Hoagland —100— Plant Nutrition 



not advanced that far. Yet it may be permissible 

 to outline in summary form certain general concepts 

 that may usefully guide future research. First of all, 

 the plant needs to be envisaged as an integrated or- 

 ganism with respect to solute absorption and move- 

 ment as well as in other respects. The growth of the 

 roots and the active absorption of salts from the ex- 

 ternal medium require a source of certain metabo- 

 lites, which must normally originate in the shoot. Root 

 metabolism is concerned in active transport mechan- 

 isms by which solutes are absorbed and carried into 

 the conducting system. The concentration of solutes 

 in the conducting system as the salt moves upward 

 depends on the active transport capacity of living 

 cells as well as on water movement. 



The distribution of mobile salt ions in the various 

 parts of the plant and their relative concentration in 

 the different tissues is not merely a necessary con- 

 sequence of the movement of water containing solutes. 

 Here, too, relative rates of growth and metabolism of 

 tissues determine the utilization or storage of the 

 solute. Thus, transpiration does not assume the de- 

 terminative role in either absorption or translocation 

 of salt that was once supposed. Neither, however, can 

 we dissociate entirely the absorption and upward move- 

 ment of solute and of water. Transpiration makes 

 possible the rapid carrying of nutrient salts over long 

 distances and the rapid removal of salt from the roots, 

 so that conditions are favorable for continued active 

 movement of salt into the upward conducting tissues. 

 As we have seen, such a view does not imply any pro- 

 portionality between salt absorbed and translocated 

 and water transpired. For purposes of plant nutrition 

 a very limited transpiration could suffice. 



There is no inconsistency in recognizing that con- 

 ditions occur under which absorption and movement 

 of water may have a role in salt absorption and move- 

 ment, and at the same time have clearly in view that 



