CAUSES OF ITS BENEFICIAL INFLUENCE. 167 



substances that are formed in a vegetable organism 

 during the process of nutrition, which are produced, 

 therefore, in consequence of the formation of woody 

 fibre, starch, albumen, gum, acids, &c., cannot 

 again serve in any other plants to form the same 

 constituents of vegetables. 



The consideration of these facts enables us to 

 distinguish the difference between the views of 

 Decandolle and those of Macaire-Princep. The 

 substances which the former physiologist viewed as 

 excrements, belonged to the soil ; they were undi- 

 gested matters, which although not adapted for the 

 nutrition of one plant, might yet be indispensable 

 to another. Those matters, on the contrary, de- 

 signated as excrements by Macaire-Princep, could 

 only in one form serve for the nutrition of vegeta- 

 bles. It is scarcely necessary to remark, that this 

 excrementitious matter must undergo a change be- 

 fore another season. During autumn and winter 

 it begins to suffer a change from the influence of 

 air and water ; its putrefaction, and at length, by 

 continued contact with the air, which tillage is the 

 means of procuring, its decay are effected ; and 

 at the commencement of spring it has become 

 converted, either in whole or in part, into a sub- 

 stance which supplies the place of humus, by being 

 a constant source of carbonic acid. 



The quickness with which this decay of the excre- 

 ments of plants proceeds, depends on the composi- 

 tion of the soil, and on its greater or less porosity. 



