OUTER cf;lls and tissues. 



29 



with stomata whereby transpiration may be regulated, in order 

 to cope with such conditions as draught ; otherwise certain plants 

 would wither in a few hours. Stomata are usually more 

 numerous on the under surface of a leaf than oa tlie'Tipper 

 aspect, and in some leaves may be greatly re'duced in number, in 

 order <^n prevent ff^css?ivc loss- of water (leaves of plants in the 

 Canary Islands, belonging to the genus Cddus)/' 



Fig. 15. — a, The formation of a Stoma (two stages) in the Leaf of 

 Prunus lauroceraius. b, A Stoma from the Leaf of Pinus, seen in 

 section. — w, Vestibule ; e, epidermal-cells ; jr, guard-cells ; Ay, hypo- 

 dermis ; rpc, respiratory cavity; c, mesopliyll cells. 



The stomata have, however, another and most important 

 function — viz., that of admitting the gases of the atmosphere 



* The stomata also undergo certain changes whereby the aperture, or 

 stoma proper, is closed at times, by variations in the turgidity of the 

 guard-cells ; this occurs at night time. 



