PHYSIOLOGY 215 



The amount of carbonic acid gas contained in the air varies at different times 

 and places. H. BUOWN found that in 10,000 litres of air it was 2'7-2'9 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 3^-3 litres in 10,000 litres of the 

 atmosphere. This weighs about 7 grammes, of which T s r is oxygen, and only T : y 

 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 INGEXHOUSS 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 3000 billion kilos, in which are held 800 billion kilos of carbon. This 

 amount would be sufficient for the vegetation of the entire earth for a long time, 

 even if the air were not continually receiving new supplies of carbonic acid 

 through the respiration and decomposition of organisms, tli rough 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. CREDNEK calculated that 460,000,000, 000 kilos of coal are burnt annual]}-, 

 yielding to the atmosphere about 1,265,000 million kilogrammes of carbonic acid 

 gas. These sources of C0 2 are, according to Beyerinck, insignificant compared 

 with the enormous amounts produced by the bacteria of the soil in putrefactive 

 processes. The whole carbon supply of tbe atmosphere is at the disposal of plants, 

 since the C0 2 becomes uniformly distributed by constant diffusion. 



Not all plants, nor indeed all parts of a plant, are thus able to 

 abstract the carbon from the carbonic acid of the air. Only such 

 organs as are coloured green by chlorophyll are capable of exercising 

 this function, for the chlorophyll bodies themselves are the labora- 

 tories in which this chemical process, so important for the whole 

 living world, is carried on. From these laboratories is derived the 

 whole of the carbon which composes the organic substance of all living 

 things, plants as well as animals. Animals are unable to derive this 

 most essential element of their bodies from inorganic sources. They 

 can only take it up in organic substances, which have been 

 previously formed in plants. Such plants, also, as are without 

 chlorophyll, as, for example, the Fungi and some of the higher 

 parasitic plants, are dependent for their nutrition upon organic 

 substances previously formed by the chlorophyll bodies of other 

 plants. 



Roots and other organs unprovided with chlorophyll, and also the 

 colourless protoplasm in the green cells themselves, are similarly 

 dependent upon the activity of the chloroplasts. 



The derivation of carbon from carbonic acid and its conversion 

 into organic substances is termed ASSIMILATION. In its broadest 



