ransitory thing, ever abandoning the _old_, and
renewed in the _young_.

Sec. 4. ANATOMY OF LEAVES.

439. The wood in leaves is the framework of ribs, veins, and veinlets
(125), serving not only to strengthen them, but also to bring in the
sap, and to distribute it throughout every part. The cellular portion is
the green pulp, and is nearly the same as the green layer of the bark.
So that the leaf may properly enough be regarded as a sort of expansion
of the fibrous and green layers of the bark. It has no proper corky
layer; but the whole is covered by a transparent skin or _epidermis_,
resembling that of the stem.

440. The cells of the leaf are of various forms, rarely so compact as to
form a close cellular tissue, usually loosely arranged, at least in the
lower part, so as to give copious intervening spaces or air passages,
communicating throughout the whole interior (Fig. 443, 483). The green
color is given by the chlorophyll (417), seen through the very
transparent walls of the cells and through the translucent epidermis of
the leaf.

[Illustration: Fig. 483. Magnified section of a leaf of White Lily, to
exhibit the cellular structure, both of upper and lower stratum, the
air-passages of the lower, and the epidermis or skin, in section, also a
little of that of the lower face, with some of its stomates.]

441. In ordinary leaves, having an upper and under surface, the green
cells form two distinct strata, of different arrangement. Those of the
upper stratum are oblong or cylindrical, and stand endwise to the
surface of the leaf, usually close together, leaving hardly any vacant
spaces; those of the lower are commonly irregular in shape, most of them
with their longer diameter parallel to the face of the leaf, and are
very loosely arranged, leaving many and wide air-chambers. The green
color of the lower is therefore diluted, and paler than that of the
upper face of the leaf. The upper part of the leaf is so constructed as
to bear the direct action of the sunshine; the lower so as to afford
freer circulation of air, and to facilitate transpiration. It
communicates more directly than the upper with the external air by means
of _Stomates_.

442. =The Epidermis= or skin of leaves and all young shoots is best seen
in the foliage. It may readily be stripped off from the surface of a
Lily-leaf, and still more so from more fleshy and soft leaves, such as
those of Houseleek. The epidermis is usually composed of a s