714 PART IV. THE PHYSIOLOGY OF PLANTS. 



of the formation of chlorophyll if the soil be supplied with iron, 

 or even if their surface is washed with a very weak solution of 

 iron. 



Potassium is found in plants in the form of salts combined with 

 various organic acids, as tartaric acid, malic acid, and oxalic acid. 

 Unless the soil contains potassium-compounds, the assin ilation of 

 carbon dioxide by plants possessing chlorophyll does not go on, 

 as is shown by the fact that, under these circumstances, the plant 

 does not increase in dry weight. Potassium-salts are especially 

 abundant in those parts of the plant which are rich in carbo- 

 hydrates such as starch and sugar, as in potatoes, beet-roots, and 

 fruits. 



Calcium and Magnesium have been shown to be necessary to the 

 normal development of plants : they are absorbed as nitrates, phos- 

 phates and sulphates, and thus serve as bases for the absorption 

 of these other important elements. Little is known as to their 

 direct use : they are of importance in neutralising the organic 

 acids (especially oxalic) formed in the plant : hence calcium, at 

 least, is of importance in connexion with the distribution of carbo- 

 hydrates in the plant. They occur in the plant as salts of lime 

 and magnesia in combination with both organic and inorganic 

 acids. 



It has been discovered by experimental cultures, that a plant 

 can be perfectly nourished if it is supplied with all those elemen- 

 tary substances which have been enumerated as essential. This 

 might be done, for instance, by growing it in a watery solution of 

 either of the two following groups of chemical compounds : 



1. 2. 



Calcium nitrate Calcium nitrate 



Potassium nitrate Ammonium nitrate 



Potassium superphosphate Potassium sulphate 



Magnesium sulphate Magnesium phosphate 



Ferrous phosphate Ferrous chloride. 

 Sodium chloride. 



In these two mixtures, as well as in others of the same aoids and 

 bases which might be formulated, all the essential elements are 

 included in forms suitable for absorption ; the proportion of mixed 

 salts should not, however, exceed about '3% by weight of the 

 liquid. 



