[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 185 



Calories.^ By using these figures for the country as a whole and adding a 

 httle extra for range and desert vegetation, we find that the green plants 

 of the United States accumulate about 10 X 10^' Calories of energy 

 annually. This is approximately 70 per cent of the energy chemically 

 bound by them in photosynthesis. 



Through the use of plants as food for himself and his domesticated 

 animals, and in several other ways, man in the United States annually 

 destroys plants containing about 6 X 10^'' Calories of bound energy. In 

 addition to this the coal, oil, and gas removed from the earth each year in 

 this country have an energy value of more than 5 X 10^^ Calories. 

 From these calculations we seem to be responsible directly or indirectly 

 for the dissipation of chemically bound energy about as rapidly as it 

 accumulates in the green plants of today. To this we must add the amount 

 released by undomesticated animals and a part of that released by non- 

 green plants.- What of the future? 



Energy transformations and conservation. In the early years of the 

 present century the chief energy transformers in industry and transpor- 

 tation were the steam engine and the horse. Coal was often used for the 

 supply of chemically bound energy in the steam engine, but the use of 

 wood obtained from nearby living forests was not uncommon. All the 

 chemically bound energy transformed into mechanical energy in the 

 horse came each year from the living plants of the farm. Horses were 

 used for practically all transportation except that upon railways and 

 the sea. The farmer not only harvested the chemically bound energy 

 which he used on his own farm, but in addition he sold enormous quan- 

 tities of it for horses employed in various ways in the city. Since the 

 horses also came from the farm, the farmer furnished both the energy 

 and the transformer. With the coming of the automobile and tractor, 

 conditions were reversed. The farmer now buys from industry not only 

 the transformer ( motor ) but also the chemically bound energy ( gas and 

 oil) that was stored in plants millions of years ago. Such drastic changes 

 in the sources of energy have been reflected in economic relations for 

 many years. Moreover, there is a limit to the natural deposits of chem- 

 ically bound energy in coal and related products. 



^ If the reader wishes to calculate the equivalent of Calories of energy in terms of grams 

 of sugar, he can do so by multiplying by 180/674 or by dividing by 3.75. Grams of sugar 

 may be stated in terms of COo used in photosynthesis by multiplying by 264/180. See the 

 equation of relative weights in Chapter XIII. 



- One may also consider this problem on the basis of whether plant residue ( humus in 

 the soil, peat, coal, etc.) is accumulating today as rapidly as it is being destroyed. It did 

 accumulate more rapidly than it was destroyed before man came upon the scene. 



