Book I. PROCESS OF VEGETABLE NUTRITION. 23S 



fifteen. Now tlie 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 whicli 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 i^enebier, therefore, may still be correct. It must be acknow- 

 ledged, however, that the subject is not yet altogether satisfactorily cleared up ; and that 

 <;arbon may certainly enter the plant in some state different from that either of charcoal 

 in solution, or of carbonic acid gus. 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. III. Process of Vegetable Nutrition, 



1511. Plants are nourished in a manner in some degree analogous to the animal econo- 

 my. 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. 



1512. 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 absorp- 

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

 former term is applied tb 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, tliat 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 the other parts of the plant. We can scarcely 

 suppose it to be effected by the dry and indurate epidermis of the bark and 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 by which it cannot be effected, because no pores are visible in their epidermis. 

 Decandolle found no pores in the epidermis of fleshy fruits, such as pears, peaches, and 

 gooseberries ; nor in that of roots, or scales of bulbs ; nor in any part not exposed to 

 the influence 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 soft parts of woody plants, absorb moisture and inhale gases from the soil or atmo- 

 sphere by means of the pores of their epidermis, and thus the plant effects the intro- 

 susception of its food. 



1513. Ascent of the sap. The means by which the plant eflTects the intro susccption 

 of its food, is chiefly that of absorption by the root. But the fluids existing in the soil 

 when absorbed by the root, are designated by the appellation of sap or lymph ; which, 

 before it can be rendered subservient to the purposes of vegetable nutrition, must either 



