LEAVES 15 



in which all the leaves are referred to a single plane, shows 

 the mosaic. In many trees in dense forests, notably in 

 the tropics, the leaves appear chiefly and sometimes ex- 

 clusively at the extremities of the branches, often producing 

 a magnificent dome-like mosaic. 



In the case of stem- exposed to direct light only on one 

 side, as the horizontal branches of trees, stems prostrate 

 on the ground, and stems against a support (as climbers 

 and twiners), the leaf-i>lades must be brought to the light 

 side so far as possible, and those that belong to the shaded 

 side must be fitted into the spaces left by those that belong 

 to the illuminated side. This is brought about in various 

 ways, as by the twisting of the stem, the twisting and 

 elongation of the petiole, the bending of the blade on the 

 petiole, etc. Looking up into a tree in full foliage, one 

 will notice that the horizontal branches are comparatively 

 bare beneath, the leaf-blades being displayed on the upper 

 side as a mosaic. The most complete leaf-mosaic is shown 

 by certain ivies, involving such an amount of twisting, dis- 

 placement, elongation of petioles, etc., as to give ample 

 evidence of the importance of securing for leaves an ex- 

 posure to light (Fig. 10). 



13. Structure. Before considering the work of the leaf 

 it will be necessary to know something of its minute 

 structure. To see this structure, not merely surface views 

 must be obtained, but also good clear sections through the 

 leaf (cross-sections) must be made; and for this purpose a 

 relatively thick spongy leaf, like that of the hyacinth or 

 the lily, gives the least trouble. 



(1) Epidermis. It is possible to peel off from the sur- 

 face of such a leaf a delicate transparent skin (epidermis}. 

 This epidermis completely covers the leaf, and generally 

 shows no green color. Examined under the compound 

 microscope it is seen to be made up of small units of 

 structure known as cells (Fig. 11). Each cell is bounded 



