FUNCTIONS OF LIFE ELEMENTS 



65 



the life phenomena through stimulation (irritability) and in- 

 hibition. It unites with carbon as carbonate of lime and is 

 contained in many of those animal skeletons which, through 

 deposition, make up an important part of the earth's crust. 



MAiNCiiUM 



i I! ! ' 



III 



i500 ro 20 



So . 410 



iiliihl:! 



I 



{-"If] 



IRON MVDROGEN 



Fig. g. Chemical Life Elemextr ix the Sux. 



Three regions of the solar spectrum with lines showing the presence of such essential 

 life elements as carbon, nitrogen, calcium, iron, magnesium, sodium, and h\'drogen. 

 From the Mount Wilson Observatory. 



In invertebrates the carbonates, except in certain brachiopods, 

 are far more important as skeletal material than the phosphates: 

 the limestones form only about live per cent of the sedimen- 

 taries. Shales and sandstones are far more abundant. 



Iron is essential for the production of chlorophyll,^ though, 

 unlike magnesium, it is not contained in it. It is present as 

 well in all protoplasm, while in the higher animals it serves, in 



1 Sachs, Julius, 188.3, p. 699. 



