stomata 



97 



Uniformly thickened, ttiose remote from the pore being relatively 

 thin, whilst those towards the aperture show a marked decrease 

 in thickness opposite the middle of the pore {e.g. Onion, Fig. 

 46, E) ; in such cases the outer ridges are well marked, and 

 there is frequently a second pair of ridges on the inner side. 

 As a result of this unequal distribution of the thickening, the 

 cavities of the guard-cells taper abruptly in the direction of the 



Fig. 46. — Structure of stomata. A, C, F, and G, in surface view ; B. D, 

 and E, in transverse section. A, B, Jlillet-Grass (MilUum effiisum). 

 C, D, Iris gennanica. E, Onion (Allium cepaj. F, JIadder (Riibia 

 peregrina). G, Sediitn spectabile. cii., cuticle ; ep., epidermal cell ; 

 g.c, guard-cells ; s.c, subsidiary cells. 



pore, so that a ma.ximum thickness is obtained towards the 

 upper and lower sides (Fig. 46, E) ; in some plants this is so 

 marked that the whole ca^dty merely appears as a slit {e.g. Grass, 

 Fig. 46, B). Beneath the stoma lies an air-space, the respiratory 

 cavity (Fig. 51, R.), serving for the immediate interchange of gases 

 and water-vapour between the intercellular system of the plant 

 and the air around. 



The vertical leaf of the Iris, in which both surfaces arc alike, 

 bears almost the same number of stomata on each. ■ Moreover, 



7 



