great complexity. 



It is proposed to analyze certain problems of the soil-plant 

 system with reference to experiments conducted in accordance 

 with the views just outlined. For the greater part the discussion 

 will be based on data recently acquired by a group of investi- 

 gators working in California. 



Nature of the Process of Accumulation of Mineral 



Solutes by Plants 



It is now generally understood that absorbing plant cells possess 

 the power of accumulating certain chemical elements derived 

 from the nutrient medium, not merely by fixing these elements 

 in organic combination, but also by storing them in vacuolar sap 

 in inorganic form. This frequently implies a movement of 

 solutes against a concentration gradient, and work must there- 

 fore be done by the cell in this process. Observations on Valonia," 

 Nitella,^ and other plant cells supporting this conclusion have 

 been discussed so often in recent years that no recital of the 

 arguments need be presented at this time. 



The conception of protoplasmic activities as indispensably 

 related to the absorption and accumulation of essential or non- 

 essential mineral solutes has been fundamentally advanced by 

 the work of Steward" in his investigation of storage tissues. It 



