250 



PRINCIPLES OF GENERAL PHYSIOLOGY 



word, i.e., utilised as actual constituents of the cells, or for energy or 

 growth, such as sugar, ammonium nitrate, phosphate, iron, and potassium, others 

 have additional functions. Tartaric acid keeps the solution acid and prevents 

 the growth -of bacteria; iron serves to neutralise, probably oxidise, injurious 

 substances formed in the process of growth. The cane-sugar is first hydrolysrd 

 by an enzyme in the mould and lactose will not replace it, since the enzyme 



Free 

 .Nitrogen 



,V 



Nitrates 



re ' ^ 



r-i ' 



-. i 



o> i 



Nitrites 



Plant 

 .protein 



Animal 

 proteinj 



Plant 

 id animal 

 residues. 



X 



O 



^ 



rO* 





Fm. 69. DIAGRAM OF THE NITROOEX CYCLE, FROM THE ATMOSPHERE THROUOH 



PLANTS AND ANIMATES. 



required to hydrolyse lactose is absent. Glucose, of course, can replace it, and 

 it is of interest that alcohol, while retarding the germination of the spores, 

 serves as an excellent source of carbon to the grown plant. 



THE NITROGEN CYCLE 



It will have become sufficiently obvious that the continued supply of carbon 

 and hydrogen to living organisms in general is sufficiently provided for by the 

 activity of the green plant in forming sugar from the carbon dioxide evolved in 



