122 THE ART OF CULTURE. 



taric, citric, oxalic, and malic acids, are not merely present for 

 the purpose of being used in druggists' shops, or in our house- 

 hold, as acid or as neutral salts. These organic acids must be 

 necessary for the formation of certain constituents in the plants. 



We have already come to the conclusion, that the carbon of 

 all plants is derived from carbonic acid ; tartaric, oxalic, citric 

 acid, &c, must, therefore, obtain their carbon from the same 

 source. But, can we conceive that the carbon forms a direct 

 and immediate combination with hydrogen for the production of 

 substances so various as sugar, starch, woody fibre, resin, wax, 

 and oil of turpentine ? Is it not much more probable that the 

 conversion of the carbon of carbonic acid into the constituent of 

 a plant proceeds in a gradual manner ; that by the union of the 

 constituents of water with carbonic acid, a substance is formed, 

 becoming gradually poorer in oxygen ; and that the carbon as-, 

 sumes the form of oxalic, tartaric, or other organic acids, before 

 it is converted into sugar, starch, or woody fibre ? 



According to this view, a ready and simple explanation is fur- 

 nished of the necessity of alkalic bases to vegetable life ; for 

 they are present for the purpose of effecting the conversion of 

 carbonic acid into a living part of a plant. The smallest parti- 

 cles of sugar, or of organic acids, when separated from plants, 

 follow their own peculiar attractions ; they form crystals, or they 

 follow the power which induces the cohesion of their atoms, but 

 still their carbon is capable of being converted into a constituent 

 of a living organ ; and, although sugar and tartaric acid have 

 been formed by vital agencies, they do not in themselves possess 

 any vital functions. 



From the preceding part of this chapter it will be seen that 

 fallow is that period of culture when the land is exposed to pro- 

 gressive disintegration by the action of the weather, for the pur- 

 pose of liberating a certain quantity of alkalies and silica to be 

 absorbed by future plants. 



The careful and frequent working of fallow land will accele- 

 rate and increase its disintegration ; for the purposes of culture it 

 is quite the same whether the land be covered with weeds, or 

 with a plant which does not extract the potash of the soil. 



