94 ELEMENTS OF PLANT ANATOMY. 
along the edges, or, as is more frequently the case, they unite, 
forming a continuous nerve near the edge. . 
The general characters of the phanerogamic leaf may be 
stated as follows: the epidermis of the upper surface is nearly, 
if not quite, without stomata; that of the under surface has 
numerous stomata connecting with large air-spaces. The cells 
between the two epidermal layers are divided into palisade and 
spongy tissues ; the former lies next the upper surface, and is 
richly provided with chlorophyll ; the latter lies below, its cells 
containing less chlorophyll, and the bundles of the vascular 
system run through its upper portion. 
Sometimes there is present a hypoderma and frequently 
sclerenchymatic!cells of different shape are scattered irregularly 
through the central portion. Thus the leaf is a dorsiventral as 
well as a bilateral organ. <A striking difference in structure is 
shown in those leaves which for some reason, such as twisting 
of the petiole, infolding of the blade or other departure from 
the ordinary position, develop the palisade tissue on what is 
morphologically the under side, and the spongy tissue above, 
thus exactly reversing the normal order of arrangement. 
In leaves which are more or less cylindrical in shape, or 
which differ greatly from the flattened organs so far described, 
the tissues are inclined to a concentric arrangement, similar to 
that found in the stem. This is true in the so-called pine 
needles and other Conifer leaves. These are mostly designed 
to live for several years instead of a single season, and there- 
fore, as might be expected, the anatomical structure differs 
from that described in the typical leaf. One example may be 
taken from the genus Abies. A cross-section of this leaf shows 
peculiarly thickened epidermal cells, and next these, a closely 
attached hypodermal layer, broken only by the stomata. These 
latter occur at somewhat regular intervals around the whole 
circumference. Next come numerous layers of green paren- 
chymatie or mesophyll cells thickly interspersed with resin 
1 See note on page 125. 
