SKCT. I 



MORPHOLOGY 



113 



perception of light. Cells of an epidermis which is smooth on the 

 surface may have their inner walls convex towards the interior of the 

 leaf. In a few cases the epidermis presents arrangements which 

 Haberlandt has termed ocelli and regards as localised organs for the 

 perception of light ; these may be either groups of cells with smooth 

 outer walls or single large cells with projecting outer walls and highly 

 refractive contents. 



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 consists of an intercellular passage perforating 

 the epidermis and bounded by two elliptical epidermal cells, termed 

 GUARD-CELLS (Figs. 120, 121 A). The guard-cells always contain 

 chloroplasts, and are also characterised by their peculiarly thickened 



Fig. 121. — Epidermis from the under side of a leaf of Tradcscantia virglnlm A, In surface view : 

 B, in transverse section ; I, colourless rudiments of clironiatophores surrounding the nucleus. 

 (X 240.) 



walls, Avhich stand in relation to the changes of shape by which the 

 functions of the stoma are carried out. These, as is best seen in 

 transverse sections, form ridge-like protuberances projecting above 

 and below from the sides of the guard-cells adjoining the air-passage 

 (Fig. 121 B). Midway between the projecting ridges, on the other 

 hand, the walls of the guard-cells remain unthickened (Fig. 122). 



The unthickened parts of the walls of the guard-cells jut out into the pore (Figs. 

 121 B, 122), and thus facilitate its closing. In addition, the external thickened 

 walls of the two adjacent epidermal cells become, in some cases, suddenly thin on 

 approaching the guard-cells. By this means a hinge-like connection is formed 

 which renders the guard-cells more or less independent of the other epidermal 

 cells (Fig. 122). At other times the same result is accomplished by raising the 

 stomata above the epidermis, or, more frequently, by sinking them below the less 

 thickened epidermal walls. The elevation serves to increase the evaporation, and 

 is on this account met with in Ferns which grow in damp situations. The de- 

 pression diminishes the transpiration by creating a still atmosphere above the 

 guard-cells. On this account it is met with in plants of dry regions (xerophytes). 

 Frequently the epidermal cells ac'joining the guard-cells are less thickened or 



