SECT. II PHYSIOLOGY 217 



utilised in assimilation is, according to Detlefsen, only 1 per cent ; according to 

 the calculations of H. BiiowN, somewhat over ^ per cent in sunlight, and over 

 2 per cent in dilluse light. That, however, light must disappear as such in COo- 

 assimilation is clear, for from what other source than the energy of light could the 

 energy be obtained that is stored uj) in the organic substance formed in assimila- 

 tion. This potential energy of the organic substance of the plant serves to 

 maintain the vital processes. The force exerted by our steam-engines also is to 

 be traced to the assimilatory activity of the plants, the wood or the carbonised 

 remains of which are burnt beneath its boiler. For in the combustion of the 

 reduced carbon compounds to carbon dioxide the energy, which was previously 

 required to transform carbon dioxide into the combustible materials, again 

 becomes free. 



The assimilatory activity of a chloroplast, like every vital function, 

 is dependent on a number of internal and external factors. To the 

 internal factors belong the presence of the pigment chlorophyll and 

 its situation in a living chloroplast. Chlorophyll itself, separated from 

 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). Further a chloroplast isolated from the rest of the cell 

 cannot continue to assimilate ; the protoplasm in which it is normally 

 embedded is thus one of the internal conditions of COg-assimilation. 



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 the existence of all organisms, would ultimately cease 

 were not fresh supplies of carbon dioxide continuously produced. 



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

 and places. H. Brown found that in 10,000 litres of air it was 2"7 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 Sg-Sj litres in 10,000 litres of the 

 atmosphere. This weighs about 7 grammes, of which y\ is oxygen, and only xt 

 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 acce})ted, 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 j)ercentage 

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

 about 3000 billion kilos, in whicli are held 800 billion kilos of carbon. This 

 amount would be suflicient 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, through the combustion 

 of wood and coal, and through vohanic activity. An adult will exhale daily about 

 900 grammes CO.^ (2-15 grammes C). The 1400 million human beings in the world 



