CAUSES OF ITS BENEFICIAL INFLUENCE. 173 



six-fold surface, capable of assimilating these bodies, 

 and of preventing the volatilization of the ammonia 

 from the soil, by completely covering it in. 



An immediate consequence of the production of 

 the green principle of the leaves, and of their re- 

 maining component parts, as well as those of the 

 stem, is the equally abundant excretion of organic 

 matters into the soil from the roots. 



The favorable 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 assim- 

 ilation. When such a field is observed for several 

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

 ered 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 alternate 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 

 leaves of which are food for animals ; for they pre- 

 pare the soil for plants of every other kind, but 

 particularly for those to which, as to rape and flax, 



15* 



