372 PHYSIOLOGY 
Yet in spite of this relative inefficiency, 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 io 15 
calories. The coal consumed in the whole world in a year, reported in 1906 as about. 
1000 million metric tons, 1 represents 8 X io is 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 -giniimr 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 sufficient intenv 
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 of 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 o 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 CO 2 and 
light would permit. Likewise in direct sunlight the temperature may 
rise so high in the interior of a leaf as to retard photosynthesis 2 ; and 
in tropical deserts, where the heat of the air itself may run to 45 C. , 
it is probable that photosynthesis is reduced thereby. 
1 The metric ton about equals the English " long " ton, 2200 Ibs. 
3 But these heating effects of direct sun are compensated in a measure by evaporation 
