140 THE LIFE OF THE PLANT 



mental law of chemistry, that matter is neither created 

 nor destroyed, we tried to discover the sources of the 

 matter composing the plant, the way in which it 

 penetrates into the plant, and the changes it undergoes 

 during the process. 



But the vegetable body is a storehouse, so to speak, 

 of energy as well as matter heat-energy for in- 

 stance. Burning a single seed of a birch-tree will not 

 warm our frozen hands for a moment, whereas a birch- 

 tree a hundred years old will serve to heat our stove 

 for many a day. A birch-tree therefore accumulates 

 heat during its life-time, which we use as such, or else 

 as a source of mechanical force. 



Where does this heat, this energy, come from ? We 

 raised a similar question with regard to matter. Just 

 as we admit that matter neither disappears nor is 

 created, so do we also assume that neither does energy 

 disappear, nor is it created. In fact just as the 

 chemists of last century came to the conclusion that 

 matter is indestructible, so also the physicists of the 

 present day have come to the conclusion that energy 

 is never destroyed. The different sources of energy 

 may suffer endless change, passing from one form into 

 another, or become concealed in a state of tension ; but 

 they are never destroyed, never created anew. 



What is this latent energy, this heat concealed in our 

 fuel, and whence does it come, since it could not arise 

 spontaneously ? In order to explain this we must 

 glance again at the chemical phenomena already 

 familiar to us that take place in the leaf, but this time 

 from the purely physical point of view of the trans- 

 formation of energy involved. 



All chemical phenomena can be divided into two 

 categories : those in which heat, light, electricity in a 

 word energy appears, is given off ; and those in which 

 energy disappears, is absorbed. Phenomena of the former 

 category take place spontaneously, or require but an 



