146 BIOLOGY OF PLANTS. 



Objections to the Theory of Liehig. The arguments 

 by which this theory is supported, are not all well founded ; 

 and if they were, they would not prove its truth, but would 

 furnish good reasons for the opposite opinion, that plants de- 

 rive a part of their carbon from other sources. 



1. This theory does not give a correct view of the compo- 

 sition of humus. According to the recent analysis of Ber- 

 zelius, the humus of soils, as we have seen, is composed of 

 humin, extract of humus, humic, crenic and apocrenic acids, 

 and some salts. Dr. C. T. Jackson makes the humus of soils 

 consist of more than twenty substances, of which humic, cre- 

 nic and apocrenic acids are the most important. These sub- 

 stances are rich in carbon, containing from fifty to sixty per 

 cent. They are rendered soluble in water, by the action of 

 the oxygen of the air and of alkalies ; they must therefore en- 

 ter the roots of plants, and may be decomposed in the vege- 

 table organs, yielding their carbon, oxygen and hydrogen, 

 while the inorganic bases with which they were united, may 

 be returned to the soil as excretory matter. Thus, according 

 to the theory itself, and to other facts, the inorganic bases, 

 which Liebig supposes remmnjixed in the plant, may be the 

 means of conveying successive portions of carbon into the 

 vegetable organs. In fact the decomposition of salts by the 

 " catalysis of life" is the most important change which takes 

 place in the soil. The bases are thus let loose upon the hu- 

 mus, combine with it, and the salts may again be decom- 

 posed. Hence, a part of the carbon must be derived from 

 the humus of the soil. 



2. But, on the supposition that no larger quantity of carbon 

 could be introduced, by means of humic acid or humates, than 

 the theory supposes, the conclusion is still unavoidable, that a 

 part of the carbon is derived from this source, or else, that large 

 quantities of carbon are taken into the vegetable organs and 

 again rejected, without being decomposed ; but in such a 

 case, they must act as poisons, and become a constant source 



