i6o 



TRANSPIRATION CHECKS 



cells having slightly thickened walls with simple pits and 

 separated by narrow intercellular spaces (Fig. 80). The 

 central region of the cladode is occupied by large water-storing 

 cells (aqueous tissue, Aq., cf. below, p. 166) which are enveloped 

 by the assimilatory tissue. The arrangement of the vascular 

 system is like that of a leaf. 



The development of a vestibule impedes the escape of 

 the water-vapour exhaled from the stoma, and consequently 



B 



D 



FIG. 81. Stomata. A, Holly (Ilex aquifolium). B, Gorse (Ulex euvopaus), 

 C, Brooklime (Veronica beccabunga). D, Yellow Pimpernel (Lysi- 

 machia,' nemorum). 



effects reduced transpiration. Protection is more commonly 

 attained by the location of the stoma at the base of a depression, 

 a condition already noted in the leaf of the Iris (Fig. 46, D, p. 97). 

 More pronounced examples are afforded by many plants with 

 leathery leaves. In the Holly (Fig. 81, A) each stoma is situated at 

 the base of a canal which is formed mainly as a result of the great 

 thickening of the cuticle. Similarly protected stomata are well 

 seen in sections through the stem-spines of the Gorse (Fig. 81, B), 

 where the cuticle also reaches an extreme development. Plants 

 of damp situations offer a marked contrast in these respects, 



