180 VEGETABLE PHYSIOLOGY 



that this cannot be the only or even the main cause of the 

 disturbance of nutrition, as the salts are not interchange- 

 able, 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. Moreover, tur- 

 gescence 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 subserve 

 different purposes. Of these the members of the first are 

 essential, because they enter into the constitution of the 

 living substance. They are sulphur and 'phosphorus. All 

 analyses of proteins show that sulphur is an essential con- 

 stituent of them, and as proteins are immediately applied 

 to the construction of protoplasm, there can hardly be any 

 doubt that sulphur is contained in living substance. Phos- 

 phorus does not seem to be present in the ordinary cyto- 

 plasm, but is undoubtedly associated with the nucleus. 

 The nature of the connection is not very clear, but all nuclei 

 contain a constituent which bears the name of nuclein. 

 This can be extracted from it by appropriate treatment, 

 and analysis shows that phosphorus enters into its molecule. 

 Nuclein occurs also in the substance of many cells, either 

 as nucleic acid, or associated with certain protein bodies. 



The second group comprises certain metals which are 

 essential to the development of a plant, but which appa- 

 rently do not ever form part of the living substance. 

 There is some little doubt about this, as the fact cannot 

 be ascertained by analysis. The members of this group 

 are potassium, magnesium, calcium, and iron. 



The third group includes several elements which are 

 not absolutely essential, but which are useful in many 

 cases, and which are very widely distributed, although not 

 universally present. Among them are sodium, silicon, 

 manganese, chlorine, bromine, and iodine. 



: The fourth group includes many other elements which 

 are only occasionally present, and which probably play no 

 part in the metabolic processes. They appear to be absorbed 



