1374. The sleep of plants depends also upon the same influence of the light. The upper leaf-cells sleep during the night, while the lower cells, especially those of the petiole, fill and consequently bend the latter upwards. The sleep of the flower must have the same cause. As likewise the alternating motion of many leaves, as in the Mimosæ.
1375. The motion also performed by the staminal filaments towards the pistil must finally depend upon this unequal replenishment of the external and internal cells.
1376. The coloration of the parts of plants is a result of the decomposition of the starch-granules in the cells by the agency of light.
1377. The operation of the heat is more intelligible than any other. That which is to move and separate itself, must have a certain degree of extension, or must be fluid, namely, aquiform. In a cold temperature the upper saps, not becoming warmer than the lower, are consequently not lighter, and on that account also do not ascend upwards. The mortal freezing of trees descends from above downwards. In other respects plants have, like animals, a self-inherent, though very feeble, process of heat. Germination proves this, in cases where many seeds lie upon each other.
1378. The air acts also mechanically and physically upon plants, by causing motion of the solid parts and by promoting evaporation. Electricity is without doubt active in the spring of the year, and evokes the antagonism between the fabric of the stem and root.
1379. The physical operation of water consists indeed, for the greatest part, in its preserving the solid parts in a supple or pliant state. Its principal office is, however, to convey nourishment to the plant.
1380. The earths act beneficially only upon plants, if they have been all mixed with each other. Mineral salts occurring in moderate quantity in the soil promote growth; alkalies and acids are injurious thereunto. The same holds good of Inflammables and metallic limes.
b. BY INTERNAL ACTIVITY.
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.