THJi] WOMK OF LEA VES 197 



dermis is then seen to be made up of a series of cells, 

 all lying in one plane, with their edges closely joined 

 except at certain points where openings exist (Figs. 

 113 and 114, s). Each opening (called a stoma, plural 

 stomata) is surrounded by crescent-shaped cells called 

 the guard-cells (r/). The openings are filled with air, 

 giving them a dark appearance. Compare the num- 

 ber of openings on the upper and the lower surfaces. 

 Does it correspond with what you have already learned 

 by means of the air-pump! 



Now let us examine the interior of the leaf.^ For 

 this purpose roll a leaf tightly, hold the roll firmly be- 

 tween the thumb and forefinger of the left hand, and 

 with a sharp razor, moistened in water, cut the thin- 

 nest possible slices across the leaf; the slices should 

 be so thin as to appear almost or quite colorless. 

 Transfer some of these with a wet camel's -hair brush 

 to a drop of water on a slide, cover and examine as 

 before. The epidermis of the upper surface (Fig. 113, 

 e) appears as a nearly colorless row of cells. The 

 function of the epidermis is to protect the leaf from 

 drying up, as well as against the attacks of parasites, 

 insect enemies, etc. For this reason its outer wall 

 (cuticle, Fig. 114, c) is thickened and made water-proof 

 (or almost so). Below the epidermis appear long, 



^ The leaf of the common Yellow Mustard is chosen here because it is easy 

 to obtain and easily prepared, while showing the parts of the leaf with great 

 distinctness: leaves of Beet, Beech, etc., are excellent: any thin, tender leaf 

 may be used. 



