The Kinds of Work That Are Done by Plants 89 



is this, — the release of energy. This release takes place in a single 

 perfectly definite way, namely, as the result of the invariable 

 physical fact of Nature that at the instant carbon unites chemi- 

 cally with oxygen, it matters not in what place or under what 

 circumstances, energy is released. It is for the release of this 

 energy that the process of respiration exists ; and the plant no more 

 respires for the purpose of absorbing oxygen and releasing carbon 

 dioxide than we kindle a fire in the grate in order to make oxygen 

 rush into the furnace or carbon dioxide pour out of the chimney. 

 The object of respiration and of building the fire (i. e., of com- 

 bustion), are one and the same, — namely, to secure that energy 

 w^hich is always released at the moment of chemical union of 

 carbon with oxygen. Respiration and combustion are strictly 

 homologous terms, applying to phenomena which are also homol- 

 ogous. In the combustion of coal, which is carbon, in a grate, 

 the energy is released chiefly as heat (with some Hght) ; and by 

 causing that release to occur underneath a suitable arrangement 

 of boilers, pistons and wheels, the energy can be made to produce 

 motion and thus do work, as every steam engine is a visible wit- 

 ness. In the explosion (which is merely a rapid combustion), of 

 gasolene and oxygen inside the cylinder of an automobile engine, 

 we have exactly the same thing wdth a very much simpler machin- 

 ery. In respiration within the cells of an animal or a plant, the 

 machinery is simpler still, but the principle remains the same; 

 the energy is released at the moment of oxidation under such 

 conditions that it acts on the simple protoplasmic machinery 

 provided by the plant in a way to secure transformation into 

 motion and work. The source of the energy of the work done 

 by the engine and plant is identically the same; it is only the in- 

 termediate machinery which is different. The nature of this 

 machinery, it is true, is not at all understood in the plant, but we 

 know that something of the kind must exist. The machinery 

 must also differ somewhat for the different kinds of work that 

 plants and animals do; but in all cases it is driven by one and the 



