PHOTOSYNTHESIS AND RESPIRATION. 85 



large Sunflower leaf may absorb, during a single hour's ex- 

 posure to sunlight, the carbon dioxide contained in many 

 cubic feet of air, making about 2 grammes of starch per hour 

 per square yard of surface. 



About half of the dry weight of a plant consists of carbon, and the 4 

 litres of carbon dioxide contained in 10,000 litres of air contain only 

 2 grammes of carbon. A tree having a dry weight of 5,000 kilograms 

 must therefore (to obtain its 2^ million grammes of carbon) deprive 

 12 million cubic metres of air of their carbon dioxide. But the 

 atmosphere contains an astounding amount of carbon dioxide, which 

 is placed at the disposal of plants owing to its uniform distribution by 

 diffusion. 



The actual supply of carbon dioxide in the air is estimated at 3,000 

 billion kilograms, containing 800 billion kilograms (about 800 million 

 tons) of carbon, which would be sufficient for the earth's vegetation for 

 several years, even if the supply were not being continually renewed by 

 the respiration and decomposition of organisms, by processes of burning, 

 etc. Since an adult breathes out daily more than 400 litres of carbon 

 dioxide, the 1400 millions of human beings in the world give to the air 

 annually about 100 billion kilograms of carbon. The amount of coal 

 burnt annually has been estimated at 400 billion (400,000,000,000) 

 kilograms, yielding to the air over 300 billion kilograms of carbon 

 (coal contains about 80 per cent, carbon). Even larger amounts of 

 carbon dioxide are doubtless produced by the Bacteria of the soil. 



12O. How Carbon Dioxide enters the Leaf. In a 



seaweed or a submerged flowering-plant carbon dioxide is 

 absorbed, and oxygen given out, by diffusion through the 

 whole surface of the plant. In the case of an ordinary plant 

 with stomate-bearing green organs, the gaseous exchanges l 

 between the atmosphere and the plant's tissues take place 

 through the stomates almost exclusively. The stomate- 

 bearing leaf is a much more efficient organ of gaseous 

 exchange than it would appear at first sight. 



The actual opening of a stomate is only about O0002 sq. mm. 

 small enough to hinder the entrance of dust or water. The 

 stomates are very numerous ; a Sunflower leaf, for example, 

 has about 12 millions. A square millimetre of Broad Bean 

 leaflet gave, as the average of several counts, 90 stomates on 



1 These exchanges are (1) entrance of carbon dioxide and escape of 

 oxygen, (2) entrance of oxygen and escape of carbon dioxide, (3) escape 

 of water vapour, in the processes of Carbon-assimilation, Respiration, 

 and Transpiration respectively. 



