{78 VEGETABLE PHYSIOLOGY 
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.c. of each of these solutions should then be added to 
9-8 litres of water. This culture-medium will contain 0:2 
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 
the plants injuriously. Too great a quantity of salts, 
however, becomes deleterious. 
The effect of omitting any particular constituent can 
be examined by making up the culture-fluid as required. 
Fig. 89 shows the effect of varying it in certain particulars. 
Pot 1 contains such a fluid as is described above ; in pot 2 
is no potassium : in pot 8 potassium is replaced by sodium ; 
in pot 4is no calcium, while from pot 5 all compounds of 
nitrogen are absent. The general character of such experi- 
ments can be seen by comparing the relative development of 
the plants under these conditions, and it is at once evident 
that the different metals and other elements employed have a 
certain functional importance. Deprivation of any of those 
mentioned affects all plants injuriously, though in different 
degrees. 
We can, however, say very little as to the way in which 
the injurious effects are produced in different cases. We 
can, as a rule, only guess at the functions of the different 
ash constituents by studying the effects thus made evident. 
In a very few cases we can associate an element with some 
definite metabolic process. An instance is afforded by the 
behaviour of iron, in the absence of which, as we have 
seen, there is no development of chlorophyll in the chloro- 
plasts. We cannot even here say very definitely how this 
inhibition is caused. It seems unlikely that it is directly 
concerned with the manufacture of chlorophyll, for all 
analyses of the latter show that iron does not enter into 
