GEORGE T. SCOTT AND HUGH R. HAYWOOD 



59 



Efif ect of External Sodium on Potassium Accumulation in Ulva. I L is of con- 

 siderable importance to determine whether or not the potassium-accumulating 

 mechanism is passive, dependent upon and secondary to the sodium-secreting 

 mechanism, or whether it is active and independent of sodium secretion. To 

 this end the following type of experiment was carried out. Samples of Ulva 

 fronds were repeatedly suspended in isotonic sucrose for 2 to 3 hours, at which 

 time the cells had lost about 85% of their sodium and potassium (figs. 19 and 

 20). At this time half the samples were transferred to complete natural sea 

 water, while the others were placed in sodium-free Allen's artificial sea water, 

 isotonicity maintained by the addition of sucrose. Potassium reaccumulation 

 was followed in the two groups by removing samples at various times over a 

 6 Of 1 1 1 1 1 1 1 



TRANSFERRED 

 TO SEA WATER AT I8°C 



200 

 HOURS 



250 



300 



350 



Fig. 18. Influence of cold (2-5°C) on the potassium and sodium content of Valonia macro- 

 physa. At the time indicated by the arrow the cells were transferred to running sea water 

 at i8°C in the light. 



24-hour period. Two features of the data are outstanding: i) the potassium 

 reaccumulation is, within experimental error, linear; and 2) the presence of 

 sodium in the external fluid made no measurable difference in the rate of potas- 

 sium reaccumulation. It is noteworthy that in other experiments the potassium 

 reaccumulation was complete to the control level. The sodium concentration 

 in those samples returned to complete sea water is seen to have reached the 

 control level within the time at which the first samples of this series were re- 

 moved; the curve for re-entry of sodium, then, is probably steeper than the 

 one shown here and need not be linear. 



The results of these experiments indicate a mechanism for potassium uptake 

 which is independent of sodium secretion. Further, since the kinetics of potas- 

 sium reaccumulation under these conditions are described by a straight line. 



