RELATION BETWEEN POSITION AND FOUM OF GREEN LEAVES. 419 



borne by them, certiiinly 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 tlio liorizontal branches occurs, 

 somewhat more often in low semi-sliruhs and heilis, than in trees and slirubs, 

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

 as in some species of speedwell (Veronica officinalis and Chavicedrys), and in 

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

 (Helianthe'mum grandiflorum), an erect branch of \vhich is illustrated in fig. 107 \ 



Fig. 109. — Leafy Twig projecting Interally from the Stem of tne Norway Maple (Acer platanoides). 



tlie leaves are arranged in pairs and placed crosswise, so that they occur ou 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, howevei', consisting 

 of a smaller number than the two side rows. 



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

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



