veness of Mimosa is crude by
comparison with that of the seedling. A plant with a perception of the
position of the centre of the earth and a power of growing along the line
so perceived is a much greater miracle than a leaf that closes its
leaflets when burnt or cut or shaken.

I shall show that certain parts of the plant have the special quality of
the perception of gravitation, but we are at present ignorant of how the
act of perception is effected. We know something of the machinery of
hearing or vision in animals, but in plants we can only guess that when a
cell is placed horizontally a resulting change of pressure on the
protoplasm produces that loss of equilibrium which is translated into
curvature. It is, however, probable that Nemec and Haberlandt are right,
and that the stimulus depends on the pressure of solid particles, _e.g._
starch-grains, on the protoplasm. {40}

The use of this gravitational sensitiveness is clear enough. It is to
the pine tree what a plumbline is to the builder, for neither plant nor
man can build high unless he builds straight. A man has a general
perception of the verticalness of his body and of surrounding objects,
but he does not trust to this sense in placing brick on brick to make a
house. He uses a plumb-line which tells him through his eye the precise
line along which he must pile his bricks. The tree has also to pile one
over another the cells or chambers in which its protoplasmic body lives,
and this too must be done along a vertical line; but the plant is guided
by the sensitiveness to gravity of which I have spoken.

It must be clearly understood that gravity does not act directly on the
growth of plants. It does not act as a magnet acts on iron, or to take a
better example, it does not simply act as gravity acts on the plumb-line
in which the string is kept in a vertical line by the weight. It might
be supposed that in some occult way the stem was mechanically kept
straight like the string, and this indeed was the view formerly held
about such roots as grow straight down into the earth. But it is not so;
the thing is not explicable mechanically. Gravitation is nothing more
than a sign-post or signal to the plant--a signal which the plant
interprets in the way best suited to its success in the struggle for
life, just as what we see or hear gives us signals of changes in the
exterior world by which we regulate our conduct.

It may be said that this is hard to pr