32 ON THE ORIGIN AND ACTION OI^ HUMUS 



own sustenance ; and when the formation of the woody su> 

 stance has advanced to a certain extent, the expenditure of th« 

 nutriment, the supply of which still remains the same, takes a 

 new direction, and blossoms are produced. The functions of the 

 leaves of most plants cease upon the ripening of their fru:» ; 

 because the products of their action are no longer needed. They 

 now yield to the chemical influence of the oxygen of the air, 

 generally sudor a change in color, and fall off. 



A peculiar transformation of the matter contained in all 

 plants takes place in the period between blossoming and the 

 ripening of the fruit ; new compounds are produced, which 

 furnish constituents to the blossoms, fruit, and seeds. 



Transformations of existing compounds are constantly taking 

 place during the whole life of a plant, in consequence of which, 

 and as the results of these transformations, there are produced 

 gaseous matters which are excreted by the leaves and blossoms, 

 solid excrements deposited in the bark, and fluid soluble substan- 

 ces which are eliminated by the roots. Such secretions are most 

 abundant immediately before the formation and during the con- 

 tinuance of the blossoms ; they diminish after the development 

 of the fruit. Substances containing a large proportion of carbon 

 are excreted by the roots and absorbed by the soil. Through 

 the expulsion of these matters unfitted for nutrition, the soil 

 receives again the greater part of the carbon which it had at first 

 yielded to the young plants as food, in the form of carbonic acid. 



The soluble matter thus acquired by the soil is still capable 

 of decay and putrefaction, and, by undergoing these process s, 

 furnishes renewed sources of nutrition to another generation oi 

 plants; it becomes humus. The fallen leaves of trees, and the 

 old roots of grass in the meadow, are likewise converted into 

 humus by the same influence. 



The carbon contained in the roots of annual plants, such as 

 the corn plants and culinary vegetables, is without doubt derived 

 principally from the atmosphere. But after the removal of the 

 crop, their roots remain in the soil, and, undergoing putrefaction 

 and decay, furnish humus, or that suL stance which is able to 

 yield carbonic acid to a new vegetation. A soil receives more 



