314 , BOTANY part i 



parallel to itself round the axis of the ^^encil. If at the comnienceiiient of the 

 revolving movement it was at the lower side of tlie pencil, it will be found after a 

 quarter of the circle has been moved through to be on the left side (as seen from 

 the outside), and after half the circle has been traversed to be on the upper side. 

 The rotation of the line thus proceeds (as seen from the outside) in the direction of 

 the hands of a watch and thus in the opposite direction to the revolving move- 

 ment. If, however, the rotating movement of the pencil, as the revolving move- 

 ment proceeds, is hindered by holding the end of the pencil, a torsion will arise at 

 the lower portion of the rubber tube. If after a time the end of the pencil is 

 liberated it undergoes the rotation tliat has been prevented, and with this the 

 torsion again disappears. If we now mark with Indian ink an angle of the 

 horizontal terminal portion of the shoot of Calystegia, this line will be seen like the 

 line in the model to move in the direction of the hands of a Avatch as the revolving 

 movement of the shoot proceeds. "When the end of the shoot has completed a 

 revolving movement against the hands of the watch of 360°, the line has also 

 moved through 360° in the direction of the hands of the watch. In other words, 

 after a complete circular movement everything is as before and there is no torsion 

 in the vertical portion of the shoot. 



The revolving movement was for long regarded as a purely autonomic move- 

 ment resulting from internal causes. In practice it is found that it only occurs 

 under the influence of gravity and ceases completely on the klinostat. Some authors 

 regard it as a geotropic movement. Noll has arrived at the following view. In 

 this case it is not an upper or lower side that is hindered in growth by terrestrial 

 gravity, but a lateral flank ; in the sinistrorse plants it is the right side (the apex 

 being looked at from above) which grows more strongly than the opposite side. 

 This "lateral geotropic" reaction does not occur in the whole shoot of the twining 

 plant ; on the contrary it is limited to the transition region between the erect 

 portion and the horizontal portion of the end of the shoot. The part that has 

 become erect is simply negatively geotropic, the horizontal portion is diageotropic. 

 If now the right flank of the curved portion of the shoot is accelerated in its 

 growth by gravity the horizontal end will begin to move in a circle. In doing 

 this, however, it must, in order to avoid a basal torsion, rotate around its axis, and 

 thus another angle will be brought to occupj' the right flank. The geotropic 

 acceleration of growth will tlius involve the several angles of the stem as they 

 successively come to occupy the right flank. 



This view of the influence of gravity is not, however, accepted on all sides. 

 Pfeffer, for instance, regards the revolving movement as an autonomic move- 

 ment, and only regards gravity as having the significance of a general condition of 

 the movement, such as heat, water, light, etc., are. 



The commencement of the revolving movement does not by itself 

 determine a twining movement. This only begins when the shoot 

 meets a more or less vertical and not too thick support. This is 

 enclosed in loose and at first very horizontal spirals which gradually 

 become more erect and steeper. The straightening results from 

 negative geotropism and leads under otherwise favourable conditions 

 Avhen the support is subsequently removed to a complete obliteration 

 of the spiral coils, the straightened stem appearing twisted. If the 

 support is not removed it leads to tightening of the spiral and 

 increased pressure on the support. The twining movement thus 



