1420. Vegetation must therefore effect an important change in the earth's electricity. The earth must be differently polarized after, to what it was before, the fall of the leaf. 1421. Thereby the northern hemisphere is differently polarized to the southern, because the latter has less soil than the former. b. _Wood-process--Nutrition._ 1422. As most of the spiral vessels are collected together in the body of wood, and finally in the leaves issue forth quite free and naked into the air, so must the wood conduct for the most part air into the plant. The polarization of the other systems, of the liber and the bark, must therefore proceed from the body of wood. 1423. The greatest amount of induration must originate in the body of the spiral vessel, because in it the process of oxydation takes place in the most active manner. From the same cause the process of nutrition must also be supported by it in the most powerful manner. The wood is the chief seat of nutrition. c. _Tracheal-process--Oxydation._ 1424. The structure of the spiral vessels, their resemblance to the tracheæ or air-tubes of insects, their distribution throughout the whole trunk, the air they contain which is found decidedly free in the plant, leave no doubt that the tracheæ are air-conveying organs, and consequently have, like the arteries in animals, the process of respiration directly intrusted to them. 1425. Now through the process of respiration the general polarity, and consequently the cause or fundamental principle of all life, enters the plant. 1426. The tracheæ penetrate or traverse the whole plant from the apex of the root to that of the flower. Their operation must therefore also extend through the whole plant. 1427. The tracheal system must also govern the plant by polarity, and thus in an immaterial manner. 1428. This polarity acts simply in the direction of the plant's longitude, not transversely, like the material fundamental processes. 1429. The tracheæ impart in a spiritual manner the antagonism between the root and fabric of the stem. 1430. As the tracheæ are, or constitute, the highest system of the plant, so must it be them upon which the light principally acts. The material processes of plants are kept in activity by the antagonism of light. 1431. By this only are the instantaneous changes, which follow upon the influence of light or section of the spiral vessels, to be explained. Upon this, therefore, depends the instantaneous elevation of the processes under the influence of a ray of light, and their depression, if only a cloud pass in front of and obscure the sun; hence too does the plant die, so to speak, upon the very spot, if the spiral fibres within the liber be cut through, but the latter left uninjured.