8 PHOTOSYNTHESIS IN NATURE CHAP. 1 



sponds to an air layer approximately 8 X 10^ meters thick (under stand- 

 ard conditions), Gut's calculations indicate a yearly consumption of about 

 22% of all carbon dioxide in the air column above the forest. 



However, the rapid exchange of carbon dioxide between atmosphere 

 and hydrosphere makes the separate comparison of the carbon utilization 

 by land plants with the amount of atmospheric carbon dioxide, and of the 

 carbon utilization by sea plants with the quantity of dissolved carbonates 

 irrelevant. Instead, we have to compare the total carbon fixation by all 

 plants, on land and in the sea, with the total carbon reserve available in 

 both the troposphere and the hydrosphere. The plants assimilate a 

 quantity of carbon equal to this reserve in 300 or 400 years. To maintain 

 the cycle, an equivalent quantity of carbon dioxide must be liberated 

 during the same period bj- respiration, and by the decay of vegetable 

 and animal matter. 



Veniadsky (1930) estimated the weight of the "biosphere"' on earth as 10'^ tons, and 

 postulated its renewal "several times a year." In other words, he assumed that an 

 average carbon atom remains in organic combination for only a few months. These as- 

 sumptions call for a rate of photosynthesis of the order of at least 10" and more probably 

 10^* tons of organic carbon per year (i. e.. from 60 to 600 times more than was allowed 

 above) and for the consumption, each year, of the whole amount of carbon available in 

 the air and in the water of the oceans. In another place, Vernadsky speaks of the living 

 organisms transforming in a single year "more than the total quantity of carbon on the 

 earth." Even if he means only the carbon content of the air and water, this estimate 

 appears much too high, if compared with the better supported figures of Liebig, Schroeder, 

 and Riley. To make his figures plausible, Vernadsky criticizes the values of Liebig for 

 average crops because of the neglect of roots and stubbles, and points out that some 

 plants can produce crops far in excess of these averages. He quotes, for example, 50 

 tons of dry organic matter per hectare (not including wood and leaves) harvested from 

 a banana plantation; 250 tons fresh tubers which Manihoi/utilissima Pohl can produce 

 per hectare, as well as other types of plants which can yield as much as 50 or 60 tons 

 of organic carbon per hectare annually. 



Table l.III shows, however, that unless Vernadsky is willing to postulate that the 

 average annual plankton production in the sea is not 3.75 tons, as estimated by Riley, 

 but 30 or .50 tons of carbon per hectare, his assumption of a yearly fixation of 10" tons 

 carbon is impossible. 



Our estimates of the rate of carbon fixation on earth can be checked 

 by calculations of an entirely different and not less interesting type — 

 based on the amount of available sun energy and its average utilization 

 in photosynthesis. The energy of the sun radiation reaching the upper 

 boundaries of the atmosphere is 1.25 X 10-'^ cal. per annum, and only 

 about 40% of this energy penetrates to the surface of the earth, the rest 

 being scattered and absorbed by the clouds and by the atmosphere. Of 

 the 5 X 10'-^ cal. which reach the earth's surface, 50% are in the form of 

 infrared and extreme red radiations, which are not used in photosynthesis, 

 and at least 20% are absorbed by rocks, sand and ploughed fields, or 



