196 TWISTED INTERNODES. [CH. VII 



termined by light, although the amount of such movement 

 is regulated by the gravitation-stimulus. 



(224) Horizontal branches. 



The horizontal position assumed by some branches is 

 according to Frank ^ due to diageotropism. The following 

 observations are worth making although they leave it 

 undecided whether the horizontal position is due to light- 

 or gravitation-stimulus. 



In the spring the developing buds of the hazel 

 {Corylus), hornbeam (Caiyiniis), elm (Ulmus), and lime 

 (Tilia) are curved so as to point downwards, and as 

 further development proceeds they move up into a hori- 

 zontal position. Select a horizontal branch of one of the 

 above plants in which the terminal bud is directed 

 vertically downwards, and fix the branch vertically up- 

 wards so that the bud is horizontal. It will be found 

 that in this case the curvature of the bud remains 

 unchanged, so that the branch into which it develops is 

 at right angles to the older part of the branch with which 

 it is continuous. 



(225) Torsion of internodes. 



In many plants, as Frank has shown ^ the arrangement 

 of the leaves on the adult horizontal branches is normally 

 and regularly produced by torsion of the internodes. This 

 is the case with Weigelia, Symphoricarpus, Philadel- 

 phus coronaria, and some species of Lonicera. The 

 original decussation of the leaves is plainly seen in the 



1 Frank, Die natilrliche ivagerechte Eichtung, etc., 1870. 



