IMPORTANCE OF PLANTS — ROBBINS 307 



equivalent of somewhere near 3X10 11 tons of coal. If these figures 

 are approximately correct, then about 0.15 percent of that part of the 

 sun's energy which falls on the earth is caught annually and stored 

 by plants. 



Riley has estimated the photosynthetic efficiency of the world as a 

 whole to be 0.18 percent and that of the land areas 0.09 percent. 



Another set of data leads to somewhat the same conclusion. Tran- 

 seau calculated that 1.6 percent of the sun's energy was utilized by a 

 field of corn in Illinois during the 100 days of its growing season. 

 Since for much of the rest of the year a cornfield lacks vegetation 

 it would appear that something less than 1 percent of the sun's energy 

 annually reaching corn land in Illinois is fixed. In some parts of the 

 Tropics and other sections of the world where vegetation is active 

 the year round this proportion would be larger ; on the other hand, in 

 the Arctic and in deserts it would be much less. We may be justified, 

 therefore, in assuming that the annual energy fixation of plants ap- 

 proximates the equivalent of 3 X 10 n tons of coal. 



This astronomical figure is at first sight quite comforting, par- 

 ticularly when we learn that in energy value it is over 200 times the 

 coal and oil burned in 1938. The difficulty is that most of this an- 

 nual income is not used. Wood, alcohol produced in fermentation, 

 and plant waste play but a minor part in furnishing heat or me- 

 chanical energy because of their inconvenience, expense, or lack of 

 adaptability to modern machinery. We depend at present upon coal 

 and petroleum, the world's capital stock of available energy, to supply 

 the amount required for this mechanical age. 



Berl has reported a method by which motor fuel equal in many 

 respects to petroleum can be produced from cellulose, starch, sugar, 

 and other carbohydrates, thus offering the possibility of replacing our 

 stock of usable stored energy by utilizing part of the current day -to- 

 day income. Carbohydrates only can be used by Berl's method; 

 lignin, protein, oils, and fats are unsuitable as crude materials. How- 

 ever, if all the carbohydrates in all the plants were used as Berl sug- 

 gests, and this is obviously impracticable, we would have but 6 times 

 the present annual consumption of petroleum and less than 2 times 

 the equivalent of the annual world consumption of petroleum and 

 coal. If all the world used coal and petroleum as we did in this 

 country in 1942, the total energy fixed by plants would be but 25 times 

 that dissipated and all the carbohydrates made each year would yield 

 about one-third the amount the world would need. We can only 

 guess what these figures would be if we knew the energy consumption 

 for the war years of 1944 and 1945. 



Two years ago the National Science Fund asked a representative 

 group of outstanding scientists to list the problems with which scien- 



