MrNTJTE STRUCTURE OF LEAVES 



153 



In the case of an apple tree, where the epidermis of the 

 lower surface of the leaf contains about 24,000 stomata to 

 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. 



163. 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 

 simpler to state at 

 once that the epi- 

 dermis serves as a 

 mechanical protec- 

 tion to the parts 

 beneath and pre- 

 vents excessive 

 evaporation, that 

 the palisade-cells 

 (which it may not be easy to make out very clearly in a 

 roughly prepared section) hold large quantities of the green 

 coloring matter of the leaf in a position where it can 

 receive enough but not too much sunlight, and 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 



Fig. 118. — A Stoma of Thyme. (Greatly magnified.) 



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

 face Tiew of stoma, ou, cuticle ; g^ guard-cellfi ; 

 «, stoma ; £, epidermal cells ; a, air chamber ; 

 tit cells of spongy parenchyma with grains of 

 chlorophyll. 



