MINUTE STRUCTURE OF LEAVES 



129 



cu 



the square inch, or the black walnut, with nearly 300,000 

 to the square inch, the total number on a tree is incon- 

 ceivably large. 



152. Uses of the Parts examined. — It will be most con- 

 venient to discuss the uses of the parts of the leaf a little 

 later, but it will 

 make matters sim- 

 pler to state at once 

 that the epidermis 

 serves as a mechan- 

 ical protection to 

 the parts beneath 

 and prevents exces- 

 sive evaporation, 

 that the palisade- 

 cells (which may 

 not be made out 

 very clearly in a 

 roughly prepared 

 section) hold large 

 quantities of the 

 green coloring mat- 

 ter of the leaf in a 

 position where it 

 can receive enough 

 but not too much 

 sunlight, and that 

 the cells of the 

 spongy parenchyma share the work of the palisade-cells, 

 besides evaporating much water. The stomata admit air 

 to the interior of the leaf (where the air spaces serve to 

 store and to distribute it), they allow oxygen and carbonic 



Pig. 94. 



A Stoma of Thyme, 

 magnified.) 



(Greatly 



A , section at right angles to surface of leaf ; B, 

 surface Tiew of stoma, cu, cuticle ; g, guard- 

 cells; 5, stoma; e, epidermal cells; a, air 

 chamber ; c, cells of spongy parenchyma Trith 

 grains of chlorophyll. 



