PHOSPHORUS CYCLES 279 



duced to gaseous nitrogen by the denitrifying bacteria, 

 and so complete the cycle. 



Sulphur and Phosphorus Cycles. — It has already 

 been explained that sulphur taken up by plants as sul- 

 phuric acid compounds is built into protoplasm and re- 

 appears during putrefaction of dead plants and animals 

 as the malodorous gas, hydrogen sulphide. This in turn 

 is eventually restored to the soil as sulphates by the 

 action of the sulphur bacteria. Phosphorus enters the 

 plant as a phosphate and becomes a part of the cells. On 

 their death and decomposition it reappears as phosphoric 

 acid and unites with lime or other earthy material to 

 form phosphate again. 



The Energy Cycle differs from the material cycle 

 just discussed in that it is not a closed one so far 

 as the earth alone is concerned. The energy for photo- 

 synthesis comes from the sun in the form of light. It 

 is locked up in food and fuel. When liberated by metabo- 

 lism or combustion it reappears in the form of chemical 

 energy, heat, or mechanical work. In these forms it is 

 eventually dissipated into space or stored up in other 

 chemical compounds or in energy of position, when ex- 

 pended in raising a weight, for example. In no case is 

 it again usable directly by green plants. In the case of 

 fuel burned to run an electric light plant, some of it could 

 be used again. Whether that which escapes into space 

 ever returns as light is a question not answerable in the 

 present state of knowledge. 



