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 stems 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-blades 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, ete. 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 sec 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 
