SEPARATION OF OXYGEN. 91 



(in 100 parts) differing from that of woody fibre only in its being 

 richer in carbon. 



Misled by this simplicity in its constitution, physiologists found 

 no difficulty in discovering the mode of the formation of woody 

 fibre ; for they say,* humus has only to enter into combination 

 with water, in order to effect the formation of woody fibre, and 

 other substances similarly composed, such as sugar, starch, and 

 gum. But they forget that their own experiments have suffi- 

 ciently demonstrated the inaptitude of these substances for assimi- 

 lation. Yet we could scarcely conceive a form more fitted for 

 assimilation than that of the substances just mentioned. They 

 contain all the elements of woody fibre, and with respect to 

 their composition in 100 parts, they correspond closely with 

 humus ; but they do not nourish plants. 



All the erroneous opinions concerning the modus operandi of 

 humus have their origin in the false notions entertained respect- 

 ing the most important vital functions of plants ; analogy, that 

 fertile source of error, having, unfortunately, led to the very 

 unapt comparison of the vital functions of plants with those of 

 animals. 



Substances, such as sugar, starch, &c, containing carbon and 

 the elements of water, are products of the life of plants which 

 live only whilst they generate them. The same may be said of 

 humus, for it can be formed in plants like the former substances. 

 Smithson, Jameson, and Thomson, found that the black excre- 

 tions of unhealthy elms, oaks, and horse-chestnuts, consisted of 

 humic acid in combination with alkalies. Berzelius detected 

 similar products in the bark of most trees. Now, can it be sup- 

 posed that the diseased organs of a plant possess the power of 

 generating the matter to which its sustenance and vigor are 

 ascribed ? 



How does it happen, it may be asked, that the absorption of 

 carbon from the atmosphere by plants is doubted by many bota- 

 nists and vegetable physiologists, and that by the greater number 

 ;he purification of the air by means of them is wholly denied ? 



These doubts have arisen from an erroneous consideration of 



* Meyen, Pflanzenphysiologie, II., S. 141. 



