178 VEGETABLE PHYSIOLOGY 



Or a convenient fluid may be prepared by dissolving 

 2O5 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 3 potassium is replaced by sodium ; 

 in pot 4 is 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 



