y,--that is, normally
presents its faces one to the sky, the other to the ground, or when the
leaf is erect the upper face looks toward the stem that bears it, the
lower face away from it. Whenever this is not the case there is
something to be explained.

125. The framework consists of _wood_,--a fibrous and tough material
which runs from the stem through the leaf-stalk, when there is one, in
the form of parallel threads or bundles of fibres; and in the blade
these spread out in a horizontal direction, to form the _ribs_ and
_veins_ of the leaf. The stout main branches of the framework are called
the _Ribs_. When there is only one, as in Fig. 112, 114, or a middle one
decidedly larger than the rest, it is called the _Midrib_. The smaller
divisions are termed _Veins_; and their still smaller subdivisions,
_Veinlets_. The latter subdivide again and again, until they become so
fine that they are invisible to the naked eye. The fibres of which they
are composed are hollow; forming tubes by which the sap is brought into
the leaves and carried to every part.

[Illustration: Fig. 112. Leaf of the Quince: _b_, blade; _p_, petiole;
_st_, stipules.]

126. =Venation= is the name of the mode of veining, that is, of the way
in which the veins are distributed in the blade. This is of two
principal kinds; namely, the _parallel-veined_, and the _netted-veined_.

127. In _Netted-veined_ (also called _Reticulated_) leaves, the veins
branch off from the main rib or ribs, divide into finer and finer
veinlets, and the branches unite with each other to form meshes of
network. That is, they _anastomose_, as anatomists say of the veins and
arteries of the body. The Quince-leaf, in Fig. 112, shows this kind of
veining in a leaf with a single rib. The Maple, Basswood, Plane or
Buttonwood (Fig. 74) show it in leaves of several ribs.

128. In _parallel-veined_ leaves, the whole framework consists of
slender ribs or veins, which run parallel with each other, or nearly so,
from the base to the point of the leaf,--not dividing and subdividing,
nor forming meshes, except by minute cross-veinlets. The leaf of any
grass, or that of the Lily of the Valley (Fig. 113) will furnish a good
illustration. Such parallel veins Linnaeus called _Nerves_, and
parallel-veined leaves are still commonly called _nerved_ leaves, while
those of the other kind are said to be _veined_,--terms which it is
convenient to use, although these "nerves" and "veins" are all t