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 (palisade tissue) or are 



sp 





a.c' 



FIG. 26. Cross section of a geranium leaf 



a, air space; a.c, air chamber; e, 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 layer 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 masses of small, 

 thick-walled cells closely crowded together and usually lying 

 about midway between the upper and lower surfaces of the leaf. 



