THE ACCUMULATION OF IONS BY PLANT CELLS 409 



dicating that increased respiration of those cells, due to adequate aeration, 

 was causally connected with the accumulation of ions. Concurrently with 

 the absorption of ions a marked diminution occurred in the starch content of 

 the peripheral layers of cells. Presumably some or all of the glucose result- 

 ing from the digestion of starch was oxidized in respiration. 



That the absorption of ions by roots is also influenced by aeration of the 

 circumambient solution has been shown by Hoagland and Broyer (1936). 

 In one experiment excised roots of barley plants were transferred to a solu- 

 tion in which potassium bromide was present in a concentration of 0.0075 ^^ 

 and calcium nitrate in a concentra- 

 tion of 0.0025 M. When a stream 

 of air was passed through the solu- 

 tion K+, Br~, and NOs" ions 

 accumulated rapidly in the cells. 

 When a similar experiment was per- 

 formed in which a stream of nitro- 

 gen was passed through the solution 

 instead of air very little absorption 

 of ions occurred. Evidently a high 

 rate of aerobic respiration is corre- 

 lated with the cellular activities 

 which bring about the accumulation 

 of both cations and anions in root 

 cells. The sugar content of the 

 roots decreased during rapid aera- 

 tion, and if the roots were first de- 

 pleted of sugars their capacity for 

 the absorption of electrolytes was 



greatly diminished. Other experiments by the same workers indicated that 

 not only is a relatively high rate of aerobic respiration required for marked 

 accumulation of ions, but the ions already present in the root cells may not 

 be retained if the rate of respiration falls too low. Low temperatures were 

 also found to result in a marked retardation in the rate of accumulation of 

 ions, presumably because of the correlated decrease in the rate of respiration. 



Prevot and Steward (1936) have demonstrated the presence of a longi- 

 tudinal gradient of accumulation in barley roots extending from the apex 

 to the region of emergence of the secondary roots. Accumulation of Br~ 

 ions was found to be greatest in the apical i cm. portion and to decrease 

 progressively with increasing distance from the tip. The same is presumably 

 true for other ions. This gradation in the accumulative capacity of the 



10 



20 30 40 50 60 70 80 

 PER CENT OXYGEN 



90 100 



Fig. 95. Relation between oxygen 

 content of the solution, relative respira- 

 tion and relative absorption of bromine 

 ions. Figures on abscissa represent per 

 cent of oxygen in the oxygen-nitrogen 

 mixtures used to aerate the solution. Data 

 of Steward (1933). 



