4IO ABSORPTION OF MLNERAL SALTS 



root cells is directly correlated with the progressive diminution in metabolic 

 activity of the cortical cells of the root with increasing distance from the 

 apex. Under the conditions of these experiments most of the accumulation 

 of ions occurred in the cortical cells. 



Not only do ions accumulate in the cortical cells of a root from an exter- 

 nal solution, but there is some evidence that the cells of the stele (tissues in- 

 ternal to the endodermis) accumulate ions from the cortical root cells 

 (Steward, 1935)- 



That there is a close correlation between the accumulation of ions by 

 plant cells and their rate of respiration seems to have been demonstrated 

 beyond doubt. This has been shown for cells from a number of very diverse 

 plant tissues. Three conditions known to be necessary for the occurrence of 

 this phenomenon are apparently essential because they contribute to relatively 

 high rates of aerobic respiration. These are: (i) an adequate supply of 

 oxygen, (2) a suitable temperature, and (3) a supply of sugars or of easily 

 hydrolyzable polysaccharides within the cells. 



In Valonia and Nitella there seems little doubt that the ions occur in 

 the cell sap in the free state. It is generally assumed that this is also true 

 in the cells of the higher plants although the evidence for this is largely pre- 

 sumptive. If this is granted it follows that the ions move into such cells in 

 opposition to a concentration gradient. Theoretically the movement of ions 

 into a plant cell in opposition to a concentration gradient would require 

 energy. Likewise the retention of ions within cells in greater concentration 

 than in the external medium also requires a continuous expenditure of energy 

 if the cell membranes remain permeable to those ions. The relation between 

 respiration and salt accumulation is certainly not a simple one, but a partial 

 explanation may be that energy of respiration is utilized in the accumulation 

 and retention of ions. 



It has also been found that rapid accumulation of an ion does not take 

 place in cells in which the concentration of that ion is already relatively high. 

 The previous metabolic history of the cells, particularly as regards electro- 

 lytes, will therefore have an important influence upon the capacity of those 

 cells for further accumulation of electrolytes. 



The phenomenon of accumulation of salts at appreciable rates seems 

 to be confined largely if not entirely to cells which have not yet lost the 

 capacity for cell division and growth (Berry and Steward, 1934). There 

 are some indications that meristematic cells possess the most marked capacity 

 for accumulation, and that with a decrease in the proportion of growing 

 cells or of cells capable of renewed growth in any tissue, its capacity for the 

 accumulation of electrolytes diminishes, even under conditions which are 



