36 INTRODUCTION TO BOTANY 
34. Internal structure of the leaf. An idea of the inner 
structure is best obtained by examining a cross section of a 
simple leaf. In general, three kinds of cells appear within the 
epidermis, and these are variously arranged in different kinds 
of plants. In simple leaves, such as that shown in figure 26, 
almost all the cells contain the green coloring matter, chloro- 
phyll (meaning “ leaf green ”). These chlorophyll-bearing cells 
are long and are arranged side by side (palsade tissue) or are 
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Fic. 26. Cross section of a geranium leaf 
a, air space; a.c, air chamber; ¢, upper epidermis; e’, lower epidermis; p, pali- 
sade cells; s, stoma; sp, spongy parenchyma (usually spongy parenchyma has 
fewer chloroplasts than the palisade tissue); v, vein. Magnified 150 times. After 
drawing by Mrs. F. E. Clements 
more irregular in form and loosely arranged (spongy tissue). 
Air spaces are abundant between these cells, but they are 
larger and more abundant in the spongy tissue than in the 
palisade tissue. When a single Jayer of palisade tissue and 
one of spongy tissue are present, the palisade tissue lies next 
to the upper surface of the leaf and the spongy tissue next to 
the lower surface; but frequently there is a lower layer of 
palisade tissue, not so well developed as the upper one. In 
cross sections of leaves the veins appear as masscs of small, 
thick-walled cells closely crowded together and usually lying 
about midway between the upper and lower surfaces of the leaf. 
