1381. That the plant imbibes water, and this indeed in great quantity, by its whole surface is a well-ascertained fact; but it has been by no means equally determined whether it obtains its nourishment simply through the water, or directly also from the air, e. g. the carbon, as well as the nitrogen, from the carbonic acid. The principal imbibition, however, takes place through the root; but experiments that have been made upon this subject leave it doubtful, whether in this case it is simply mucus, extract from the humus or vegetable mould, or simply carbonic acid that has been absorbed. 1382. It is moreover a fact that the green parts of plants exposed to direct sun-light consume or take in carbonic acid, and develop or give out oxygen; on the contrary, during the night, and even in cloudy or gloomy days, they absorb oxygen and exhale or develop carbonic acid. Now, as there are far more gloomy or at least cloudy than clear days, it thus becomes evident that far more oxygen has been taken up, than separated, from the air. During germination oxygen gas is consumed and, on the other hand, carbonic acid developed. 1383. The saps ascend upwards, and chiefly indeed, in the liber; on its passage different substances forming from it, which appear especially in the fruit in greatest proportion and variety. 2. _Processes._ 1384. The tissues of plants form three formations, which must be similar in their functions, and can only exhibit subordinate differences. The cell-formation is displayed in the cellular tissue, in the bark and root. The vascular formation in the vascular tissue, in the liber and stalk. The tracheal formation in the tracheal tissue, the wood and the leaves. There can accordingly be only three principal functions in the vegetable stem, and of these each will display minor differences. A. _Cellular Processes._ a. ROOT-PROCESS--ABSORPTION. 1385. As the root is the cellular organ proper, so in it principally resides the water-process or the commencement of chemical elaboration and analysis. Now the chemism in an organic body is called digestion. 1386. The root is the mouth or pharynx of the plant, and is therefore principally concerned with absorption. Its process is therefore the formation of mucus, or as it were of salivation. The root cannot, however, create mucus, as it was created at the conclusion of the earth-metamorphosis in the sea; it can absorb it or in the highest degree compound it out of the constituent parts. 1387. The process of the formation of mucus is a process of putrefaction; the function of the root consists accordingly in supporting a constant process of putrefaction. The soil in which the root stands must contain substances susceptible of, and the conditions necessary to, putrefaction. These substances are organic matters and water; the conditions heat and access of air. Such a soil is called humus or mould. In a pure, dry earth, no root can thrive.