DIV. ii PHYSIOLOGY 251 



the plant, is as little able to decompose the carbon dioxide as is a 

 chloroplast which for any reason has not developed the characteristic 

 pigment (chloroplasts developed in the dark or in the absence of iron, 

 leucoplasts of subterranean parts or of epidermal cells) or has lost it 

 (chromoplasts). Since, however, assimilation is not proportional to the 

 amount of chlorophyll, it is necessary to assume with WILLSTAETTER ( 30 ) 

 that in addition to the pigment another factor is essential, whether 

 this is the protoplasm of the chloroplast or an enzyme which it contains 

 (p. 264). 



Among external factors sunlight as referred to above must be 

 mentioned first, and next the presence of carbon dioxide. Since the 

 latter is only present in small proportion in the air, the life of plants, 

 and with this tl\e existence of all organisms, would ultimately cease 

 were not fresh supplies of carbon dioxide continuously produced. 

 Estimating the amount of carbon dioxide in the atmosphere at 2100 

 billion kilogrammes and the annual consumption by green land plants 

 at 50-80 billion kg., the supply would be used up in some thirty 

 years ( 30a ). 



The air is continually receiving new supplies of carbonic acid through the 

 respiration and decomposition of organisms, through the combustion of wood and 

 coal, and through volcanic activity. An adult will exhale daily about 900 grammes 

 C0. 2 (245 grammes C). The 1400 million human beings in the world would thus 

 give back to the air 1200 million kilos of C0 2 (340 million kilos C). The C0 2 

 discharged into the air from all the chimneys on the earth is an enormous amount. 

 In Germany alone in 1911, besides 73 million tons of brown coal, 161 million 

 tons of coal were used ; the latter would produce some 400,000 million kg. of 

 carbon dioxide, which is about 1/5000 of the total amount in the atmosphere. 

 Animals produce large amounts of carbon dioxide in respiration, as also do plants, 

 including fungi and bacteria (especially the bacteria of the soil). 



The fixation of carbon dioxide by green plants and the production of carbon 

 dioxide in the ways referred to are approximately equivalent. The amount of 

 carbonic acid gas contained in the air varies at different times and places. It 

 has been found that in 10,000 litres of air it was 27 to 2'9 litres in July, 3 '0-3 '6 

 litres in the winter ; close to the ground 12-13 litres were present in the same 

 volume. The average amount is about 3J-3 litres in 10,000 litres of the 

 atmosphere. This weighs about 7 grammes, of which T 8 T is oxygen, and only T 3 T 

 carbon. Only 2 grammes of carbon are thus contained in the 10,000 litres of air. 

 In order, therefore, for a single tree having a dry weight of 5000 kilos to acquire its 

 2,500,000 grammes of carbon, it must deprive 12 million cubic metres of air of their 

 carbonic acid. From the consideration of these figures, it is not strange that the 

 discovery of INGENHOUSS was unwillingly accepted, and afterwards rejected and 

 forgotten. LIEBIG was the first in Germany to again call attention to this discovery, 

 which to-day is accepted without question. The immensity of the numbers just 

 cited are not so appalling when one considers that, in spite of the small percentage 

 of carbonic acid in the atmosphere, the actual supply of this gas is estimated at 

 about 2100 billion kilos, in which are held 560 billion kilos of carbon. The 

 whole carbon supply of the atmosphere is at the disposal of plants, since the C0 2 

 becomes uniformly distributed by constant diffusion. 



Submerged water plants absorb the C0 2 dissolved in water. Its amount varies 



