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 

 always 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 portula- 

 coides, Fig. 44, A, h.). 

 In times of drought 

 the adjacent assimi- 

 latory tissues with- 

 draw moisture from 

 these cells, which 

 consequently con- 

 tract, the side walls 

 becoming undulated ; 



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 (Nepeta when water again be- 

 glechoma). C, Surface section of leaf of comes plentiful, the 

 Onion. D, Surface section of stem of 

 Runner Bean (Phaseolus multiflorus}. 

 St., stomata. 



cells fill and the walls 

 gradually straighten 

 out. The epidermis 



is not uncommonly aided in the storage of water by the 

 development of one or more additional layers, which may be 



