66 ON THE ORIGIN AND ACTION OF HUMUS. 



of carbonic acid ; and, on the other hand, the quan- 

 tity of their food is diminished by every difficulty 

 which 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 atmo- 

 spheric 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 atmosphere 

 are formed, the carbonic acid of the soil is no fur- 

 ther 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 the atmosphere 

 as much as is requisite for the process of assimila- 

 tion. 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 curi- 

 osities, 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 Teltow,^ when placed in a soil which yields 

 as much nourishment as it can take up, increases to 

 several pounds in weight. 



The size of a plant is proportional to the surface 

 of the organs which are destined to convey food to it, 



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

 vated in a sandy soil; they are much esteemed, and weigh rarely 

 above one ounce. — L. 



