14 TWINING PLANTS. Chap. L 



other internodes ; for a line painted on the convex 

 surface first becomes lateral and then concaye ; but, 

 owing to the youth of these terminal internodes, the 

 reversal of the hook is a slower process than that of the 

 revolving movement.* This strongly marked tendency 

 in the young, terminal and flexible internodes, to bend 

 in a greater degree or 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 impor- 

 tant) it causes the extremity of the shoot to embrace 

 the support much more closely than it could otherwise 

 have done, and thus aids in preventing the stem from 

 being blown away during windy weather, as I have 

 many times observed. In Lonicera hraehypoda the 

 hook only straightens itself periodically, and never 

 becomes reversed. I will not assert that the tips of 

 all twining plants when hooked, either reverse them- 

 selves or become periodically straight, in the manner 

 just described ; for the hooked form may in some cases 

 be permanent, and be due to the manner of growth of 

 the species, as with the tips of the shoots of the com- 

 mon vine, and more plainly with those of Cissus dis- 

 color — plants which are not spiral twiners. 



The first purpose of the spontaneous revolving 

 movement, or, more strictly speaking, of the con- 



* The mechanism by which the H. de Vries (ibid. p. 337) : he 

 end of the shoot remains hooked concludes that "it depends on the 

 appears to be a difficult and relation between the rapidity of tor- 

 complex problem, discussed by Dr. sion and the rapidity of nutation ' 



