66 



L. LENORE CONOVER 



attained the horizontal position, placing its formerly lower 

 side uppermost and making a small torsion in doing so. These 

 positions were the final ones for these shoots. 



A series of rather brief tests showed that any flank of the hor- 

 izontal portion of the immature shoot may be made to act as 

 the permanent upper side. As already stated, the side that had 

 been the lower will serve as the upper if the plant is inverted. 

 If, however, the bent portion of the young shoot is brought 

 into the horizontal position, with its lateral branches in the 

 vertical position, the central axis will retain its position, while 

 the lateral branches will bend into the horizontal position. To 

 determine that the main axis in such a test as the last named 

 does not twist so as to bring the lateral branches into the hori- 

 zontal position, a hne of ink was drawn along the upper side 

 of the central axis before the plant was displaced from its nor- 

 mal position. When the plant had regained its equilibrium 

 after displacement, the Line was found still straight. 



III. NUTATION OF TIP OF MAIN .\XIS AND OF LATERAL BRANCHES 



While the young orthotropic shoot is growing upward, there 

 is, especially as the time nears fer the diageotropic curve, a 

 diurnal nutation due to the action of light and of gravitation. 

 During the day, the shoot-tip bends more or less toward the 

 horizontal, controlled by the direction of the strongest light,' 

 and erects itself again to the vertical position as darkness comes 

 on. 



Besides the heliotropic and geotropic nutation there is appar- 

 ently an autonomic nutation of the tip of the main axis and 

 of the lateral branches while the diageotropic position is being 

 assumed. The tips of these various members descend 45 de- 

 grees to 90 degrees below the horizontal and rise again to the 

 horizontal twice or more times a day. The path pursued by 

 one of the tips in these oscillations does not lie in a single plane 

 but varies from a zigzag course to an ellipse. These oscilla- 

 tions often seem to be induced partly by alternating wilting 

 and recovery. The matter needs further study. 



