66 



THE VEGETATIVE ORGANS 



shaped cells joined end to end, leaving a pore or mouth 

 between them (Fig. 29, 1 and 4 s). These openings are 

 called stomata (sing., stoma, Gr. stoma = the mouth), and 

 their function is to communicate between the interior of 

 the leaf and the air outside. On the upper skin (Fig. 29, 2), 

 however, these are almost or entirely absent. 



Fig. 29. Structure of a Box Leaf. i, part of lower epidermis ; 

 2, part of upper epidermis ; 3, cross-section of Box leaf ; 4, part 

 of 3 at the point u, I, highly magnified ; 5, a cell of the mesophyll ; 

 a, air-chamber; b, bast; c, cuticle of epidermis; ch, chloroplast ; 

 g, guard cells ; I, lower surface ; l.e, lower epidermis ; n, nucleus ; 

 P, protoplasm ; P.t, palisade tissue; s, stoma ; sp, spongy tissue ; 

 u, upper surface ; v, vein ; w, wood. 



A thin transverse section of the leaf should be examined 

 with a lens or microscope and the details shown in Fig. 29, 

 3 and 4, identified. The green tissue between the two skins 

 is seen to be arranged in two distinct layers ; the upper 

 one, called the palisade tissue, consists of perpendicular 

 elongated cells, closely packed together and attached above 

 to the upper skin (Fig. 2g,4p.t). The cells below, known 

 as the spongy tissue, are loosely arranged, leaving large 



