PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 273 



with a problem in metabolism and not with permeability. Plant 

 nutrition is a question not only of getting certain salts and not 

 others into the cell but of accumulating them. The law of 

 diffusion states that substances pass in excess from a region 

 of higher activity to one of lower activity, concentration being a 

 factor. If we view the problem of accumulation as one of 

 permeability, then the diffusion law is apparently not adhered 

 to; but if we recognize that in all forms of metabolism one sub- 

 stance is used more than another, then accumulation is quite 

 easily explained. The fresh-water alga Nitella, with cells up 

 to 6 in. in length, lives in pond water where the amount of 

 potassium is very slight, yet potassium may occur within the 

 cell sap at a concentration one thousand times that of the 

 potassium in the surrounding water. Hoagland and Davis first 

 established these facts with accuracy. Osterhout and his 

 coworkers continued the work on the large marine algae Valonia 

 and Halicystis, both one-celled, bladder-like plants two or three 

 centimeters in diameter, with a large central vacuole containing 

 several cubic centimeters of sap. The contents of a number of 

 cells will yield sufficient sap to permit an accurate chemical 

 analysis. The results show that the concentration of potassium 

 is over forty times as great within the Valonia cell as without, 

 while sodium is only one-fifth and calcium one-seventh as con- 

 centrated within the cell as in the sea water; in other words, while 

 there is forty times as much sodium as potassium in sea water, 

 there is five and one-half times as much potassium as sodium 

 within the cell. The total concentration of ions is approximately 

 the same within and without the cell, only the proportions differ. 

 Starfish, sea-urchin eggs, and many other cells contain a higher 

 concentration of potassium than of sodium, though this relation- 

 ship is reversed in the salt content of the sea water. Apparently, 

 the cell has a mechanism by means of which it can hold more 

 potassium than occurs in its surroundings and yet at the same 

 time exclude sodium and calcium. If the problem is still con- 

 sidered as one in permeability, diffusion cannot be the explana- 

 tion. Potassium may enter Valonia as much as two hundred 

 times as rapidly as sodium, while, on the basis of ionic mobilities 

 (the comparative rate at which ions move by diffusion), potas- 

 sium should enter only six times as fast. 



Now we come to the most extraordinary feature of the whole 

 story. Growing in company with Valonia is its very closely 



