166 A TEXTBOOK OF BOTANY [Ch. IV, 8 



duces a union of oxygen with the carbon in coal, whereby, 

 with formation of carbon dioxide, energy is released in 

 the form of heat ; and this heat expands the steam which 

 drives the engine and does the work. Both respiration and 

 combustion are essentially energy-releasing processes, acting 

 alike through oxidation of carbon ; for it is one of the most 

 fundamental of cosmic facts, that whenever and wherever 

 carbon is allowed to unite chemically with oxygen, energy is 

 set free, and can do work, whether in the cells of a plant, 

 the muscles of a man, or the boiler of an engine. Respiration 

 is in principle nothing but a slow regulated combustion 

 within the bodies of plants and animals under protoplasmic 

 machinery capable of turning its energy into work. 



What now is the original source of the energy thus con- 

 tained in the food? Energy exists in nature in two forms, 

 kinetic or active, like heat, light, and electricity, and potential 

 or latent, as in wound springs, raised weights, explosives, 

 and unoxidized chemical substances in general. In fact, 

 potential energy exists in all unsatisfied affinities, physical or 

 chemical. When kinetic energy is used to wind springs, 

 lift weights against gravitation, or separate substances from 

 close chemical unions, it goes thereby into the potential 

 form, and thus remains until the spring unwinds, the weight 

 falls, or the chemical unions again take place, during which 

 processes the potential energy again becomes kinetic, and 

 can be used to do work, as clocks, water power mills, and 

 artillery illustrate. The kinetic energy of the sunlight in 

 photosynthesis originally dissociated the carbon dioxide and 

 water into their constituents, thereby passing into potential 

 form. The oxygen was released into the air, but the carbon 

 and hydrogen remained in the food, and thus were trans- 

 ferred throughout the bodies of plants and animals, carrying 

 the store of potential energy, which, on chemical access of 

 oxygen in respiration, again becomes kinetic and does work. 

 The process has an accurate parallel in the case of the storage 

 battery, now familiar in the modern automobile. A charge 



