4 6S PHYSIOLOGY 
that the stimulus, instead of finally affecting the side of the stem next 
the earth, as it does in the younger stages of development, now affects 
the flank, determining there more rapid growth. According as the right 
flank or the left grows faster, the tip will be swung like the hands of a 
clock or in the opposite direction. The twining may then be desig- 
nated as clockwise or counter-clockwise (see Part III, fig. 957). There 
is no fundamental reason, apparently, for one direction rather than 
the other. While usually the same species of plant twines always in 
the same fashion, closely allied species will differ in this; there are some 
species that twine indifferently in either direction; and there are a few 
in which the individual plant may change the direction of twining in 
the course of its development. 
Rotation and revolution. When growth of a given flank has swung 
the free tip around, this very act, by twisting the stem on its own axis, 
brings a new segment of the stem into the flank position and so exposes 
it to excitation. 
This may be understood by representing the stem by a hexagonal pencil. If 
the side on which the name is stamped face the right with the pencil horizontal 
and the point away from the body, then this right flank may be imagined to be the 
one whose growth is accelerated; by that the point would be swung to the left, and 
by the time it has passed over 90 the pencil would be rotated on its axis through 
90, so that the stamped side would now, face upward and the angle that was first at 
the bottom would now be the flank. This rotation may be imitated, if it cannot be 
seen to be a mechanical necessity when a horizontal portion of an erect stem is so 
rotated, by sticking the end of a pencil into a piece of rubber tubing just stiff enough 
to bend into a quadrant under its weight. Now upon swinging this apparatus without 
torsion, as can be done by holding the end of the tube and pushing the test pencil 
around with another, the rotation will become at once evident, being complete when 
one revolution is completed. 
The new flank thus brought under the influence of gravity has its rate 
of growth increased, which swings the tip further, rotates the free part 
of the axis, and so brings another segment into the flank position. Given 
the sensitiveness of the flank to gravity, the revolving movement follow? 
as a necessity. 
The support. When a stem is swinging thus, if it come into contact 
with some obstacle near the tip, flexure may carry it past the object ; 
but if it strikes the obstruction further back, the bending may not be 
sufficient to carry the axis' past the obstacle, particularly if it be of moder- 
ate size. Instead, curvature will soon occur in the part projecting 
beyond it, and the revolving movement will be continued by the apical 
