THE ORIGIN OF ORGANIC MATTER 49 



would in time be withdrawn from the atmosphere, and that the rates of 

 change in the carbon dioxide cycle would be greatly retarded. The effect 

 of such a state of affairs on the life of our planet is impossible to imagine. 

 An important factor in the production of carbon dioxide by decaying 

 animal and vegetable matter is that it is produced under or on the surface 

 of the ground where it soon becomes directly available again for photo- 

 synthesis. Air analyses show that the carbon dioxide content near the 

 ground is often many times that at several feet higher. The amount of 

 carbon dioxide produced by lower organisms cannot be calculated with 

 any degree of accuracy. Over a period of years it would, of course, not 

 be in excess of the amount of carbon dioxide reduced by the chlorophyllous 

 plants, though probably it is greater than the amount produced by higher 

 plants and animals. 



The higher chlorophyll-bearing plants cannot be regarded as carbon 

 dioxide producers, for in the total span of their life history they reduce 

 carbon dioxide and convert it largely into carbohydrates. Of course all 

 of the carbon dioxide reduced by a chlorophyllous plant is not perma- 

 nently laid down as carbohydrate. The plant is a living organism and 

 requires energy for its life processes ; this it obtains from the oxidation 

 of carbon compounds previously produced. Thus a portion of the ma- 

 terial produced in photosynthesis is oxidized by the plant and is in a sense 

 the operating expense of the factory. This factor varies with different 

 species, but in most cases 15 to 20 per cent of the material synthesized is 

 consumed in this manner. The carbon dioxide production of germinating 

 seeds is very high. In some cases as much as 25 per cent of the dry 

 material is consumed within 24 hours. In this manner most of the stored 

 material is depleted by the time the plant becomes a self-supporting 

 organism. But taken as a whole and during the entire course of its 

 life the chlorophyllous plant is on the minus side of the carbon diox- 

 ide cycle. 



The carbon dioxide production of animals is in a sense a reversal of the 

 photosynthetic process. The food of animals is directly or indirectly a 

 product of photosynthesis. The amount of carbon dioxide which can 

 be produced by the respiration of animals is naturally dependent upon the 

 quantity of material produced by photosynthesis. The respiration of 

 animals, therefore, tends to maintain the constancy of the atmospheric 

 carbon dioxide. Not all of the carbon in the food of animals is exhaled 

 as carbon dioxide. In mammals about 5 per cent of the carbon is excreted 

 in the form of organic compounds. The latter are in turn broken down by 

 micro-organisms. The rate of carbon dioxide production and the degree 

 of oxidation of the organic food materials varies greatly with different 

 animals. 



Some idea can be gained of this source of carbon dioxide from a 

 calculation of the amount produced by man. While the amount varies 

 greatly with weight and muscular work, we can take the value of 900 

 grams of CO.. produced per individual per day. The 1750 million in- 



