Book I. PROCESS OF VEGETABLE NUTRITION. 165 



first place, it is known that carbonic acid gas is soluble in water ; in the second place, it 

 is known to be contained in the soil, and generated by the fermentation of the materials 

 composing manures ; and, in the next place, it is known to be beneficial to vegetation 

 when applied artificially to the roots, at least in a certain degree. This is evident from 

 the following experiment of Ruckert, as well as from several experiments of Saussure's, 

 previously related. Ruckert planted two beans in pots of equal dimensions, filled with 

 garden-mould ; the one was moistened with distilled water, and the other with water im- 

 pregnated with carbonic acid gas. But the latter appeared above ground nine days sooner 

 than the former, and produced twenty-five beans ; while the former produced only fifteen. 

 Now the result of this experiment, as well as the preceding facts, is evidently favorable to 

 the presumption of Senebier, and shows that if carbonic acid is not the state in which car- 

 bon enters the plant, it is at least a state preparatory to it ; and there are other circum- 

 stances tending to corroborate the opinion, resulting from the analysis of the ascending 

 sap of plants. The tears of the vine, when analysed by Senebier, yielded a portion of 

 carbonic acid and earth ; and as the ascending sap could not be supposed to have yet un- 

 dergone much alteration, the carbonic acid, like (he earth, was probably taken up from 

 the soil. But this opinion, which seems to be so firmly established upon the basis of ex- 

 periment, Hassenfratz strenuously controverts. According to experiments which he had 

 instituted with an express view to the investigation of this subject, plants which were yaised 

 in water impregnated with carbonic acid differed in no respect from such as grew in pure 

 water, and contained no carbon that did not previously exist in the seed. Now if this 

 were the fact, it would be decisive of the point in question. But it is plain from the ex- 

 periments of Saussure, as related in the preceding section, that Hassenfratz must have 

 been mistaken both with regard to the utility of carbonic acid gas as furnishing a vegetable 

 aliment, and with regard to the augmentation of carbon in the plant. The opinion of 

 Senebier, therefore, may still be correct. It must be acknowledged, however, that the 

 subject is not yet altogether satisfactorily cleared up; and that carbon may certainly enter 

 the plant in some state different from that, either of charcoal in solution, or of carbonic 

 acid gas. Is not the carbonic acid of the soil decomposed before entering the plant ? 

 This is a conjecture of Dr. Thomson's, founded upon the following facts : — the green 

 oxide of iron is capable of decomposing carbonic acid ; and many soils contain that oxide. 

 Most soils, indeed, contain iron, either in the state of the brown or green oxide, and it 

 has been found that oils convert the brown oxide into green. But dung and rich soils 

 contain a quantity of oily substance. One effect of manures, therefore, may be that of 

 reducing the brown oxide of iron to the green, thus rendering it capable of decomposing 

 carbonic acid gas, so as to prepare it for some new combination, in which it may serve as 

 an aliment for plants. All this, however, is but a conjecture ; and it is more probable 

 that the carbonic acid of the soil enters the root in combination with some other substance, 

 and is afterwards decomposed within the plant itself. 



Sect. III. Process of Vegetable Nutrition. 



739. Plants are nourished in a manner in some degree analogous to the animal economy. 

 The food of plants, whether lodged in the soil, or wafted through the atmosphere, is taken 



' up by intro-susception in the form of gases or other fluids : it is then known as their sap ; 

 this sap ascends to the leaves, where it is elaborated as the blood of animals is in the 

 lungs ; it then enters into the general circulation of the plant, and promotes its growth. 



740. Intro-susception. As plants have no organ analogous to the mouth of animals, they 

 are enabled to take up the nourishment necessary to their support only by absorption, or in- 

 halation as the chyle into the animal lacteals, or the air into the lungs. The former term is 

 applied to the intro-susception of non-elastic fluids ; the latter to that of gaseous fluids. 

 The absorption of non-elastic fluids by the epidermis of plants does not admit of a 

 doubt. It is proved, indisputably, that the leaves not only contain air, but do actually 

 inhale it. It was the opinion of Priestley that they inhale it chiefly by the upper surface. 

 And it has been shown by Saussure, that their inhaling power depends entirely upon the 

 organisation. It has been a question, however, among phytologists, whether it is not also 

 effected by the epidermis of die other parts of the plant. We can scarcely suppose it 

 to be effected bv the dry and indurated epidermis of the bark of aged trunks, of which 

 the original organisation is obliterated ; nor by that of the larger and more aged branches. 

 But it has been thought there are even some of the soft and succulent parts of the plant 

 bv which it cannot be effected, because no pores are visible in their epidermis. Decan- 

 dolle found no pores in the epidermis of fleshy fruits, such as pears, peaches, and goose- 

 berries ; nor in that of roots, or scales of bulbs ; nor in any part not exposed to the in- 

 fluence of air and light. It is known, however, that fruits will not ripen, and that roots 

 will not thrive, if wholly deprived of air ; and hence it is probable that they inhale it by 

 their epidermis, though the pores by which it enters should not be visible. In the root, 

 indeed, it may possibly enter in combination with the moisture of the soil ; but in the 

 other parts of the plant it enters no doubt in the state of gas. Herbs, therefore, and the 



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