194 Elementary Biology., 



Importance. Iron is associated in some way with the 

 formation of chlorophyll. Without it no chlorophyll is 

 formed, and yet if more than a minute trace be present it 

 acts as a poison on the plant. It does not, however, form 

 a constituent of chlorophyll. It must, therefore, act simply 

 as an essential agent in its production. 



Ill, Non-essential elements in the plant organism, 

 present in variable amount. 



All these substances are not found in the same plant, 

 nor at the same time. Some, indeed, are invariable consti- 

 tuents though they do not seem essential to life, e.g. sodium. 



Sodium. This metal might be said to be ubiquitous, 

 and yet it cannot be said to be of essential service to the 

 plant. It is obtained from salts in the soil, and is taken up 

 chiefly in the form of sodium chloride. 



Chlorine. Chlorine, as has been mentioned above, is 

 necessarily taken into the plant in the process of the absorp- 

 tion of potassium and sodium, but it does not seem to be 

 necessary to the life of the plant. 



Silicon, however, enters in considerable quantity into 

 the composition of many plants of the grass tribe ; whilst 

 many of the lower plants have large deposits of silica 

 (SiO- 2 ) in the cells of the thallus. The silicious covering of 

 the Diatoms and allied forms are composed almost entirely 

 of this compound (p. 72). 



Iodine and bromine are found in tolerably large quanti- 

 ties in certain of the marine Algae, whence, indeed, a large 

 proportion of the iodine and bromine of commerce is 

 obtained. 



With regard to the other elements mentioned under 

 this head, we need only say that they are found rather as 

 accidental constituents than as essentials in the plant organ- 

 ism. They are probably not peculiar to plant life at all. 



The forms in which these various elements appear in the 



