188 VEGETABLE PHYSIOLOGY 



first are essential,' because they enter into the constitution 

 of the living substance. They are sulphur and phosphorus. 

 All analyses of proteids show that sulphur is an essential 

 constituent of them, and as proteids 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 cytoplasm, 

 but it 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 proteid 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 because they are present in the particular soil in 

 which the plant happens to be growing, and have the 

 power of osmosing through the walls of the root-hairs, and 

 passing their plasmatic membranes. Many of them have 

 only been found in a few plants. Among them may be 

 mentioned aluminium, zinc, copper, cobalt, nickel, zirco- 

 nium, fluorine, and lithium. 



What is frequently spoken of as the selective power of 

 plants is often misunderstood. If a substance is present 



