The Plant as a Metabolic Unit 237 



root cells, no matter how intense their metabolism, and the con- 

 tinuance of absorption from a nutrient solution depends upon 

 movement of solutes from the root to the shoot. 



This raises once more the question of the path of translocation 

 of mineral solutes. Doubdess, most investigators in the field of 

 botany feel that the assumption may be safely made that these 

 solutes are always moved upward in the transpiration stream, 

 but this assumption is not accepted by all investigators. Curtis* 

 has presented evidence with respect to woody plants which indi- 

 cates very strongly, he thinks, that movement occurs primarily 

 in the living cells of the phloem and not in the xylem system. 

 Experiments with certain annual plants, however, lead to the 

 conclusion that movements of inorganic solutes of large magni- 

 tude do occur in the xylem system, and this may be the chief, 

 possibly the sole, path of conduction in the plants concerned. If 

 so, transpiration would have a function in the removal of solutes 

 from the root system and would therefore influence the effi- 

 ciency of root activities in the process of absorption of mineral 

 solutes. Whatever may be the path or the mechanism of trans- 

 location, growth and metabolism must be concerned in the ac- 

 cumulation or utilization of mineral solutes in green cells just 

 as in root cells. The distribution of mineral elements in different 

 tissues cannot be explained without reference to the metabolic 

 activities of living cells, in relation to solute accumulation, and 

 to the effects of climatic environment upon these activities. 



With respect to another aspect of translocatioii, it has been 

 demonstrated that a very direct relationship frequendy exists 

 between root pressure and rate of accumulation of mineral ele- 

 ments from solution.* The rapid accumulation of electrolytes 

 by the root system establishes an osmotic gradient resulting in 



* Unpublished data by D. R. Hoagland and T. C. Broyer. 



