THE CONSTITUENTS OF THE ASH OF PLANTS 185 



the localisation of different functions in particular organs 

 which is so much more clearly characteristic of the animal 

 organism, it becomes very difficult to ascertain the exact 

 nature of the part played by any of these ash constituents. 

 We can more easily determine what is the effect produced 

 by variations in the amount supplied or by the total 

 absence of any of them. This effect is usually, however, 

 only the general effect upon the plant, and the experiments 

 leave us still quite in the dark as to the way in which any 

 general effect is produced, whether directly, or indirectly 

 by affecting the health of the plant and thus leading to 

 secondary changes in its tissues. 



The experiments in question are preferably carried out 

 by means of water-culture, the general nature of which we 

 have already explained. Plants will grow very well in 

 water containing small quantities of various, inorganic salts, 

 and these can be varied at will for the purpose of definite 

 inquiries. The composition of such a culture-solution is 

 given by Pfeffer as under : 



Calcium nitrate . . 4 grms. 



Potassium nitrate . . 1 grm. 



Magnesium sulphate . 1 grm. 



Potassium acid phosphate 1 grm. 



Potassium chloride , . *5 grm. 



Ferric chloride solution . a few drops 



Water .... 7 litres 



Or a convenient fluid may be prepared by dissolving 

 2O 5 grms. magnesium sulphate in 350 cc. of water, and 

 40 grms. calcium nitrate, 10 grms. potassium nitrate, and 

 10 grms. acid phosphate of potassium in another 350 cc. ; 

 100 cc. of each of these solutions should then be added 

 to 9'8 litres of water. This culture-medium will contain 

 2 per cent, of salts, and will need only the further addition 

 of a few drops of ferric chloride solution. 



This percentage is generally satisfactory, though the 

 concentration may be increased twofold without affecting 



