320 EFFECT OF CHEMICAL AGENTS [Ch. XI 



ride ; to that of the other, both sodium chloride and potassium 

 salts. After 26 days the dog fed on the potassium salts as well 

 as the sodium chloride was about 30% heavier than the other, 

 and this difference was reasonably ascribed to the beneficial 

 effects of the potassium. 



The particular part which potassium takes in growth is still 

 somewhat doubtful. The recent observations of Copeland 

 ('97) with seedlings reared in water cultures in which sodium 

 replaces potassium, lead him to the conclusion that potassium 

 is necessary to turgescence. The potassium salts become 

 lodged in the cell-sap, as analysis shows, and are therefore, 

 perhaps, one of the principal causes of imbibition. 



Rubidium and Ccesium. — These rather rare metals are 

 important only because of the fact that they may replace 

 potassium in the growth of some fungi. Winogeabsky ('84) 

 recognized this to be the case with rubidium in yeast cultures. 

 Nageli ('80) found that in molds rubidium and cEesium gave 

 even greater growth of dry substance than potassium cultures, 

 a conclusion abundantly confirmed by the studies of Benecke 

 ('95). Whatever, therefore, is the significance of potassium 

 for growth, rubidium and csesium seem to have the same 

 significance. 



Earthy Metals. — Under this head are included the elements 

 calcium, strontium, and barium, which form compounds having 

 closely similar molecular structure and properties. We might 

 therefore expect them to be in some degree mutually replace- 

 able. 



Calcium shares with potassium and phosphorus the position 

 of one of the most abundant elements of organisms. It is found 

 in every tissue and seems to be a constant accompaniment of 

 protoplasm. 



Its constant occurrence is indicative of its importance as 

 food for both plants and animals. It is apparently indispensa- 

 ble and unreplaceable bj^ any other element in phanerogams ; 

 but MOLISCH ('94) finds that growth can take place in certain 

 molds (Penicillium, Aspergillus) as well in its absence as in 

 its presence, and in some algee, but not in all (MoLisCH, '95), 

 calcium is apparently of little importance. In animals, calcium 

 can be replaced by other elements only to a very slight extent. 



