RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



419 



borne by them, certainly assist in the completion of this peculiar arrangement of 

 the leaves; the above illustration will demonstrate other particulars far better than 

 the most detailed description. 



The elevation of individual leaf-stalks above the horizontal branches occurs, 

 somewhat more often in low semi-shrubs and herbs, than in trees and shrubs, 

 whose shoots, furnished with decussate leaves, come to lie flat on the ground, 

 as in some species of speedwell (Ve^^onica officinalis and Chamcedrys), and in 

 many species of Rock Rose (Helianthemwm). In the large-flowered Rock Rose 

 {Helianthemum grandiflorum), an erect branch of which is illustrated in fig. 107 ^ 



Fig. 109. — Leafy Twig projecting laterally from the Stem of tae Norway Maple {Acer platanoides). 



the leaves are arranged in pairs and placed crosswise, so that they occur on the 

 stem in four rows. If such a shoot bends down over the ground, a slight twisting 

 of the leaf -stalks occurs first of all, so that their leaf-blades come to lie parallel to 

 the soil; but another alteration is yet to be noticed. In every alternate pair of 

 leaves one of the leaf -stalks rises up, and its blade is bent down almost at a right 

 angle and lies above the horizontal stem as shown in fig. 107 ^. In consequence of 

 this alteration of position the leaves no longer form four rows as on the erect 

 shoots, nor two as. in Diervilla, but three rows, the middle one, however, consisting 

 of a smaller number than the two side rows. 



The fourth case, which still remains to be discussed, is the increase in length of 

 individual leaf-stalks. It may be very well seen in maple-trees, especially in the 

 Norway Maple {Acer platanoides), and this species will therefore serve us as an 

 example. Fig. 106 shows an erect branch of this maple. The stalks of every pair 

 of opposite leaves are of equal length on the erect branch. But how entirely 

 diiFerent in respect to length are those leaf-stalks which embellish the horizontally- 



