242 D. R. Hoagland 



and acidic ions, but the metabolic activities of the plant govern 

 the rates at which absorption can occur. The maintenance of 

 suitable internal hydrogen ion concentrations in plant tissues is 

 dependent on the proper functioning of the system as a whole. 

 The measurement merely of the reaction of the external solution 

 gives but a very faint insight into the role of hydrogen ion con- 

 centration in plant growth, and undue simplification of the prob- 

 lem may readily lead to fallacious conclusions. 



General Conclusions 



This survey is very incomplete, yet it is believed that the evidence 

 presented by way of illustration suffices to establish the general 

 thesis proposed in the Introduction. Many problems of the soil- 

 plant system can be reoriented if the view be adopted that the 

 plant not only occupies the soil, but also constitutes a metabolic 

 unit in the whole environment, aerial and subterranean. 



This is not a vague and sterile generalization, but on the con- 

 trary, one which leads to specific and fruitful suggestions for 

 future experimentation. The soil is regarded as a medium which 

 must provide a suitable environment for root metabolism. Soil 

 temperature and aeration assume a definite importance in the 

 study of solute absorption. More specific reasons than have been 

 apparent heretofore are found for investigating the relation of 

 leaf metabolism and of the demands of reproductive processes to 

 the metabolism of root cells. The indirect effects of the quality, 

 intensity, and duration of light on root development are recog- 

 nized as requiring far more intensive study than has yet been 

 accorded to them. Transpiration becomes of interest in relation 

 to changes in the concentration of solutes in the soil solution and 

 to the aeration of the soil. Transpiration or other factors influ- 

 encing rate of movement of mineral solutes out of the root sys- 



