306 M. WICHURA ON THE WINDING OP LEAVES. 



account in the winding leaf. For since the weight always 

 draws downward, if it were this that compelled the winding 

 leaf to curve, the latter must, in every half revolution, according 

 as it turned this or that side downward, curve, alternately, at 

 one time to this, at another to that side. It must consequently 

 — see § 31. — turn sometimes one, sometimes the other side to 

 the interior of the helix. But it is just one peculiarity of wind- 

 ing leaves, that they always turn one side to the interior of the 

 helix, no matter how many revolutions the leaf may make. The 

 cause of the curvature can only be sought therefore in the 

 leaf itself, and not in any one-sided force acting upon it from 

 without. 



§127. 



The unequal tension of the margins of a leaf in relation to the 

 axis, and the parts lying next to it, is produced by the revolu- 

 tion around the axis. It lies in the nature of the curve, as a 

 bent line, that the heliacally wound lateral parts of a leaf with 

 a straight axis must have passed over a longer course, and there- 

 fore must be longer, than this axis itself, which travels through 

 about the same distance by a straight path. If a leaf winding 

 round its straight axis is unrolled, the margins are thrown into 

 waves and folds, and thus present clearly to the eye the over- 

 plus of longitudinal development which was expended in the 

 formation of the heliacal curve. But such forms of the leaf are 

 comparatively rare. In the great majority of leaves the borders 

 and axis exhibit a perfect uniform longitudinal development, and 

 no obstacle is offered to their parts being spread out in one plane. 

 Hence it is clear, that, when such a normally formed leaf be- 

 comes affected by revolution of its axis, its first effort must be 

 directed to supply the want of predominant longitudinal develop- 

 ment by elongation of the lateral part of the leaf. The extensi- 

 bility of vegetable fibre is certainly capable of this up to a certain 

 point. Vegetable fibre is, however, at the same time elastic, and 

 thus the force which expands it is met at once by another force, 

 which strives to draw it back again into its former dimensions. 

 Only a part of this form can be taken off by the curvature of the 

 axis, since the two forces do not act in diametrical opposition, 

 but obliquely against each other. Another part of the force 



