274 PROTOPLASM 



related cousin Halicystis. Analysis of the sap of this alga reveals 

 that instead of there being five times as much potassium as 

 sodium within the cell, as in the case of Valonia, there is eighty- 

 seven times as much sodium as potassium. 



Should such differences in ionic concentration remain constant, 

 they could be used to distinguish species, especially those species 

 which show little difference in their morphological characteristics. 

 We should then have a classification of plants based on chemical 

 or physiological distinctions similar to the protein classification 

 of Mez (page 504) and Moyer (page 385). 



A further step has been made in this direction by S. C. Brooks, 

 who finds that Valonia utricularis growing at Naples shows little 

 preference for potassium over sodium. V. macrophysa, on the 

 other hand, shows a preference for potassium over sodium to the 

 extent of 2.77 times. Species off Florida have a concentration of 

 potassium five to twelve times that of sodium. V. ventricosa at 

 Tonga has from four to ten times as much potassium as sodium. 

 Brooks asks, Are we here dealing with physiological variants or 

 with distinct valid species? Blinks has utilized these chemical 

 distinctions in segregating part of the genus Halicystis as a new 

 species. 



A mechanism that permits two closely related plants growing in 

 the same environment to show marked differences in their pref- 

 erence for sodium and potassium has been suggested by I. W. 

 Bailey and C. Zirkle. It is assumed that acidity determines the 

 rate of penetration of the two ions and that pH values within and 

 without the cell are different. If potassium enters more rapidly 

 on the alkaline side while sodium enters more rapidly on the acid 

 side of neutrality, and if sea water is more alkaline than the cell 

 sap (sea water has a pH of 8.2 and cell sap 5.8), then potassium 

 will enter the cell easily; but once within, it cannot leave readily 

 because of the acid condition of the cell contents. Sodium, on the 

 other hand, will presumably enter slowly because of the unfavor- 

 able alkaline environment without but will leave readily because 

 of the favorable acid environment within. The explanation is 

 not based on permeability of the membrane ; for if the membrane 

 is permeable for potassium in one direction, it should be so in the 

 other direction (if electrical forces are ignored). A difference in 

 ionic activity is postulated on the assumption that potassium 

 has a greater activity in an alkaline medium than in an acid 



