48 
THE LEAF 
water, thus preventing excessive evaporation from the cells. 
Usually the cuticle is thicker, and the stomata fewer on the 
upper side of the leaf, while the cells of the upper mesophyll 
usually stand with their long axes vertical, forming a palisade 
tissue ; this is most marked in leaves exposed to sunshine. 
In the leaf the one or few vascular bundles that enter it 
from the stem commonly branch out a great deal, forming 
the veins or nerves . These, as may be seen in a skeletonised 
leaf picked up in winter, branch and rejoin ( anastomose ) 
repeatedly, thus forming a fine network all over the leaf. 
In the meshes of the network the final endings of the 
bundles may be seen as little blind branches. These run 
among the green cells and consist even at their smallest of 
a xylem and a phloem portion ; the former supplies water to 
the assimilating cells, the latter carries away the products 
of assimilation to other parts of the plant. Except in the 
finer ramifications they are usually accompanied by a certain 
number of fibres whose function is primarily mechanical, the 
bundles being rendered elastic and comparatively rigid by 
their presence, so that the thin and delicate green parts 
of the leaf are stretched out smoothly and are less liable to 
injury by tearing. 
The structure of a leaf is admirably suited to the per- 
formance of the great functions of assimilation and transpi- 
ration, the maximum of cell-surface being exposed to the air 
with the minimum of waste or of mutual interference of 
parts. Such, or something like it, is the structure of most 
leaves growing in sunny places with a plentiful supply of 
water from the roots. In very dry climates the transpiration 
would be too great for the water supply and the structure 
has to be modified to suit the new conditions (see Xero- 
phytes, Chap. III.). Similar considerations explain the 
other modifications of structure found in water-plants, &c. 
Storage of reserve-materials is only performed to a slight 
extent by such a leaf ; the rapidity of assimilation during the 
day causes a temporary excess of the products, which are 
stored in the cells till night, when they are carried away. In 
many plants the systematic storage, for long periods, of large 
quantities of stuff, is undertaken by the leaves, which ex- 
hibit a more or less fleshy or succulent character (see Chap. 
III., Herbs, Xerophytes, &c.). 
