48 ORIGIN AND ACTION OF HUMUS. 



opposes the renewal of air. A plant itself effects 

 this change of air at a certain period of its growth. 

 The carbonic acid, which protects the undecayed 

 humus from further change, is absorbed and taken 

 away by the fine fibres of the roots, and by the 

 roots themselves ; this is replaced by atmospheric 

 air, by which process the decay is renewed, and a 

 fresh portion of carbonic acid formed. A plant at 

 this time receives its food, both by the roots, and 

 by the organs above ground, and advances rapidly 

 to maturity. 



When a plant is quite matured, and when the 

 organs, by which it obtains food from the atmo- 

 sphere, are formed, the carbonic acid of the soil is 

 no further required. 



Deficiency of moisture in the soil, or its com- 

 plete dryness, does not now check the growth 

 of a plant, provided it receives from the dew 

 and the atmosphere as much as is requisite for 

 the process of assimilation. During the heat of 

 summer it derives its carbon exclusively from the 

 atmosphere. 



We do not know what height and strength 

 nature has allotted to plants ; we are acquainted 

 only with the size which they usually attain. 

 Oaks are shown, both in London and Amsterdam, 

 as remarkable curiosities, which have been reared 

 by Chinese gardeners, and are only one foot and a 

 half in height, although their trunks, barks, leaves, 

 branches, and whole habitus, evince a venerable age. 



