CUTICLE AND STOMATA 



303 



external walls of these cells, in contact with the outer 

 air, consists of a waterproof covering, the cuticle, com- 

 posed of cutin (see p. 284). If the cuticle were perfectly 

 continuous over the whole surface of the leaf it would 

 prevent the interchange of gases between the mesophyll 

 and the outer air, for instance the supply of carbon 

 dioxide which the chloroplasts use as part of the raw 



Fig. 50. — Part of transverse section of dorsiventral foliage leaf : cut., 

 cuticle ; M.e^., upper epidermis ; >»e«., mesophyll ; pal., palisade tissue ; 

 s^., spongy tissue; iwi., intercellular space', U.S., vascular bundle 

 (in longitudinal section, passing out of the plane of section to 

 the right — further to the right a bundle is seen cut in transverse 

 section) ; sh., bundle sheath; x, xylem; ^A., phloem ; l.ep., lower 

 epidermis ; St., stoma. The black arrows indicate the passage 

 of liquid water from the xylem tracheids through the sheath 

 into the mesophyll cells and epidermal cells. The white arrows 

 the evaporation of water from the mesophyll cells into the inter- 

 cellular spaces and diffusion out through the stomata. 



material of photosynthesis. The epidermis, with its 

 cuticle, is in fact pierced by minute holes {stomata), 

 each enclosed by two speciaUsed epidermal cells {guard 

 cells), and these pores lead into the system of intercellular 

 space of the mesophyll (Fig. 50). 



Transpiration. — The pores of the stomata not only 

 allow of the diffusion of carbon dioxide from the air 



