CAUSES OF ITS BENEFICIAL INFLUENCE. 167 



might be serviceable and even indispensable to the 

 existence of several other plants. But 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 dis- 

 tinguish the difference betw^een the views of Decan- 

 dolle and those of Macaire-Princep. The substances 

 which the former physiologist viewed as excrements, 

 belonged to the soil ; they were undigested matters, 

 which although not adapted for the nutrition of one 

 plant might yet be indispensable to another. Those 

 matters, on the contrary, designated as excrements 

 by Macaire-Princep, could only in one form serve for 

 the nutrition of vegetables. It is scarcely necessary 

 to remark, that this excrementitious matter must un- 

 dergo a change before another season. During au- 

 tumn 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 effect- 

 ed ; and at the commencement of spring it has be- 

 come converted, either in w^hole or in part, into a 

 substance which supplies the place of humus, by be- 

 ing a constant source of carbonic acid. 



The quickness with which this decay of the ex- 

 crements of plants proceeds depends on the com- 

 position of the soil, and on its greater or less po- 

 rosity. It will take place very quickly in a calcareous 

 soil : for the power of organic excrements to attract 

 oxygen and to putrefy is increased by contact with 

 the alkaline constituents, and by the general porous 

 nature of such kinds of soil, which freely permit the 

 access of air. But it requires a longer time in heavy 

 soils consisting of loam or clay. 



The same plants can be cultivated with advantage 

 on one soil after the second year, but in others not 



