204 THE FOOD AND NUTRITION OF PLANTS. 



general statement, that, by so much as plants grow, they decom- 

 pose carbonic acid and give its oxygen to the air ; or, in other 

 words, purify the air. 



359. Every six pounds of carbon in existing plants has with- 

 drawn twenty-two pounds of carbonic acid gas from the atmos- 

 phere, and replaced it with sixteen pounds of oxygen gas, occupy- 

 ing the same bulk. To form some general conception of the 

 extent of the influence of vegetation upon the air we breathe, 

 therefore, we should compute the quantity of carbon, or charcoal, 

 that is contained in the forests and herbage of the world, and add 

 to the estimate all that exists in the soil as vegetable mould, peat, 

 and in other forms ; all that is locked up in the vast deposits of 

 coal (the product of the vegetation of bygone ages) ; and, finally, 

 all that pertains to the whole existent animal kingdom ; and we 

 shall have the aggregate amount of a single, though the largest, 

 element which vegetation has withdrawn from the atmosphere. By 



when in full health and vigorous vegetation, impart no carbonic acid to the air 

 during the night. See Pepys, in Philosophical Transactions, for 1843. 

 They deteriorate the air only in their decay, and in peculiar processes, dis- 

 tinct from vegetation and directly the reverse of assimilation ; as in germina- 

 tion, for instance, where, as will hereafter be explained, the proteine induces 

 the decomposition of a portion of the store of assimilated matter, in order that 

 the rest may be brought into a serviceable condition. For at the beginning, it 

 must be recollected, the plant or the shoot grows, not by assimilation, but by 

 consuming and appropriating a store of nourishment which was assimilated by 

 the parent. The evolution of carbonic acid by plants, therefore, which has so 

 long been taken for granted, and misinterpreted, has no existence as'a gen- 

 eral phenomenon. And it is by a false analogy that this loss which plants sus- 

 tain in the night has been dignified with the name of vegetable respiration, 

 and vegetables said to vitiate the atmosphere, just like animals, by their respi- 

 ration, while they purify it by their digestion. If, indeed, this were a con- 

 stant function, in any way contributing to maintain the life arid health of the 

 plant, it might be properly enough compared with the respiration of animals, 

 which is itself a decomposing operation. But this is not the case. And herein 

 is a characteristic difference between vegetables and animals : the tissues of the 

 latter continue to live and act through the lifetime of the animal, and there- 

 fore require constant interstitial renewal by nutrition, new particles replacing 

 the old, which are removed and restored to the mineral world by respiration: 

 while in plants there is no such renewal, but the fabric once completed re- 

 mains unchanged, ceases to be nourished, and consequently soon loses its vital- 

 ity ; while new parts are continually formed farther on to take their places, to 

 be in turn abandoned. Plants, therefore, having no decomposition and recom- 

 position of any completed fabric, cannot have the function of respiration. 



