BOTANY 



carbon will be found to make up about half the dry weight (when 

 freed from water) of the plant. 



Whence do plants derive this carbon 1 The " humus " theory, 

 accepted for a long time, assumed that the humus of the soil was the 

 source of all the supply ; and that carbon, like all the other nutrient 

 substances, was taken up by the roots. That plants grown in pure 

 sand free from humus, or in a water-culture, increase in dry substance, 

 and consequently in carbon, clearly demonstrates the falsity of this 

 theory. The carbon of plants must therefore be derived from other 

 sources ; and, in fact, the carbon in humus is, on the contrary, due to 

 previous vegetable decomposition. The discovery made at the end of 

 last and the beginning of the present century, that the carbon of 



PLANTS IS DERIVED FROM THE CARBONIC ACID OF THE ATMOSPHERE, 



and is taken up by the action of the green leaves, is associated with 

 the names of the Dutchman Ingenhouss, and the Geneva Professors 

 Senebier and Theo. de Saussure. This discovery is one of the 

 most important in the progress of the natural sciences. It was by 

 no means easy to prove that the invisible gaseous exchange between 

 a plant and the atmosphere constitutes the chief source of nourish- 

 ment ; and it required the courage of a firm conviction to derive the 

 thousands of pounds of carbon accumulated in the trees of a forest, 

 from the small proportion contained in the atmosphere. 



10,000 litres of air contain only 4-5 1. of carbonic acid, which weigh 8-10 grams ; 

 f T of this weight is oxygen, however, and only T 3 T carbon. Accordingly, 10,000 litres 

 of air contain only 2 grams of carbon. In order, therefore, for a single tree, having 

 a dry weight of 5000 kilos, to acquire its 2,500,000 grams 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 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, 

 through the combustion of wood and coal, and through volcanic activity. An 

 adult will exhale daily about 900 grams C0 2 (245 grams 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. The Krupp works at Essen, 

 according to Hansen, send out daily into the atmosphere about 2,400,000 kilos of 

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

 as 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 



