430 



PROTOPLASM 



subjected to the rigors of outdoor life, it succumbs. Thus, 

 laboratory cultures indicate that silica is not necessary for 

 grasses (many if not all of which contain silica) but they develop 

 well without this element only in culture. In the field, silica 

 appears to play the role of a protector from parasites. Wheat 

 and rye, grown in laboratory cultures deficient in silicic acid, 

 suffer severely from "rust." 



Fig. 174. — Tobacco plants in culture solutions: 1 without nitrogen; 2 without 

 phosphorus; 3 without potassium; 4 without calcium; 5 without magnesium; 

 6 all of the necessary elements included; 7 without boron; 8 without sulphur; 

 9 without manganese; 10 without iron. {From J. E. McMurtrey.) 



We now come to the function of these elements. The first 

 four in the preceding list, viz., carbon, hydrogen, oxygen, and 

 nitrogen, are necessary for the manufacture of foods, such as 

 sugars, fats, and proteins. 



Potassium. — That potassium is necessary for plant life is 

 apparent from the fact that it has long been recognized as one 

 of the three principal fertilizer constituents, nitrogen and phos- 

 phorus being the other two. The role of potassium in organisms 

 appears to be primarily a catalytic one, as is true of numerous 

 other elements that do not enter into the composition of tissue 

 or serve directly as food for producing energy. 



