1006 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



experimental yields of Noddack and Komor, and about equal to the highest 

 short-time averages of Purevich. Putter attributed these comparatively 

 high values to conditions that favor large-scale field experiments (particu- 

 larly to the carbon dioxide supply from the ground) . However, it seems more 

 likely that his conversion yields were overestimated. An error by a factor 

 of about 1.5 could have been caused by the use of too low a factor for the 

 conversion of lux into energy units (in chapter 25 we stated that, in sun- 

 light, one lux corresponds to about 10 erg/cm.^ sec, while Putter used a 

 factor of 6.3). Another error in the same direction may have been intro- 

 duced by the comparison of exceptional crops with average insolation data. 



Spoehr (1926), who made calculations similar to those of Putter, but 

 took into consideration only the grain in the field crops and the utilizable 

 timber in the forests, obtained much lower values of energy conversion — 

 c. g., 0.13% of total incident radiation for a wheat field, and 0.35% for a 

 forest of fast-growing eucalyptus trees. Similar figures were obtained, for 

 forest trees, by Boysen-Jensen (1932). These calculations were intended 

 to estimate the practical efficiency of plants as converters of solar energy — 

 so that stalks, husks and roots of the wheat plants, and leaves and roots 

 of the trees were neglected altogether; but the consideration of these 

 terms could scarcely more than double the calculated conversion yields — ■ 

 which would thus become comparable to the results of Noddack and Ko- 

 mor, but could never approach the much higher figures of Putter. To sum 

 up, it seems that 1% of total incident solar energy, and about 2.5% of ab- 

 sorbed visible radiation, represent a fair estimate of the average utilization 

 of light energy by field crops and forests, during the summer vegetation 

 period, under moderate climatic conditions. The average quantum yield 

 of photosynthesis under these conditions is of the order of 0.01 (1 molecule 

 COo reduced per 100 visible quanta absorbed). 



Analysis of the data on plankton production in the sea (of. Table l.II) 

 led Riley (1941) to the conclusion that the average utiUzation of light energy 

 falling on the surface of the sea is between 0.6 and 0.8%,* i. e., similar to 

 average utilization of light by fields and forests. However, in the ocean, 

 vegetation develops more or less uniformly throughout the year; and ex- 

 cept for the part of the Arctic seas covered by ice, there are no large barren 

 regions in the ocean comparable to the deserts or glaciers on the surface of 

 the earth. These differences weigh heavily in favor of the oceans as the 

 main producers of organic matter on earth (of. Table l.III). 



In chapter 1 (page 9) we made one more step and calculated the 

 total production of organic matter on earth by assuming that the average yield 

 of conversion of the energy of visible radiations absorbed by the plants is 

 2% (corresponding to 0.8% of the total incident light energy). Now, 



* Lanskaja and Sivkov (1950) gave much higher figures, 3-14%. 



