468 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 oo° 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 follows 

 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 



