CH. VIl] DIAHELIOTROPISM. 185 



Syringa vulgaris, and especially the shrubby Veronicas 

 such as V. traversi and salicifolia. The drawing, fig. 35, 

 represents the position of the leaves of the latter species, 

 the light being supposed to fall obliquely from the left. 

 It will be seen that the leaf T which points towards 

 the lighted side has a curve in its petiole which brings 

 the surface of the leaf at right angles to the light; 

 the leaf F is also at right angles to the light, but 

 it is brought into that position by the petiole being 

 above instead of below the horizon. The leaf 0, 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. 

 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 

 being tied to an upright stick so that no curvatures, 

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

 by the leaves with the horizon should be noted, and 

 should again be measured after a few^ days. 



(217) The movements due to specific sensitiveness. 



It was at one time believed that the diaheliotropic 

 position was simply the result of a balance struck between 

 such opposing tendencies as apheliotropism, apogeotropism, 

 epinasty, &c. &c., and that diaheliotropism as a specific 

 form of sensitiveness was non-existent. This view has 

 now given way to the belief that a leaf in placing itself 



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

 drawing clearer. 



