METABOLISM. 27 



fall on the leaf are absorbed in some form, and 

 it becomes very probable that much of the solar 

 energy, other than that we term light, is retained 

 in the leaf for other purposes than assimilation 

 or, at least, no other conclusion seems possible in 

 view of all the facts. Engelmann's researches with 

 purple bacteria are almost conclusive on this point, 

 and we may regard it as extremely probable that 

 the plant makes other uses of rays, perceived by us 

 as heat-rays, as sources of energy. Researches on 

 the influences of temperature on assimilation and 

 other functions point to the same conclusion ; 

 and Pfeffer and Rodemann definitely state that 

 heat is converted into work in the osmotic cells. 

 And the study of the absorption bands in the 

 spectrum of the living leaf becomes more in- 

 telligible in the light of these conclusions. More- 

 over, the fact that a plant still carries on pro- 

 cesses of metabolism when active transpiration 

 has lowered its temperature below that of the 

 surrounding air and the plant therefore receives 

 heat from the environment points to similar 

 conclusions. 



The importance of the conclusion is immense, 

 for even if the plant had no other sources of energy 

 than the darker heat rays of the solar spectrum, 

 it is clear that it ought to be able to do work. 



The above may suffice for the general establish- 

 ment of the conclusion that the plant absorbs 

 more radiant energy than it employs solely for 

 assimilation, and emphasises our deduction that it 

 is a machine for storing energy. 



