190 



XV. THE LEAF VASCULAR BUNDLE ENDINGS. 



of the epidermis by means of surface-sections. These can be 

 obtained by laying a leaflet flat over the index-finger of the left 

 hand, and held down at its two extreme edges by the thumb and 

 second finger ; or upon a piece of bottle cork one portion of the 

 curved surface of which has been cut off to somewhat flatten it. 

 We readily note that the upper side (Fig. 75, A) has no, or at the 

 most but few, stomata ; while these, on the other hand, are 

 numerous on the under side (B). Elongated stomatic pits, filled 

 with air, lead up to the stomata. Above the glands, as can be 

 determined upon either upper or under epidermis, lie usually four 

 cells (A, sc), which form the middle of a shallow depression. In 

 thicker parts of the section, where the glands are not opened by 



FIG. 75. Epidermis and underlying tissue of the leaf of Rnta graveolens. A , epi- 

 dermis of the upper side ; sc, epidermal cells over an internal gland ; _/?, palisade cells ; 

 B, epidermis of the under side ; s, spongy parenchyma. In A the intercellular spaces, 

 filled with air, are shaded ; in B they are left clear (x 240). 



the razor, we can see in them a highly refractive yellow drop. 

 With still deeper focussing we can determine that immediately under 

 the epidermis of the upper side lies a green tissue of cells, which 

 appear round in optical section (A, p). These cells are almost 

 completely separated from one another laterally, and the inter- 

 cellular spaces are filled with air. Below the under epidermis 

 are situated cells, likewise green and rounded in optical section, 

 but in much smaller number (B, s). These cells, also, are sepa- 

 rated by air-spaces, which, under the stomata, take the form of 

 specially wide air-chambers the respiratory, aerating or venti- 

 lating chambers. 



After obtaining this general information as to the surface 



