Page:An introduction to physiological and systematical botany (1st edition).djvu/237

 Rh ago been observed by Sennebier to be coloured even in the dark, apparently because their colour depends on a different principle from the green of leaves.

Light acts beneficially upon the upper surface of leaves, and hurtfully upon the under side; hence the former is always turned towards the light, in whatever situation the plant may happen to be placed. Trees nailed against a north wall turn their leaves from the wall, though it be towards the north, and in direct opposition to those on a southern wall over against them. Plants in a hot-house all present the fronts of their leaves, and this influences even the posture of the branches, to the side where there is most light, but neither to the quarter where most air is admitted, nor to the flue in search of heat. If the branches of a trained fruit-tree in full leaf be disturbed in their position, the leaves resume their original direction in the course of a day or two. The brighter the day, the more quickly is this accomplished. If the experiment be often repeated, they continue to turn, but more weakly, and are much injured by the exertion. Black spots