318 EFFECT OF CHEMICAL AGENTS [Cn. XI 



Salts of Alkalis. - - This group contains one or two of the 

 most important elements of which organisms are composed. 

 The elements are only slightly replaceable by one another. 



Lithium seems normally to have little importance for growth, 

 although slight traces of it have been found spectroscopically 

 in the blood (HOPPE-SEYLER, '81, p. 453J). 



Sodium is probably of constant occurrence in organisms. 

 The quantity of it in the body is widely different in different 

 species ; and as our table on page 297 shows, it may vary in its 

 position from one of the most abundant of the elements to one 

 of the least. Whereas sodium is not essential to the nutrition 

 of plants, it is necessary in the form of sodium chloride to 

 certain higher animals. Sodium is also found in all the tissues 

 of the body, and perhaps enters into the albuminoid molecule 

 (NENCKI, '94). The fact that, as we have seen on page 303, 

 soda is a prominent constituent of milk, indicates its impor- 

 tance in the growth of mammals. Finally, HEEBST ('97) has 

 been able to demonstrate its indispensability for the growth of 

 marine animals. 



Potassium constitutes an important part of all organisms. 

 It forms the largest part of the ash of nearly all phanerogams 

 (see page 297) and is markedly abundant in yeast and in many 

 invertebrates. It occurs in the body as chloride and as sul- 

 phate, and probably also in combination with albumen and 

 various organic acids ( VINES, p. 134). 



That it is an essential and unreplaceable food for all organ- 

 isms is indicated by trustworthy experiments upon fungi, algae, 

 phanerogams, invertebrates, and vertebrates. RAULIN ('69) 

 first showed that only culture-solutions containing this metal 

 permit the growth of fungi. 



The experiments of BENECKE ('96) upon the growth of 

 certain molds, e.g. Aspergillus, are worth citing in detail as an 

 illustration of the method. 



He sowed spores in culture-vessels made of a glass which analysis had 

 shown to be free from potassium. Two of these vessels contained a nutri- 

 tive aqueous solution consisting of 3% cane sugar and 0.25% magnesium 

 sulphate. To the one of these solutions was added 1.2% potassium nitrate, 

 and 0.26% potassium phosphate; and to the other 1% sodium nitrate and 

 0.5% sodium phosphate. After four days the first culture was covered by a 

 sheet of fungi and in five weeks the whole surface was black with spores ; in 



