POTASSIUM 173 



of protein compounds, and is more especially abundant in the 

 protein of the nucleus of plant-cells. 



Besides being met with as a constituent element of organic 

 compounds, it is often present in the form of inorganic phosphates. 



Phosphorus constitutes a large proportion of the ash of seeds, 

 and without an adequate supply of this element, the formation 

 and development of seeds do not take place satisfactorily. 

 The amount of phosphorus calculated as phosphoric acid in the 

 ash of wheat-grains averages from 45 to 50 per cent., and in 

 beans about 40 per cent. : in the ash of the vegetative parts the 

 amount is considerably smaller, e.g., in wheat-straw about 5, in 

 turnips 7, in hay 6, and in potato tubers about 17 per cent. 



Phosphorus is absorbed by plants from the soil in the form 

 of phosphates of potassium and calcium. 



(v) Sulphur enters into the composition of proteins, although 

 the amount is small, rarely exceeding 2 per cent. It is also 

 a constituent of 'mustard-oil' obtained from many cruciferous 

 plants, and is found in the form of inorganic sulphates in which 

 condition it is absorbed from the soil. 



(vi) Potassium. — This element is specially abundant in the ash 

 of the young actively-growing part of plants where cell-division 

 is going on, and probably is an essential constituent of the 

 protoplasm of all cells. It also exists combined with tartaric, 

 oxalic, malic, and other organic and inorganic acids in the cell-sap. 



Tissues containing large reserves of carbohydrates are frequently 

 rich in this element ; for example, in potato tubers 2 "3 per cent., 

 in grapes about 3 per cent., and in mangels 4 per cent, of the dry 

 matter is potash (KjO). 



It is taken up from the soil chiefly as nitrate, chloride, car- 

 bonate, sulphate and phosphate. 



The part which potassium plays in the economy of the plant 

 is not known with certainty. According to De Vries its salts 

 are especially concerned with the maintenance of the turgidity 

 of the cells, and as the latter condition is essential for growth 



