Book I. PROCESS OF VEGETABLE NUTRITION. 233 



periments of Hassenfratz. But there seem to be reasons for doubting the legitimacy of the 

 conclusion that has been drawn from it ; for Senebier found that plants whose roots were 

 immersed in water took up less of the fluid in proportion as it was mixed with water from 

 a dunghill. Perhaps then the charcoal of water from a dunghill is held merely in sus- 

 pension, and enters the plant under some other modifi.cation. But if carbon is not 

 soluble in water in the state of charcoal, in what other state is it soluble ? It is soluble 

 in the state of carbonic acid gas. But is this the state in which it actually enters the 

 root ? On this subject phytologists have been somewhat divided in opinion. Senebier 

 endeavours to prove that carbonic acid gas, dissolved in water, supplies the roots of plants 

 with almost all their carbon, and founds his arguments upon the following facts : In the 

 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 

 favourable to the presumption of Senebier, and shows that if carbonic acid is not the state 

 in which carbon enters the plant, it is at least a state preparatory to it ; and there are 

 other circumstances 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 undergone much alteration, the carbonic acid, like the earth, was probably taken 

 up from the soil. But this opinion, which seems to be so firmly established upon the 

 basis of experiment, Hassenfratz strenuously controverts. According to experiments 

 which he had instituted with an express view to the investigation of this subject, plants 

 which were raised 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 experiments of Saussure, as related in the preceding section, that Has- 

 senfratz 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 acknow- 

 ledged, 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 carbonic acid of the soil decomposed before 

 entering the plant? This is a conjecture of Dr. Thomson's, founded upon the fol- 

 lowing 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. Ill, Process of Vegetable Nuti'ition. 



1537. Plants are nourished in a nuinner in some degree analogous to that in which animals 

 are sustained. The food of plants, whether lodged in the soil, or wafted through the atmo- 

 sphere, is taken up by introsusception 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. 



1538. Introsusception. 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 absorp- 

 tion or inhalation, as the chyle into the animal lacteals, or the air into the lungs. The 

 former term is applied to the introsusception 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 their organisation. It has been a question, however, among phytologists, whether 



