IN THE VEGETABLE KINGDOM. 195 



carbon, hydrogen and oxygen in the same proportion as they 

 exist in water, and which have, therefore, received the irrational 

 designation of carbo-hydrates. The first origin of these substances, 

 which we meet with in their more advanced stages of development 

 as dextrin, sugar, starch, and cellulose, has, with apparent cor- 

 rectness, been referred to the decomposition of carbonic acid 

 under the influence of light. The opinion can scarcely be main- 

 tained in these days that the carbonic acid in the green cells of 

 the plant is instantaneously decomposed, and that the separated 

 carbon combines with undecomposed water to form dextrin, or 

 sugar. Unless we have recourse to the direct intervention of a 

 vital principle, or to some metabolic force of the cell, we must 

 admit it to be highly improbable that the bonds which hold the 

 oxygen and carbon in close combination should be suddenly rent 

 asunder. As we are still very ignorant of the proportion existing 

 between the absorbed carbonic acid and the exhaled oxygen, we 

 can only regard the view as tenable in very general terms, that a 

 decomposition of water is associated with a partial deoxidation of 

 the carbonic acid. Liebig has indicated the special grounds 

 which support the view, that the decomposition of water exerts an 

 influence on the separation of oxygen during the action of solar 

 light upon the leaves. Liebig's opinion that those organic acids 

 which we meet with in various quantities in all plants, as oxalic, 

 tartaric, citric, and malic acids, may be originally formed by the 

 simultaneous decomposition of water and carbonic acid, gains a 

 certain amount of probability from the confirmation or explanation 

 which it furnishes in relation to several other facts. This hypo- 

 thesis derives special support from the fact, that the alkalies occur 

 in accurately limited quantities in plants, and especially in their 

 green parts ; for if only a definite quantity of certain bases is 

 necessary to the life of the plant, we may readily understand that 

 they will in the first place be employed for the saturation of the 

 acids, and that when the acid by subsequent forces has been con- 

 verted into dextrin, sugar, or other indifferent matters, the same 

 amount of bases may again serve for the saturation of newly 

 formed acid; and it may even be assumed that the alkali itself 

 contributes towards the metamorphosis of the acid into these 

 indifferent substances. We should find no lack of attempted 

 explanations drawn from analogies of better known chemical pro- 

 cesses, were we to advance further into the domain of pure hypo- 

 thesis ; but it must be borne in mind, in endeavouring to support 

 such conjectures, that this process of deoxidation extends its 



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