14 



PRACTICAL BOTANY 



most of the carbon dioxide used by the plant. The guard 

 cells press closely together, or they may separate until a cir- 

 cular opening is formed, and in thus closing and opening they 

 influence the interchange of air between the interior and the 

 exterior of the leaf. This obviously affects the interchange of 

 such gases as carbon dioxide and oxygen, as well as the outgo 

 of moisture from the leaf. 



15. Leaves : internal structure. The interior cells of the 

 leaf, except those of the veins, are colored green by chlorophyll, 



FIG. 11. 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 



which means " leaf green." The cells are not uniformly green, 

 but the chlorophyll is contained in special small bodies, 

 known as chloroplasts or chlorophyll bodies (Fig. 11), which 

 are within the cells. It must be clearly understood that the 

 chloroplast and the chlorophyll are not the same. Plastids 

 (plasts) may or may not contain chlorophyll, just as a sponge 

 may or may not contain water. It is evident, therefore, that 

 a plastid can properly be called a chloroplast only when it 

 contains chlorophyll. 



