VITAL FUNCTIONS. 8 I 



substances in a state of eremacausis " (slow decay) " makes 

 it probable that the same is true of light." 



In considering the doctrine here laid down as to the 

 identity of the chemico-vital forces concerned in the nutri- 

 tive processes of plants with the sun-light and sun-heat, 

 the student must guard himself against confounding the 

 necessary condiiions of a phenomenon with its cause. The 

 chemical and calorific rays of the sun are doubtless essen- 

 tial to the performance by plants of their vital functions ; 

 but it does not follow that they are the only forces resi- 

 dent in the vegetable organism, or, indeed, that they are 

 the most important ones. The true difficulty of the pro- 

 blem lies in this very transformation of purely physical 

 energy into chemico-vital forces. How do plants convert 

 sun-light into the chemical affinity by which they are enabled 

 to raise certain stable inorganic materials to the height of 

 unstable organic compounds ? How, and in virtue of what, 

 do plants convert sun-heat into the vital force by which 

 they can increase their organised structures " to an almost 

 unlimited extent?" Here lies the true problem, and* it is 

 one from the solution of which we are very far as yet. It 

 is no real explanation to say that the mechanism or the 

 material of the plant is such as to produce this change, just 

 as when we transmit heat through a given apparatus and it 

 becomes electricity, or through another and it is converted 

 into light. No one doubts the possibility, and truly the 

 daily occurrence, of such transformations, but this affords 

 no true explanation in the case of the plant. In the first 

 place, this explanation begs the very question at issue, for 

 it assumes what cannot be proved — namely, that the change 

 is effected by the " physical basis " of the plant, instead of 

 by some special power residing in the organism. It as- 

 sumes, also, that all the forces expended by the plant in 

 its vital work are the exact equivalent of the solar heat and 

 light which it receives — an assumption which may be highly 

 probable, but is nevertheless incapable of proof. In the 



