50 PHOTOSYNTHESIS 



habitants ®* of the earth would thus produce 1575 million kilograms of 

 carbon dioxide or 575 X 10^ kilograms per year. Compared to the 

 atmospheric supply of 2 X 10^^ kilograms this is a very insignificant 

 amount, about 0.02 per cent; it nevertheless corresponds to 156 X 10^ 

 kilograms of carbon or approximately 230,000,000 tons of coal. 



It is impossible to estimate the quantity of carbon dioxide produced 

 by other animals. In general, however, it is probably safe to state that 

 of the total of what chlorophyllous plants produce in the form of carbon 

 compounds through photosynthesis, the greater portion is reconverted 

 into carbon dioxide not by animals, but by lower plants and micro- 

 organisms. 



Turning now to the other side of the carbon dioxide cycle, that of 

 COa-consumption, photosynthesis by the chlorophyllous plants is the factor 

 of foremost importance. While some attempts have been made to esti- 

 mate the amount of photosynthesis on the surface of the entire earth, this 

 involves so many variables that the results of such calculations must be 

 regarded as subject to more or less drastic revision. Such a calculation, 

 it has been claimed, would be of value in determining the total amount 

 of food the earth is capable of producing and consequently contribute to 

 the general probleni of world population. But different plants differ 

 enormously in their behavior under like conditions which, together with 

 differences between tropical and temperate regions, makes such a calcula- 

 tion, with the available data, impossible. 



The amount of carbon dioxide which plants remove from the atmos- 

 phere is very considerable. Thus Noll ^^ has calculated that a tree of 

 5000 kilograms dry weight contains about 2500 kilograms of carbon. 

 In order to have obtained this amount of carbon the tree must have re- 

 moved the carbon dioxide from about 12 million cubic meters of air. 

 Similarly the old calculations of Sachs, ^^ while they cannot be taken as 

 an exact measure of photosynthetic activity, give an idea of the order of 

 magnitude of the function of plants. The leaves of an ordinary sun- 

 flower plant have an area of about 1.5 square meters. Sachs found 

 that such a plant absorbs 660 cc. or 1.3 grams of carbon dioxide per hour. 

 In a ten hour day the plant would absorb about 400 grams of carbon 

 dioxide per month. If the entire land area of the earth were covered 

 with sunflowers so that on each square meter there was one plant or a 

 million plants to each square kilometer, the plants covering the 135 million 

 square kilometers of land would absorb 54 X 10^^ kilograms carbon 

 dioxide in a month. At this rate, the 21 X 10" kilograms carbon dioxide 

 of the atmosphere would last about 40 months. The limited significance 

 of such calculations requires no further comment. The concentration of 

 atmospheric carbon dioxide represents a condition of dynamic equilibrium 

 in which photosynthesis is but one factor. 



East, E. M., Mankind at the Crossroads. N. Y., 1924, p. 111. 



Noll, Strasburger, Lehrbuch der Botanik. Jena, 1906. 8th Edition, p. 181. 



Sachs, J., Arbeiten aus dem bot. Inst. Wilrzburg, 3, 1-33 (1884). 



