CH. VIl] 



DIAHELIOTEOPISM. 



169 



it is brought into that position by the petiole being 

 above instead of below the horizon. The leaf which 

 points towards the observer^ is twisted on its petiole and 

 thus reaches the position of maximum illumination by a 

 third movement differing from' those of either T or F. 



Fig. 33. Exp. 216. 



To observe the occurrence of the above movements it is 

 only necessary to transplant a Lamium, or to fix a twig of 

 Veronica in a bottle of water, the stem in either case 

 must be tied to an upright stick so that no curvatures, 

 except those of the leaves, may occur. The angles made 



^ In the figure the leaf opposite to has been removed to make the 

 drawing clearer. 



