represents in miniature a forest of vertical stems. Now
suppose the flower-pot upset and left lying on its side for a few hours:
the seedlings will be found to have all recovered the vertical position,
and they have done so by a bend which is just as much a case of movement
as the flexure of a man's arm, though it is effected by a very different
mechanism. Not everyone realises how rapid this movement is. Fig. 2 is
from a diagram made in the ordinary course of class-work at Cambridge,
and illustrates this point. A shoot of Valerian was placed horizontally
at 2.17, and a black line painted like a silhouette on a vertical sheet
of glass to record its position at 2.30; similar lines were painted at
intervals, forming a record of fairly rapid movement. If greater
delicacy of observation had been practised, it would have been easy to
show that the plant begins to curve up within a few minutes of being
placed horizontally.

It is a remarkable fact that the plant should be stimulated, or stirred
up, to a definite curvature by merely placing it horizontally. The
curvature tends to bring the plant into the upright position, and when
the whole stem has reached the vertical, the stimulus ceases to exist.
It is as though the plant were in a condition of content when vertical,
and of discontent in any other position, and as though the discontent
expressed itself in curvature.

But the plant does not gain the vertical by a single continuous
curvature; at first it overdoes the thing (see Fig. 2), and the end of
the shoot may pass beyond the vertical by 20-30 degrees. But this new
position, inasmuch as it is not vertical, originates a new stimulus, and
the new curvature which follows brings the shoot back towards the upright
position. It may again overshoot the mark, but by repeated corrections
it finally attains the normal upright posture.

It is this power of correcting the line of growth whenever it deviates
from the upright that enables the pine tree to grow straight upwards.
And this is what I meant when I said that its habit of growth depends on
regulated curvature, to which no one can refuse the name of movement.

[Picture: Fig. 2.--A Valerian stem curving geotropically]

The pine and the seedling have, in fact, a wonderful kind of
sensitiveness--a sensitiveness to the force of gravity. To those
accustomed to think of Mimosa as the sensitive plant _par excellence_ my
words may sound strange. But the sensiti