138 BIOLOGY OF PLANTS. 



full discussion of the theories of those chemists and physiolo- 

 gists, which are best entitled to confidence. 



In the present state of our knowledge, it will be more use- 

 ful to present the arguments for and against the most favor- 

 ite theories, and to state the practical deductions which natu- 

 rally grow out of them. 



I. Carbon. The most abundant substance in vegetables, 

 is carbon. From whence is it derived, and what are the 

 changes which take place in its assimilation ? 



History. It has been the general opinion of agricultural 

 writers, that vegetable mould or humus is the principal source 

 of the carbon of plants ; and hence, the cause of the fertility 

 of soils. 



Humus or mould is a brown substance, easily soluble in 

 alkalies, and but slightly soluble in water. It results from 

 the decomposition of vegetable matter, when subjected to the 

 agency of water and air, but its formation may be promoted 

 by the action of alkalies, alkaline earths, metallic oxides, and 

 in some cases by acids. Chemists have designated this sub- 

 stance by several names ; sometimes including all the decom- 

 posed organic matters of the soil under the term humus or 

 geine, and sometimes only the soluble parts of it. Berzelius, 

 in 1833, divided the organic matters of the soil into extract 

 of humus, geine and carbonaceous mould ; and in 1841 he 

 made the following division ; extract of humus, humic acid, 

 humin, crenic and apocrenic acids. Now the gci?ie of 1833 is 

 the hujnic acid of 1841. Dr. Dana calls all the decomposed 

 organic matter of the soil geine. This consists of two parts ; 

 that which is decomposed by, or is soluble in alkalies, and 

 which is a definite compound, he calls soluble geine, and as 

 it exhibits the properties of an acid, geic acid, answering to 

 humic acid ; and that which is insoluble in the same solvent, 

 he calls insoluble geine. He also admits the existence of 

 crenic and apocrenic acids. 



Dr. C T. Jackson denies the existence of any such defi- 



