180 ORIGIN OF ORGANIC MATTER 



as is done (hiring the day by those parts of a plants which are not 

 green, led many physiologists to doubt whether the principal source 

 of the carbon in plants was derived from the deoxidation of car- 

 bonic acid, which takes place in solar light; whilst, moreover, 

 Saussure's experiments seemed to prove that at least one-twentieth 

 of the carbon absorbed by the plants during this process could not 

 be derived from the carbonic acid. It was believed that there 

 must be some truth in the popular notion that the humus, that 

 is to say, the decaying remains of vegetable and animal matter, 

 serves the living plant as a highly carbonaceous nutrient substance, 

 at least in respect to this one-twentieth. Although it cannot be 

 denied that a certain number of plants, amongst which we may 

 reckon many of the parasitical plants, and all plants which are not 

 green, cannot draw all their carbon from the carbonic acid of the 

 air and water, this no more proves the incorrectness of Liebig's 

 view than the fact that oxygen is exhaled in solar light by certain 

 green infusoria, as Euglena (which, moreover, contain a starch-like 

 substance) refutes the view that the vital process in animals is 

 constantly combined with an absorption of oxygen and an exhala- 

 tion of carbonic acid. Although many plants may thrive better in 

 a soil rich in humus than in one in which they merely obtain the 

 necessary mineral nutriment, this beneficial effect may be owing to 

 many other conditions besides the amount of carbon contained in 

 the soil; for as the humus consists of substances undergoing 

 decomposition, it must of itself supply an abundant source for the 

 formation of carbonic acid. 



Liebig refers this nocturnal development of carbonic acid to a 

 purely mechanical cause. It is well known that plants absorb indis- 

 criminately all substances held in solution in water, but that they 

 give off, either by their roots or through other parts, all matters 

 which may injure their vital activity ; and that all terrestrial as 

 well as atmospheric water contains larger or smaller quantities of 

 carbonic acid, which, according to Liebig,* is not assimilated 

 during the night, but is again evaporated in an unchanged con- 

 dition through the leaves with the water. But whilst Liebig 

 regards the development of carbonic acid as purely mechanical, he 

 considers the nocturnal absorption of oxygen to be a purely 

 chemical process, and shows that the variations in the quantity of 



* Die Chemie in ihrer Anwendung auf Agricultur u. Physiologie, 6 Aufl. 

 1846, S. 3-253 [or English translation, London, 1840, pp. 1-2) 5"| ; Chemische 

 Briefe, 1851, S. 240 ff. u. 629 if. [or Letters on Chemistry, 3rd edition, 1851, pp. 

 176 and 506]. 



