[Chap. XX BIOLOGICAL RELATIONS OF GREEN PLANTS 183 



More than 90 per cent of it may be traced back through various energy 

 transformations to the hght energy that was chemically bound in sugar 

 during photosynthesis. Many questions relevant to these facts may be 

 considered. 



How efficient are plants as transformers of light energy? How rapidly 

 is man releasing the chemically bound energy of plants? Is the present 

 civilization releasing it more rapidly than it is being bound by plants 

 today? Is all the energy bound by plants available to man? Has the re- 

 placement of the horse by the automobile and tractor — which use the 

 energy bound by the plants of the past — altered the value of the energy 

 bound by plants of the present? What of the future? 



The answer to any one of these questions must be an approximate 

 one; but an estimate based upon available data is so much better than a 

 mere guess or no answer at all, that space will be given to a tabular 

 summary of a few data and certain general statements based upon data 

 too numerous to be presented here ( Table 7 ) . 



The acre of corn referred to in the table was located in central Illinois. 

 The yield of 100 bushels of grain on this acre is three times the average. 

 The whole plant is considered in the calculations. The acre of apple 

 trees was located at Ithaca, New York. 



Most all of the chemically bound energy that accumulates in the com 

 plant and apple tree would ultimately be available to non-green plants, 

 for as a group of organisms they may digest and oxidize most of the 

 substances made in the green plants. Only about one-fourth of the 

 energy that accumulates in a corn plant is available to man as a source 

 of bound energy in food, and still less of the energy that accumulates 

 in the young apple tree is similarly available. As a source of energy in 

 fuel, however, the amount that is available to man depends upon how 

 meticulously he harvests and uses the different parts of the plant. In 

 one country the roots may be left in the soil and the smaller branches 

 may be heaped in piles and burned, while in another where the supply 

 of chemically bound energy is scarce and expensive the smaller twigs, 

 roots, and even the leaves may be collected for fuel. In the early days 

 pioneers on our own prairies and plains sometimes used the stems and 

 leaves of native grasses as fuel. But regardless of how meticulously man 

 harvests green plants, other animals and the non-green plants will take 

 a certain toll. The insects in a pasture, for instance, may eat almost as 

 much grass as do grazing animals. 



