APPENDIX 339 



means of energy obtained free of cost, but also work most effec- 

 tually. They are regulated according to the very need for 

 water : the amount of water is regulated by the amount of the 

 radiant energy of the sun, hence also according to the severity of 

 the drought.^ 



We can prove easily enough from what has been said that 

 sunlight is a powerful source of energy, and that this very energy 

 decomposes carbonic acid in plants. The plant is unable to 

 provide itself with the energy necessary for the purpose ; it only 

 plays the part of a mechanism, if I may so say, of a connector 

 for transmitting the energy of the sun. 



The plant therefore presents a regular contrast to the animal 

 from the physical as well as the chemical point of view. The 

 life of the plant consists in a continual transformation of the 

 energy of sunlight into latent chemical energy ; while the life 

 of the animal, on the contrary, manifests the transformation 

 of chemical latent energy into heat and motion. The spring is 

 wound up in the one to unwind in the other. 



We should be wrong, however, if we imagined that this function 

 of the sunlight became intelligible the moment Ingenhousz dis- 

 covered its participation in the process of the dissociation of 

 carbonic acid. More than half a century had to pass before the 

 actual mechanical details of the process were worked out. This 

 achievement science owes to Mayer and Helmholtz. While 

 light in former days was talked of only as an incomprehensible 

 though beneficial influence, Mayer was the first to state that sun- 

 light is actually used up in the literal sense of the word, and is 

 absorbed by the plant ; that the energy of the ray transforms 

 itself into chemical tension ; that in burning fuel, and in the vital 

 processes of our organism, we use the stored up energy of the sun. 

 It would be better to listen to his own eloquent way of putting it : 

 ' Nature, says he, seems to have set itself to capture the light 

 that falls upon our planet, to transform the most mobile of all 

 forces into an immobile form, and to conserve it as such. With 

 this end in view it«#as covered the crust of the earth with 

 organisms, which, during their life-time, absorb the sunlight and 

 form at the expense of this energy the stores of latent chemical 



* See my article on the ' Struggle of Plants with Drought ' in my • 

 Russian book, Agriculture and the Physiology of Plants. Moscow, 1906. 



