MORPHOLOGY 



103 



GUARD-CELLS (Figs. 110, 111 A). The guard-cells always contain 

 chloroplasts, and are also characterised by their peculiarly thickened 

 walls. These, as is best seen in transverse sections, form ridge-like 

 protuberances projecting above and below from the sides of the 



Fid. 111. Epidermis from the under side of a leaf of Tradescantia virginiai. A, In surface view ; 

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

 (X 240.) 



guard-cells adjoining the air-passage (Fig. Ill B). Midway between 

 the projecting ridges, on the other hand, the walls of the guard-cells 

 remain unthickened (Fig. 112). 



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

 Ill , 112), and thus facilitate its closing. In addition, the external thickened 



FIG. 112. Transverse section of the epidermis of Aloe nigricans. i, Inner, uncutinised thickening 



layer, (x 240.) 



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. 

 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 



