Shut a plant up in a room into which light is only admitted through
a small hole in the window-shutter, and set the plant out of the
direction of this light; it will, in a short time, turn itself, and
even grow downwards, that it may expose its leaves to the light.
If plants be kept in darkness, they will soon become bleached; then, if
they be exposed to the sun for three, four, or five hours, the leaves
and stalks will become as intensely green as if the plants had been
reared in the sun. Again, if a lighted lamp be introduced into a dark
room, wherein a plant has been shut up and bleached, it will become
green, and direct itself towards the lamp. If such a plant be removed
from the room, exposed for some time to the sun, and then returned to
darkness, it will no longer support the privation of light, but will
fade and perish.