30 ON THE ORIGIN AND ACTION OF HUMUS. 



change, is absorbed and taken away by the fine fibres of the 

 roots, and by the roots themselves ; this is replaced by atmo- 

 spheric air, which, by its oxygen, renews the process of decay, 

 and forms a fresh portion of carbonic acid. 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 atmosphere are formed, the carbonic acid 

 of the soil is no further required. 



Deficiency of moisture in the soil, or its complete dryness, 

 does not now check the growth of a plant, provided it receives 

 from the dew and from 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. The small parsnep grown at Tcltow,* when 

 placed in a soil which yields as much nourishment as it can take 

 up, increases to several pounds in weight. 



The size which a plant acquires in a given time is pro- 

 portional TO THE SURFACE OF THE ORGANS DESTINED TO CONVEY 



good to it. When the surfaces of two planls are equal, their 

 increase depends upon the length of time that their absorbing 

 powers remain in activity. The absorbing surfaces of fir trees 

 are active during the greater part of the year, so that (c&teris 

 paribus), they increase more than those trees which part with 

 their foliage in autumn. Each leaf furnishes to a plant another 

 mouth and stomach. 



The power possessed by roots of taking up nourishment does 

 not cease as long as nutriment is present. When the food of a 

 plant is in greater quantity than its organs require for their own 



* Teltow is a village near Berlin, where small parsneps are cultivated 

 in a sandy soil : they are much esteemed, and weigh rarely above one 

 ounce. 



