420 RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



directed branches of this species. Here one of the pair always appears con- 

 siderably longer than the other; and it is not a rare occurrence for it to be 

 three times as long as its neighbour, as may be seen in figure 109. And why 

 this striking dissimilarity? The reason is again the same as in all the previous 

 cases. If all the leaf -stalks were to retain the same length on the horizontal twigs 

 which they have on the erect branches (see fig. 106), then one of the leaves of 

 every alternate pair would come to be very unfavourably situated in its neighbour's 

 shadow. This detrimental condition must be prevented, and this may be effected 

 most simply by the leaf-stalk increasing in length until the blade it carries is 

 projected beyond the area of the shadow. 



Fig. 110.— Leaf-mosaics of Unsyninietrical Leaves. 

 i Begonia Dregei growing in front of a vertical walL 2 picus scandens, growing on a vertical wall 



It may be expected that alterations of direction, shortenings and elongations, 

 similar to those just described in the case of the horizontal leafy twigs of the lower 

 boughs of trees, shrubs, and bushes, will be found on those plants which are 

 attached to a steep face of rock, a vertical wall, or to the bark of an upright tree- 

 trunk. As a matter of fact all the instances discussed here are again met with in 

 various climbing and twining growths, as well as in those whose stem is parallel to 

 a vertical wall without being attached to it, e.g. as in Bhamnus pumila, and in 

 many begonias. But here the leaf-blades do not place themselves parallel to the 

 ground, but to that surface on which the plants in question are supported, or which 

 they adjoin. In these plants another peculiarity is often observed which it will be 

 most fitting to speak of here, viz. the want of symmetry of the leaves. While in the 

 majority of plants each foliage-leaf is divided by a midrib, running from the apex 

 to the leaf -stalk, into two similar or almost similar halves, in the begonias, many 

 climbing figs, in Celtis occidentalis, elms, and numerous other plants, the two 



