172 VEGETABLE PHYSIOLOGY 
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 germ. 
Potassium chloride . : 0°5 erm. 
Ferric chloride solution . a few drops 
Water. : ‘ F 7 ~~ litres 
Or a convenient fluid may be prepared by dissolving 
20°5 grms. magnesium sulphate in 350 c.c. of water, and 
40 grms. calcium nitrate, 10 grms. potassium nitrate, and 
10 grms. acid phosphate of potassium in another 350 c.c. ; 
100 ¢.c. 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 
