214 BOTANY i AI:I i 



The Absorption of Carbon (Assimilation) 



In any attempt to distinguish the relative importance of substances 

 utilised in plant nutrition, carbon undoubtedly ranks first. Every 

 organic substance contains carbon, and there is no other element 

 which could supply or take part in the formation of so many <>r 

 such a variety of substances, in living organisms as in the chemical 

 laboratory. Organic chemistry, in short, is merely the chemistry uf 

 carbon compounds. Living beings which are composed of organic 

 substances owe the possibility of their existence primarily to the 

 properties of carbon. 



It requires no chemical analysis to realise that plants actually 

 contain carbon, although in an imperceptible form. Every burning 

 splinter of a match shows, by its charring, the presence of this 

 element. An examination of a piece of charcoal in which the finest 

 structure of the wood is still distinguishable, shows how abundant is 

 the carbon and how uniformly distributed. Estimated by weight, the 

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

 freed from water) of the plant. The great quantities of coal in the 

 deeper strata of rocks are the remains of ancient plants ; both in 

 peat and in. some cases in coal the macroscopic and microscopic 

 structure gives proof of this origin. 



AVhence do plants derive this carbon? 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 

 the eighteenth and the beginning of the nineteenth 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 INGENHOUSS, SENEBIER, THEO. DE 

 SAUSSURE, and SACHS. This discovery is one of the most im- 

 portant 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 nourishment ; 

 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 (0'03 per cent) of carbon dioxide contained 

 in the atmosphere. 



