M. WICHURA ON THE WINDING OF LEAVES. 309 



their two surfaces, in contact with moisture. The curvature is 

 thus removed, and in the same proportion as the leaf stretches 

 itself straight, the heliacal winding only possible under the hy- 

 pothesis of a certain degree of curvature, is unrolled. As soon as 

 the leaf dries and again curls, under the influence of the again in- 

 creasing difference of length of its two surfaces, the heliacal wind- 

 ing is restored. The whole of this movement depends therefore 

 upon a play of mere mechanical forms, totally distinct from that 

 originally causing the revolution of the axis. They nevertheless 

 have been confounded in many cases. The heliacal winding of 

 awns, the fruit-stalks of Mosses, &c., has been generally looked 

 upon as an effect of their hygroscopic nature, without separating 

 the two different movements contained in it, and the mechanical 

 explanation of the whole process following as a consequence of 

 this view, may have contributed to have drawn away the atten- 

 tion of botanists from the striking physiological phaenomenon of 

 revolution of the axis, which is exhibited so clearly in these very 

 structures. 



XV. Causes of the Revolution of the Axis. 

 §131. 

 To cause the revolution of the axis, w^e must imagine a force 

 circulating in a direction perpendicular to the longitudinal di- 

 rection of growth. It is an immediate revelation of the vital 

 force acting in the interior of the plant, and therefore stands in 

 the closest connexion with the growth as its regular expression. 

 Such a connexion may be detected in some cases by the cir- 

 cumstance, that in the plants mentioned in § 45, the lateral 

 direction in which solitary leaves follow one another in a spiral 

 line, acts in determining the direction of the heliacal winding of 

 the leaf, in other cases it becomes observable in the remarkable 

 inverse proportion between transverse growth and revolution of 

 the axis, through which leaves in which the growth in breadth 

 has become developed in the angle-nerved venation, exhibit no 

 revolution of the axis, while parallel-nerved leaves in which 

 growth in breadth is deficient, develope that force rotating at 

 right angles to the direction of longitudinal growth, requisite for 

 the revolution of the axis. That this inverse proportion between 

 growth in breadth and revolution actually does exist, and that 

 the parallel- nerved leaves do not exclusively wind, merely 



