372 PHYSIOLOGY 



Yet in spite of this relative inefi&ciency, the total product is enormous 

 and invaluable, because of the limitless store of energy pouring upon 

 the earth constantly from the sun, beside which the artificially released 

 energy of fuel is absolutely a negligible quantity. 



The solar energy received by the earth in a second is represented by 250 X 10" 

 calories. The coat consumed in the whole world in a year, reported in 1906 as about 

 1000 million metric tons,' represents 8 X 10"' calories. The plant can afford, so to 

 speak, to be inefficient. 



Source of light. — The source of light is quite a matter of indifference. 

 In nature, of course, the primary source, the sun, is alone to be con- 

 sidered, since the light of even the full moon (only ^d^obd that of the 

 sun) is too weak to effect photosynthesis to a measurable extent. Va- 

 rious secondary sources may be used in experiments, some electric lamps 

 and the incandescent mantles (with gas) giving light of sufiicient inten- 

 sity when near the plants. Attempts to " force " plants, by enabling 

 them to make food by night with electric arc illumination, have been 

 successful with certain sorts, showing that there is no need for rest at 

 night, and that a greater supply of food permits more rapid develop- 

 ment; but there will be no incentive for commercial application of this 

 result until the cost of electric energy is vastly less than now. 



Temperature. — A suitable temperature has usually been considered 

 merely a condition of photosynthesis, and not a source of energy for 

 the process. This is evidence that our knowledge of the energy rela- 

 tions of this process is vague, and that the matter needs investigation. 

 At present, however, it is not possible to describe in terms cf energy 

 the effect of heat upon photosynthesis, so we must be content with a 

 brief statement on temperature as a condition. 



Experiments show that even at temperatures approaching 0° C. 

 some plants can make carbohydrates; the algae of arctic waters are 

 conspicuous examples. Yet for most plants such a low temperature 

 practically stops photosynthesis; while even at several degrees higher 

 it may be the limiting factor, less food being made than the CO2 and 

 light would permit. Likewise in direct sunlight the temperature may 

 rise so high in the interior of a leaf as to retard photosynthesis ' ; and 

 in tropical deserts, where the heat of the air itself may run to 45° C, 

 it is probable that photosynthesis is reduced thereby. 



' The metric ton about equals the English " long " ton, 2200 lbs. 



^ But these heating effects of direct sun are compensated in a measure by evaporation. 



