HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC, 121 



148. Structure of Stomata and Guard-cells. Since 

 the stomata of various Monocotyledons are very large, 

 transverse sections and strips of epidermis should be 

 taken from the leaves of Lily, Tulip, Narcissus, or 

 Hyacinth. 



(1) In surface view, note that each stoma in these 

 plants has a definite position, being intercalated between, 

 the ends of two of the elongated ordinary epidermis cells. 

 Focus down on a stoma with high power, and note that 

 the opening, which is flush with the surface of the leaf, 

 narrows downwards like a funnel and then opens out 

 again below. 



(2) In section, note that the wall of each guard- cell is 

 thin where it adjoins the other epidermal cells, while on 

 the side adjoining the pore it is thickened (except at the 

 middle where the pore is narrowed) and is produced into a 

 ridge above and below these ridges, with the projection 

 that narrows the pore at the middle, divide the pore into 

 an outer and an inner chamber. 



149. The development of stomata is readily followed in the 

 young leaves of Hyacinth or Narcissus. Dissect a resting-bulb or 

 one just beginning to sprout, and from the young leaves tear off 

 strips of epidermis, or cut tangential sections, at different points 

 of the leaf. Starting from the base and working up to the apex 

 of the young leaf, note that the cells are in longitudinal rows 

 and differ in size, some being elongated and others short and square ; 

 further up, each short cell divides by a longitudinal wall into 

 two cells (guard-cells) ; this median wall then becomes thickened 

 and finally splits to form the pore, while the guard-cells curve out- 

 wards on either side. 



150. Pall of the Leaf. The formation of the absciss- 

 layer, by which the fall of the leaf is effected in autumn, 

 and the cork which closes over the stem and forms the 

 leaf-scar, may be studied in various deciduous trees, e.g. 

 Horse Chestnut or Sycamore. In autumn cut across the 

 base of the petiole of a leaf which has changed colour ; 

 after cutting across the stem above and below, split the 

 whole in halves by a longitudinal cut passing through 

 petiole base and stem, and prepare longitudinal sections 

 as shown in Fig. 33. 



