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 



B.J.S 



D 



FIG. 8 1 . Stomata. A, Holly (Ilex aquifolium) . B, Gorse ( Ulex europceus] . 

 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 mainty 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, 



