50 ORIGIN AND ACTION OF HUMUS. 



of nutritive power, and a consequent further deve- 

 lopment of new leaves and branches. Leaves, twigs, 

 and branches, when completely matured, as they 

 do not become larger, do not need food for their 

 support. For their existence as organs, they require 

 only the means necessary for the performance of 

 the special functions to which they are destined 

 by nature ; they do not exist on their own account. 



We know that the functions of the leaves and 

 other green parts of plants are ' to absorb carbonic 

 acid, and with the aid of light and moisture, to 

 appropriate its carbon. These processes are con- 

 tinually in operation ; they commence with the 

 first formation of the leaves, and do not cease with 

 their perfect development. But the new products 

 arising from this continued assimilation, are no 

 longer employed by the perfect leaves ^in their own 

 increase : they serve for the formation of woody fibre, 

 and all the solid matters of similar composition. 

 The leaves now produce sugar, amylin or starch, and 

 acids, which were previously formed by the roots 

 when they were necessary for the development of 

 the stem, buds, leaves and branches of the plant. 



The organs of assimilation, at this period of their 

 life, receive more nourishment from the atmo- 

 sphere than they employ in their own sustenance, 

 and when the formation of the woody substance 

 has advanced to a certain extent, the expenditure 

 of the nutriment, the supply of which still remains 

 the same, takes a new direction, and blossoms are 



