CAUSES OF ITS BENEFICIAL INFLUENCE. 173 



the stem, is the equally abundant excretion of 

 organic matters into the soil from the roots. 



The favourable influence which this exercises on 

 the land, by furnishing it with matter capable of 

 being converted into humus lasts for several years, 

 but barren spots gradually appear after the lapse of 

 some time. Now, it is evident, that after from six 

 to seven years the ground must become so impreg- 

 nated with excrements that every fibre of the root 

 will be surrounded with them. As they remain for 

 some time in a soluble condition, the plants must 

 absorb part of them and suffer injurious effects in 

 consequence, because they are not capable of assi- 

 milation. When such a field is observed for several 

 years, it is seen, that the barren spots are again 

 covered with vegetation, (the same plants being 

 always supposed to be grown,) whilst new spots 

 become bare and apparently unfruitful, and so on 

 alternately. The causes which produce this alter- 

 nate barrenness and fertility in the different parts 

 of the land are evident. The excrements upon the 

 barren spots receiving no new addition, and being 

 subjected to the influence of air and moisture, they 

 pass into putrefaction, and their injurious influence 

 ceases. ' The plants now find those substances, 

 which formerly prevented their growth, removed, 

 and in their place meet with humus, that is, 

 vegetable matter in the act of decay. 



We can scarcely suppose a better means of pro- 

 ducing humus than by the growth of plants, the 



