THE ORIENTATION OF FOLIAGE-LEAVES 



259 



commencement of an orienting torsion in the petiole. It is by a torsion of this kind 

 that the young leaves on hanging branches of Fraxinus^ Caragana, Salix, and Betula 

 assume their proper positions. 



If the stern is placed in a horizontal position the lateral leaves perform an 

 epinastic backward curvature, and then by torsion and a forward movement come to 

 face upwards with the lamina parallel or obliquely inclined to the stem. The leaves 

 on the upper side may attain a suitable position by the primary epinastic and geotropic 

 backward curvature, but frequently they do not reach or retain this position, lateral 

 curvatures coupled with torsion bringing them into positions similar to those assumed 

 by the lateral leaves. The same applies to the leaves on the under side. 



It is in this way that the leaves on plagiotropic shoots of Vine a, Glechoma, 

 Lysimachia nummutan'a, Buxus, Acer, and Taxus assume a more or less complete 

 dorsiventral arrangement, whereas on erect shoots they are radially arranged l 



FIG. 48. Euonymus radicans. A t a vertical shoot with decussate leaves. B, a horizontal shoot. 



(Fig. 48). The decussate leaves of Deutzia, Lonicera and Philadelphia, as well as 

 the spirally-arranged ones of Spiraea salicifolia and Kerria japom'ca, are caused to 

 assume an exact two-rowed arrangement in sloping and horizontal shoots by the 

 twisting of the internodes, so that the individual leaves need only twist slightly to place 

 themselves in a horizontal position. This torsion only begins in each internode 

 when that in the precedent one is completed, so that unnecessary torsion is avoided 2 . 

 The torsion is not only produced by gravity, but also in erect shoots by unilateral 

 illumination 3 , and since the leaves then exercise no torsion moment on the stem, it is 



1 Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870, pp. 14, 37, 57, 64. See 

 also the figures in Kerner's Natural History of Plants, 1894, Vol. I, pp. 417-23. 



2 Frank, 1. c., p. 16. 3 Schwendener u. Krabbe, 1. c., p. 320. 



S 3 



