with the leaves, unless the concoetion in them is unusually powerful; and leaves are in respect of strength midway between bark[1] and wood. But greenness does not persist, nor indeed come into existence without the presence of moisture, and is of the element of earth, and is the intermediate colour between that of earth and water. This can be illustrated by the fact that when the bark of trees dries up it turns black, and the wood inside the tree becomes white, and the green, which conies between these two colours, is the colour presented by the outward appearance of the plant.
The shapes of plants fall under three classes. Some spread upwards, others downwards, while others are intermediate in height between the two. The upward extension is due to the fact that the nutritive material makes its appearance in the marrow of the plant, and the heat draws it up, and the air, which is present in the rarities of the plant, compresses it, and it assumes a pyramidal form, just as fire assumes a pyramidal form in bodies in which it is present and rises upwards. Downward extension is due to the blocking of ducts in the plant, and, when the nutritive material is assimilated, the water, which is in the marrow of the plant, will thicken, and the rarefied portion proceeds on its upward course, while the water returns 828ato its former position in the lower portion of the plant, and by its weight presses the plant downwards. In the plants which are intermediate between the two classes already mentioned, the moisture is rarefied and the natural state of the plant is very nearly a temperate condition during the process of concoction, and the ducts are open through the middle of the plant, and the nutritive material spreads upwards and downwards. There is a double process of concoction; the first takes place below the plant, while the second takes place in the marrow which comes out of the earth and is in the middle of the plant; afterwards the nutritive materials make their appearance fully matured[2] and are distributed through the plant, and do not undergo
