STEMS, ROOTS, AND LEAVES 



115 



the epidermis. The remaining cells of the mesophyll are called 

 spongy cells, since they are more loosely arranged than the 

 palisade layer, with large intercellular spaces like the canals 

 and pores of a sponge. This canal system of intercellular spaces 

 within the mesophyll of the leaf contains air, with water vapor 

 and gases. Since the composition of the air within the leaf 

 differs from the external air in the relative amount of water 

 and gases contained in it, we may properly designate it as the 



Epidermis 

 ./*= 



Palisade parenchyma 



Midcein 



Spongy parenchyma rw 



Gtianl cell 



Air 



Guard cells 

 C 



FIG. 62. Microscopic structure of a leaf of the milkweed (Asclepias) 



a, a transverse section of a milkweed leaf; 6, a diagrammatie-drawing illustrating 

 the structure cf the guard cells ; c, the guard cells of the milkweed enlarged 



internal atmosphere of the leaf. The gases and water of this 

 internal atmosphere come in contact with the external atmosphere 

 outside the leaf through the stomata, with which the intercellular 

 spaces of the leaf are directly connected. All of the mesophyll 

 cells are also living cells, in which green chloroplastids are 

 embedded in a living cytoplasmic sac surrounding the large 

 central water vacuole. The chloroplastids are thus placed at 

 the outside of the cell, where they are exposed to sunlight and 

 to the constituents of the internal atmosphere. The importance 

 of these internal arrangements of the leaf will be more fully 

 appreciated when we consider its function in starch manufacture 

 and in the evaporation of water. 



The veins are composed of two kinds of tissue cells; namely, 

 supporting and conducting cells. The thick-walled supporting 



