The rate of entrance of potassium into Nitella flexilis has been investigated, and it has been shown that (a) at the concentrations studied the rate is independent of the external pH between 6 and 8 but it is possible that at lower concentrations a dependence may be found; (b) that it does not vary much with the external potassium concentration between 0.01 and 0.001 M, but appears to vary more with the potassium concentration below this limit.
It has also been shown that the rate is independent of the illumination, in contrast with the penetration of halides into Nitella clavata studied by Hoagland.
It has been found that potassium leaves the cells in distilled water, and since this does not seem to be the result of injury, there is apparently a concentration between 0 and 0.0001 M at which potassium neither enters nor leaves the cell. In Valonia increase of external potassium increases the rate of entrance as shown in the increase in moles of potassium in the sap. In Nitella this is true below an external concentration of 0.001 M. In Valonia this increase is paralleled by the increase in entrance of water so that little or no change in concentration occurs, but in Nitella no growth occurred during the experiment and in consequence the concentration of potassium in the sap increased.
It has been shown that the potassium content of the raw gelatinous sap is no greater than that of its ultrafiltrate, so that it is not possible to assume that any of the potassium is bound up in the cell in colloidal compounds.
It has been pointed out that all the gradients between the sap and the external solution are unfavorable to the entrance of potassium except the potassium bicarbonate gradient. However, on other grounds entrance as potassium bicarbonate is not considered to be very probable.
Various modes of entrance are discussed and it has been concluded that the subject must be investigated further before a definite answer can be given.