THE LEAVES OF VASCULAR PLANTS. 237 



stomata, the appearance of the nerves, the degree of 

 their greenness, and the appearance of their tissue ; we 

 observe this in several herbaceous plants. 



But as the two surfaces present an aspect either very 

 different or very similar, they do not appear less destined 

 to perform a special function, for their natural position 

 cannot be disturbed. If one tries to turn a leaf so as 

 to direct its lower surface upwards and its upper down- 

 wards, it always tends to regain its natural position ; 

 and if it be retained by any mechanical means in this 

 unnatural position, it dies very soon. In trees with 

 naturally drooping branches, the leaves turn round 

 more or less completely to regain their true position. 

 This fact appears to indicate that the return of leaves 

 placed in a position contrary to their natural one, is not 

 simply owing, as might be believed, to their petiole 

 endeavouring to untwist itself by the mechanical effect 

 of elasticity, for here their petiole twists spontaneously 

 to place the two surfaces in the position which is proper 

 for them. There are also some leaves which, without 

 belonging to drooping branches, are naturally placed in 

 a position contrary to that which seems natural ; thus, 

 Meyer has observed this in the limb of the leaves of 

 Festicca trinervata, and some other grasses. The cause 

 of this phenomenon is as yet unknown. 



The edge of the leaf is caused by the commissure of 

 the two cuticles : in flat leaves there is no difficulty in 

 recognising it, but in three-cornered ones, such as those 

 of Mesemhryanthemum acinaciforme, the upper surface 

 is represented by the upper part of the triangle, although 

 it may sometimes be very narrow; the lower angle or 

 the keel represents the longitudinal nerve ; the two lower 

 surfaces are the two sides of the lower surface of the 

 leaf; the upper angles are consequently the commissures 

 of the two surfaces, that is to say, the true edges of the 



