gion in which the force of gravity is perceived by the plant. Our
method of proof does not hold good, but our conclusions are true after
all. When gravitation is the stimulus, the experiment is much more
difficult than when light is in question, because now that fairy
godmothers are extinct we must not hope for a substance opaque to
gravitation, a substance with which we might shelter the root-tips from
the force of gravity as the tips of the _Setaria_ seedlings were
sheltered from light.

The plan adopted by us was simply to cut off the extreme tip of the
roots, and fortunately (or unfortunately) the result was just what was
expected--the tipless roots had lost the sense of gravitation, and were
unable to curve downwards towards the centre of the earth. It was
natural to believe that the tipless roots failed to bend because their
sense-organs--their percipient parts--had been removed. As a matter of
fact they had been removed, but it was fairly objected that the operation
of removing the delicate tissues at the tip of the root is a severe one,
and that the roots which refused to grow downwards were suffering from
shock, and not from the absence of their sense-organs.

The subsequent history of the inquiry is an instance of the unwisdom of
prophesying unless you know. In 1894 an able summary of the question was
published in a German journal, in which the impossibility of solving the
problem of the gravitational sensitiveness of the root-tip was dwelt on,
and immediately afterwards Section K of the British Association had the
satisfaction of hearing Pfeffer read a brilliant paper giving the
long-hoped-for proof that the tip of the root is a sense-organ for
gravitation. {47}

Like many other experiments, it depends on a deception or trick played on
the plant. The root is forced to grow into a minute glass tube closed at
one end and sharply bent in the middle, resembling a little glass boot;
the extreme tip being thus kept at right angles to the main body of the
root. If the theory we are testing is the right one, a root with its
motor region horizontal and its tip vertical ought to continue to grow
horizontally, because the tip being vertical is not stimulated by
gravity; it is in a quiescent, or, as it were, a satisfied condition, and
no bending influence is being sent to the motor region. And this is what
Pfeffer and Czapek found. On the other hand, if the main body of the
root points vertically down while th