NUTRITION 99 



has been withdrawn. From this it appears that potassium 

 compounds are intimately concerned in the construction of 

 protoplasmic matters. What compounds these are is not 

 now known, though it is evident that potassium may be a 

 component of reserve foods. Nobbe's hypothesis* that po- 

 tassium salts are directly concerned in the translocation of 

 the starch formed in chlorophyll-containing cells is interest- 

 ing, because it suggests one of their possible uses in the 

 plant, but that this is their main function follows neither 

 from his experiments nor his arguments. 



According to Copeland,t the potassium salts are 4 'an im- 

 portant part of the osmotically active material which keeps 

 the cell and plant turgid," and "there is no experimental 

 ground for attaching this significance to any other con- 

 stituent of the mineral food." 



Potassium salts are never very abundant in soils. They 

 occur as sulphate, phosphate, and chloride, freely soluble, 

 and hence easily washed away as well as absorbed. It is 

 necessary frequently to manure soils with potassium salts 

 when tobacco and other crops demanding considerable 

 amounts of potassium are cultivated year after year on 

 the same soil. 



CALCIUM is neither a constituent of protoplasm nor neces- 

 sary to all plants. For the fungi, and perhaps for certain 

 algae, I it is entirely unnecessary, magnesium successfully 

 taking the place of calcium, besides performing its own part 

 in nutrition. All other plants demand both calcium and 

 magnesium. Unlike the elements already considered, the 

 salts of calcium are found in tissues which have attained 

 their full growth and in which work of another kind is 

 especially going on, namely, in the chlorophyll-containing 

 food-making cells of the leaves and cortex. In organs in 

 which elaborated foods are stored, and in such dead parts 



* Nobbe, in Landwirtschaftliche Versuchsstationen, Bd. 13, 1870. 



t Copeland, E. B. Relation of nutrient salts to turgor. Botanical 

 Gazette, Vol. 24, 1897. 



t Molisch, H. In Botanisches Centralblatt, Bd. 60, 1894, and Sitzungs- 

 ber. d. Wiener Akademie, Bd. 103, 104, 105, Abth. I., 1894, 1895, 1896. 

 Benecke, W-, in Botanisches Centralblatt, Bd. 60, 1894. Jahrbuch f. 

 wiss. Botanik, Bd. 28, 189, and Botanische Zeitung, 1896. 



