SPIUAL TWI>'EBS. "* 9 



on the convex surface becomes lateral and then concave ; but, 

 owing to the youth of these terminal internodes^ the reversal of 

 the hook is a slower process than the revolving movement. This 

 strongly marked tendency in the young terminal and flexible in- 

 ternodes to bend more abruptly than the other internodes is of 

 service to the plant; for not only does the hook thus formed 

 sometimes serve to catch a support, but (and this seems to be 

 much more imj^ortant) it causes the extremity of the shoot to 

 embrace much more closely its support than it otherwise could 

 have done, and thus aids in preventing the stem from being 

 blown away from it during windy weather, as I have many times 

 observed. In Lonicera hracJiypoda the hook only straightened 

 itself periodically, and never became reversed. I will not assert 

 that the tips of all twining plants, when hooked, move as above 

 described ; for this position may in some cases be due to the 

 manner of growth, as with the bent tips of the shoots of the com- 

 mon vine, and more plainly with those of Cissus discolor; these 

 plants, however, are not spiral twiiiers. 



The purpose of the spontaneous revolving movement, or, more 

 strictly speaking, of the continuous bending movement succes- 

 sively directed to all points of the compass, is, as Mohl has re- 

 marked, obviously in part to favour the shoot finding a support. 

 This is admirably effected by the revolutions carried on night and 

 day, with a wider and >vider circle swept as the shoot increases in 

 length. But as we now understand the nature of the movement, 

 we can see that, when at last the shoot meets with a support, the 

 motion at the point of contact is necessarily arrested, but the free 

 projecting part goes on revolving. Almost immediately another 

 and upper point of the shoot is brought into contact vnth. the sup- 

 port and is arrested ; and so onwards to the extremity of the shoot ; 

 and thus it winds round its support. "When the shoot follows the 

 sun in its revolving course, it winds itself round the support from 

 right to left, the support being supposed to stand in front of the 



beholder ; when the shoot revolves in an opposite direction, the 

 line of winding is reversed. As each internode loses from age its 



power of revolving, it loses its power of spirally twining round a 

 support. If a man swings a rope round his head, and the end hits a 

 stick, it will coil round the stick according to the direction of the 

 swinging rope ; so it is with twining plants, the continued contrac- 

 tion or turgescence of the cells along the free part of the shoot 

 replacing the momentum of each atom of the free end of the rope. 

 All the authors, except Yon Mohl, who have discussed the 



