THE PROTOPLASM CYCLE 



41 



The Protoplasm Cycle 



Living protoplasm is ultimately built up by living organisms 

 from certain substances which are found free in their environ- 

 ment. The most important of these substances are water, 

 carbon dioxide and nitrogen; though small quantities of 

 inorganic substances containing phosphorus, sulphur, sodium, 

 potassium, calcium, iron, etc. are also required, as mentioned 

 in Chapter II. 



All living things absorb water directly from their environ- 

 ment, and they continually return water back to it as water- 



Nt TRIFYlKG- 

 BftCTERIft.^ 



ftftCTCRlRL 



COMBINED 

 NlTROGrEiN. 



ATMOSPHERIC 

 NlTRO<^EfM, 



i MATTER. 



4^ 

 J3EftTH OR 



SV ETXCRETJON. 



CO, 



LIV/INCt 

 PROTOPLf^SM 



BY 



ENtTRlFYIM G/ 

 BRCTERlft. 



vapour or as liquid water. There is thus a continual inter- 

 change of water between living protoplasm and the environ- 

 ment, that is, a "water cycle." 



Carbon dioxide and nitrogen have a more complicated 

 history in protoplasm formation. Carbon dioxide is found free 

 in the atmosphere and is utilized in building up proto])lasm 

 through the medium of the chlorophyll of plants ; without 

 chlorophyll carbon dioxide would no longer be available for 

 protoplasm formation, and life — at any rate as we know it — 

 would cease. When protoplasm dies, putrefactive bacteria set 

 free the carbon dioxide again, thus completing tlie "carbon 

 dioxide cycle.' 



?3 



