e sensitive tip is horizontal, a
curvature results, because as long as the tip is horizontal it is
stimulated, and the stimulus is transmitted to the motor region.

This experiment proves not only that the tip of the root is the
sense-organ for gravity, but also that the motile part is not directly
sensitive; in other words, that gravitation is perceived exclusively in
the tip of the root. Since the publication of Pfeffer's and Czapek's
papers I have been lucky enough to hit on another way of investigating
percipient organs for gravitation, {48} and I am not without hopes that
botanists may become in this question as fertile as Cyrano with his seven
ways of flying to the moon.

There is a certain kind of inverted action familiarly known as the tail
wagging the dog, and it is on this principle of inversion that my
experiment is designed. Inversion may in some cases be practised without
altering the final result. For instance, it does not much matter whether
the thread goes to the eye (the rational masculine plan) or _vice versa_,
as in the feminine way of threading a needle. In other cases you create
what is practically a new machine by inversion, as in a certain apparatus
in which the hand of a clock stops still while the clock itself rotates.
The effect is still more striking with my plants, for the inversion
practised on them entirely changes the character of their movement.

The result may be shown with the seedling _Setarias_ of which I have
spoken, or with _Sorghum_, as in Fig. 4. If one of these is supported by
its seed with its stem projecting freely in the horizontal plane, the
gravitation stimulus makes it bend upwards until the tip is vertical,
when the stimulus ceases to act and the curvature comes to an end. If
the conditions are reversed, if the seedling is supported in a horizontal
position _by its tip_, while the seed projects freely, the result is at
first the same, though finally it comes to be strikingly different. The
basal end of the seedling is carried upwards by the curvature of the
stem; but according to the theory we are testing, the tip of the seedling
is the only part of the plant which feels the gravitational stimulus, and
the tip of the seedling remains horizontal in spite of the curvature of
the stem. Therefore the tip of the seedling is not freed from
stimulation as it was in the first case, where the curvature brought the
tip into the vertical position. The horizontal tip therefo