308 M. WICHURA ON THE WINDING OF LEAVES. 



margin — as seen from below ; and in left-wound leaves with the 

 lower face turned inwards, and also in right- wound leaves with 

 the upper face inwards, the left margin — must be rendered more 

 tense than the other, and this the more, the broader the leaf is in 

 proportion to its length. But the leaves which, as we here pre- 

 suppose, are curved by the pressure arising from the revolution 

 of the axis, cannot, for that very reason, be very broad or very 

 thick, since otherwise they would not give to the certainly very 

 shght pressure acting upon them. The very slight breadth, in 

 proportion to the length, is, as already remarked, a characteristic 

 peculiarity of all winding leaves. The unequal tension of the 

 surfaces and borders of the leaf caused by curled wound move- 

 ment, is consequently really so inconsiderable, that it is doubt- 

 less mostly compensated by the extensibility of vegetable tissue. 



§129. 



The unequal length of the two surfaces, and the curvature of 

 the leaf caused by this, is the effect of a hygroscopic difference 

 of these surfaces, on account of which their relative lengths alter 

 in the drying-up of the leaf: whether this happens through an 

 elongation or shortening of one of the two faces, I must leave 

 undecided, from the want of accurate observations. This hy- 

 groscopic curvature of the leaf acquires especial importance in 

 regard to the revolution of the axis, when this requires a curva- 

 ture of the leaf to bring it to light, not being strong enough in 

 itself to produce this. The heliacal winding of the leaf is thus 

 first made possible through the addition of the hygroscopic cur- 

 vature to the revolution of the axis. Since the former, however, 

 depends upon the desiccation of the leaves, only making its ap- 

 pearance towards the end of the life of the plant, the revolution 

 of the axis in this case makes its first appearance in the latest 

 stages of growth as the last expression of an expiring vitality. 



§ 130. 



Such winding leaf-structures, which are very remarkable, like 

 the awns of the Grasses, the appendages of the carpels of the 

 Geraniece, the carpels of Dorcoceras hygrometrica, Bunge, &c., 

 possess the property of recovering the originally equal length of 



