242 LEAVES 



from the fact that the edges of the leaves, as viewed from above, 

 fit together Yike the httle tiles of a real mosaic. The fitting 

 together in this way is the best arrangement for the individual 

 leaves in a large mass to receive light. (Fig. 226.) A general 

 mosaic arrangement of leaves may be observed in connection with 

 almost every broad leaved plant, but is most noticeable in the 

 Ivies where their mosaic of leaves often completely cover the 

 surface of a wall. In case of stems exposed to direct light on 

 only one side, as the horizontal branches of trees, and stems 

 prostrate on the ground or in contact with a support, such as 

 Cucumbers, Melons, and climbing vines, the petioles of those 



Fig. 227. — Maple twig, showing mosaic arrangement of leaves. 



leaves on the under side of the stem usually curve so as to bring 

 the blades to the light. For example, in looking up into a tree 

 in full foliage, one will notice that the horizontal branches are 

 comparatively bare underneath, the leaf blades being displayed 

 on the upper side as a mosaic. {Fig- 227.) 



When plants receive light from only one side, as plants grown 

 in a room near a window, the entire plant usually bends toward 

 the light, thus bringing the leaf blades into a better position for 

 exposure. {Fig. 228.) 



General Structure of Leaves 



Although diverse in form and arrangement, foliage leaves 

 show much uniformity in structure, being so constructed as to be 

 adapted to the function of food-making. In general, they have 



