174 VEGETABLE PHYSIOLOGY 



medium from which potassium is absent, (3) one in which 

 sodium is made to replace potassium, (4) one in which 

 there is 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 



