174 VEGETABLE PHYSIOLOGY 
medium from which potassium is absent, (3) one in which 
sodium is made to replace potassium, (4) one in which 
there ig no calcium. The general character of such 
experiments 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 directly causes 
the failure of the etiolin to be converted into chlorophyll, 
for all analyses of the latter show that iron does not enter 
into its molecule. It is probably an indirect effect arrived 
at through faulty nutrition set up in the absence of the 
metal. 
At first sight it seems as if the absence of inorganic 
salts may be effective by interfering with the maintenance 
of the turgid condition of the cells, as all the compounds 
mentioned have osmotic properties. It is evident, how- 
ever, that this cannot be the only or even the main cause 
of the disturbance of nutrition, as the salts are not inter- 
changeable, and a salt of sodium in concentration quite 
sufficient to maintain the condition of turgor is unable 
to replace the salts of potassium normally required. More- 
over, turgescence can be maintained by organic acids in 
the total absence of the normal constituents of the ash. 
We can divide the latter into four groups which sub- 
serve different purposes. Of these the members of the 
