456 



TRANSFORMATION OF ENERGY 



a revolving motion, since the horizontal apex rotates round the fixed vertical 

 basal region like the hand of a clock. This movement continues so long as the 

 shoot remains capable of growth and as a rule is always in one definite direction. 



In the majority of cases the revolution, looked 

 at from above, is opposite in direction to the 

 clock-hand, i.e. to the /^/^ ; the direction of 

 motion in the hop and honeysuckle is, how- 

 ever, on the contrary, to the right, while 

 alternating left and right revolutions have 

 been observed in Bowiea voliibilis and Loasa 

 lateritia. Plants which revolve to the left 

 (Fig. 142) twine round their supports to the 

 left, and those which revolve to the right 

 twine to the right (Fig. 143). Since there is 

 obviously a close connexion between the mode 

 of twining and the revolving motion it will be 

 necessary to study this new type of movement 

 somewhat more closely. 



The rotatory movement may be most 

 easily explained by means of a simple model. 

 Fasten a thick-walled caoutchouc tube by 

 its base to a vertical peg, and place in its 

 apex a piece of lead tubing sufficiently heavy 

 to bend the tube over into the horizontal 

 position, and rotate with the hand the apex 

 of the tube in the reverse direction to that of 

 the clock-hand (Fig. 144). If a small indi- 

 cator, say a needle, be fastened to the end 

 of the lead piping, pointing downward at 

 the commencement of the movement, we 

 find that this needle points to the left after 

 a quarter of a revolution (looked at from 

 without), after half a revolution it points 

 upwards, and so on ; but one also sees that 

 the torsion of the horizontal part on its long 

 axis, as seen from without, is in the direction 

 of the clock-hand. One may easily convince 

 oneself of the fact that the torsion is in the 

 reverse direction to the rotation by drawing 

 a line on the long axis of the tube which is seen 

 to revolve round the horizontal part of the 

 tube. The advancing surface of the tube thus 

 changes moment by moment as the revolution 

 proceeds. If we desire the movement to 

 be carried out in such a way that the same 

 surface always lies in front, we must hold the 

 apex of the tube firmly during the revolution, 

 but if one revolution be completed under 

 such conditions a torsion ensues in the lower 

 end of the tube, a torsion which is immedi- 

 ately undone as soon as the end is released, 

 in consequence of which there develops at 

 once the same torsion in the apical region as we have previously noted during 

 the complete revolution. If now we mark on the horizontal and the bent region 

 of a shoot of Calystegia one definite surface, by painting a line on it with indian- 

 ink, we may note that, just as in the model, this line twists round in the 



Fig. 142. Fig. 143. 



Fig. 142. Left-handed twining shoot of 

 Pharbitis. Fig. 143. Right-handed twining 

 shoot of Myrsip/tyl/utn asparagoides. From 

 the Bonn Textbook. 



/ 



V 





Fig. 144. Model to illustrate twining. 



