MORPHOLOGY 



strictly carried out, and the epidermal cells contain chloroplasts. The 

 epidermis of Impatiens parriflora (Fig. 102) has tolerably large but 

 only slightly green chromatophores, and thus occupies an intermediate 

 position between the two extremes. The cell sap of epidermal cells is 

 often coloured red ; in many cases it has been demonstrated that 

 plants thus acquire a protection from excessive illumination. 



The formation of stomata in the epidermis is characteristic of all 

 parts of the more highly-developed plants which are exposed to the 

 air. Each stoma thus forms an intercellular passage perforating the 

 epidermis and bounded by two elliptical epidermal cells, termed GUARD- 

 CELLS (Figs. 103 A, 104 A). The guard cells always contain chloro- 



Fio. 103.— Epidermis from the under side of a leaf of Iris florentina. A, In surface view; B, in 

 transverse section ; /, vestibule ; s, opening ; c, cuticle ; a, respiratory cavity. (X 240.) 



plasts, and are also characterised by their peculiarly thickened walls, 

 which form ridge-like protuberances projecting above and below from 

 the sides of the guard-cells adjoining the air-passage (Figs. 103 B, 

 104 B). Midway between the projecting ridges, on the other hand, 

 the walls of the guard-cells remain unthickened (Fig. 105). 



The guard-cells themselves jut out into the air-passage (Figs. 103 B, 104 B, 

 105), and thus facilitate its closing. In addition, the external thickened walls of 

 the two adjacent epidermal cells become, in some cases, suddenly narrowed on 

 approaching the guard-cells (Figs. 103 B, 105). By this means a hinge-like con- 

 nection is formed which renders the guard-cells independent of the other epidermal 

 cells. At other times this same result is accomplished by raising the stomata 

 above the epidermis, or, which has the same effect, by sinking them below the 



