94 



EPIDERMIS 



In plants of dry habitats the epidermal cells may attain a 

 considerable size {e.g. in the Sea Purslane, Arenaria peploides, 



and in the Prickly 

 Saltwort, Salsola 

 kali), and serve for 

 the storage of water, 

 which is possibly 

 alwaj/s a function of 

 this layer, though 

 here to a much 

 greater extent than 

 is normally the rule. 

 In extreme cases 

 water may be stored 

 in localised enlarge- 

 ments, which often 

 project as water- 

 containing hairs or 

 bladders {e.g. the Ice- 

 plant, Mesembryan- 

 themum crystallinum, 

 and the Silver Goose- 

 foot, Obione portnla- 

 coidcs, Fig. 44, A, //..). 

 In times of drought 

 the adjacent assimi- 

 latory tissues with- 

 draw moisture from 

 these cells, which 

 consequently con- 

 tract, the side walls 

 becoming undulated ; 

 when water again be- 

 comes plentiful, the 

 cells fill and the walls 

 graduallj' straighten 



Fig. 44. — Structure of the epidermis. A, 

 Transverse section of leaf of Silver Goose- 

 foot (Obione portulacoides) showing the 

 bladder-like hairs [h.), the aqueous tissue 

 (a.), and the palisade layer (p.) B, Sur- 

 face section of leaf of Ground Ivy (Nepeia 

 glechoma). C, Surface section of leaf of 

 Onion. D, Surface section of stem of 

 Runner Bean [Phaseolus innltifioriis). 

 St., stomata. 



out. The epidermis 



is not uncommonly aided in the storage of water by the 



development of one or more additional layers, which may be 



