Dr. Hugo Mohlj on Liebig's Organic Chemistry. 91 



sequently no humjc acid (which stands next to them in a chemical 

 point of view), are ;not used as food by plants, but would rather 

 impede the vital process and kill the plant, (p. 116.) In analysing this 

 string of assertions, Dr. Mohl observes, in the first instance, that 

 the absorption of azotised compounds as food by plants is a fact 

 doubted by no one ; but he doubts whether the quantitative propor- 

 tion between the absorption and digestion of azotised and unazotised 

 substances, assumed by Liebig, be based on fact. L. says that 

 this proportion must exist in the grains of Wheat. But Hermb- 

 stadt's fine experiments on the different sorts of grain show quite 

 the contrary ; for one sort of Wheat contains 4 1 per cent of starch 

 to 34 per cent, of gluten, whilst another sort shows a proportion of 

 65 to 9. But this varying proportion of azotised and unazotised 

 substances also occurs in the organs of vegetation themselves, as may 

 be seen in the roots of Beet cultivated on a soil rich in vegetable 

 mould, or in one manured with animal matter. Chemistry, therefore, 

 does not support Liebig's assertions. Indeed, he himself, in some 

 degree, contradicts his own statements, when he says (p. 119) that 

 if plants obtain a greater proportion of carbon than of nitrogen, then 

 the carbon will not be used for forming gluten, or albumen, or wood, 

 nor for any constituent part of an organ, but will be secreted in the 

 form of sugar, starch, oil, wax, resin, mannite, gum &c. 



The assumption (continues Dr. Mohl) that the organs of plants 

 consist of gluten, albumen, and wood, and that other constituents, 

 like sugar, starch, &c,, are mere secretions, is decidedly wrong in 

 an anatomical point of view, for the solid substance of all organs 

 consists of woody fibre alone, and all the other ingredients are 

 merely preserved in the cellular substance, &c. The same objection 

 may be raised against the opinion, that starch, gum, &c, preserved 

 in the cells, are mere excrements, and not constituent parts of the 

 organ. On the other hand Dr. Mohl considers Liebig's opinion, 

 that a greater amount of nitrogen is required for forming woody 

 fibre, than sugar, gum, starch, &c, as perfectly true. This is cor- 

 roborated by the experiments of Payen, who found in all young 

 organs, while in a state of vigorous development, an abundance of 

 nitrogenous juices — which leads to the conclusion that nitrogenous 

 substances are essential to the development of new elementary 



