1373. The _light_ likewise acts upon the direction of plants and especially that of their leaves; not simply from its promoting growth by elevation of temperature and by decomposition, but obviously in a mechanical manner also; for not only do the branches of plants in a green-house grow towards the window, but most leaves turn themselves the whole day in obedience to the course of the sun. This turning must, nevertheless, have one and the same kind of cause with the growing towards light; it also is only a conatus or effort unto growth. The upper leaf-cells, being illuminated by the sun, become lighter, and are therefore directed at once, like the apices of the branches, toward the influence of light. The cells that stand perpendicularly upon the surface of the leaf are to be regarded as branches conjoined by growth. 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.