136 IS CARBONIC ACID ABSORBED FROM THE SOIL? 



We have seen reason to conclude (p. 63) that, while plants derive 

 much of their sustenance from the air, they are also fed more, or less 

 abundantly by the soil in which they grow. From this soil they ob- 

 tain through their roots the carbonic acid which is continually given off 

 by the decaying vegetable matter it contains. This carbonic acid will 

 ascend to the leaf, and will there undergo decomposhion along with that 

 which is absorbed by the leaf itself. At least we know of no function 

 of the root or stem by which the carbonic acid derived from tiie soil can 

 be decomposed and deprived of its oxygen before it reaches the leaf. 



It is distinctly stated, indeed, by Sprengel, [see above, p. 92,] that 

 when the roots of a plant are in the presence of carbonic acid, the oxy- 

 gen given off by the leaf is greater in bulk than the carbonic acid ab- 

 sorbed. But there is one observation in connection with this point which 

 it seems to me of importance to make. The leaves supply carbon to 

 the plant only in the form of carbonic acid, and they give off a bulk of 

 oxygen gas not exceeding that of the acid taken in, [see note, below.] 

 But if the carbon derived from the soil be also absorbed in the form of 

 carbonic acid, and if the oxygen contained in this portion of acid is also 

 given off by the leaf — either the quantity drawn from the soil must be 

 small, compared with that inhaled from the air, or the oxygen given off 

 by the leaf must, in the ordinary course of vegetation, be sensibly great- 

 er than the bulk of the carbonic acid which it absorbs. 



We are too little familiar with the chemical functions of the several 

 parts of plants to be able to pronounce a decided opinion on this point; 

 but it appears evident that one or other of the three following conditions 

 must obtain : — 



(a). Either in the general vegetation of the globe the bulk of the oxy- 

 gen gas given off by the leaf during the day must always be considera- 

 bly greater than that of the carbonic acid absorbed by it ; or 



(b). The root or stem must have the power of .decomposing carbonic 

 acid and of separating and setting free its oxygen ; or 



(c). The plant can derive no considerable portion of its carbon from 

 the soil, in the form of carbonic acid. 



If the experiments hitherto made by the vegetable physiologists be 

 considered of so decisive a character as to warrant us in rejecting the 

 two former conditions, the third becomes also untenable. 



sary proportions of oxygen and hydi-ogen derived from the water of the sap. This opinion 

 implies that, in the leaf, carbonic acid (CO2 ) is flecomposed into carbonic oxide and oxy- 

 gen (CO -f O), and that water likewise is decomposed,— the oxyiien produced by both de- 

 compositions bein^ given off either into the air by the leaves, or into the soil by the roots. 

 The production of grape sugar, therefore, according to this hypothecs, would be thus repre 

 sented :— There are retained, and given off. 



From 12 of Carbonic Acid = 12C02 - - - C12 O12 O12 



From 12 of Water. • - = 12HO - - - H12 O12 



C12 H12 Ol2 O24 



grape sugar 



Of the 24 of oxygen thus given off, the opinion of Persox is, that only one-half is evolved 

 by the leaf, — and the principal fact on which his opinion rests is that observed by De Saus- 

 sure, that plants of Vinca minor gave off by their leaves, in his experiments, only two-thirds 

 of the oxygen contained in the carbonic acid they absorbed. Tliis result has led BerzeUus 

 also to conjecture that the loaves of plants do not retain merely the carbon of the carbonic 

 acid, but some compound of carbon wit!) oxygen, containing much less of this element than 

 the carbonic acid does(7Vai7e de C/iCwie, V, p. 69). Tlie principal objection to this view, 

 however, is the quantity of oxygen it supposes to be rejected by the root. The experimentt 

 on which it ia founded require confirmation and extension. 



