DIOSMOTJC PROPERTIES OF CUTICLE AND CORK 117 



with a film of excreted wax cannot be wetted, nor so long as this is the 

 case will any water be absorbed by them. 



The increased impermeability of the cuticle acquired under conditions 

 in which transpiration is very active, and the formation and regeneration of 

 the cuticle on surfaces exposed externally, are evident signs of its regulatory 

 and adaptive development 1 . In no less degree cork is formed in direct 

 correlation with the developmental progress and needs of the aerial organs, 

 &c., and it is developed over areas exposed either in the normal progress of 

 events (viz. leaf-fall) or by accidental injury. 



As the permeability to water decreases so also does the permeability to 

 dissolved substances diminish, so that the cuticle may be almost impermeable 

 both to water and to dissolved salts. At the same time it is possible that 

 the permeability to water may be diminished more markedly than to other 

 substances. Thus films of india rubber or of waxed paper allow alcohol, 

 carbon dioxide, and oxygen to pass slowly through, but are almost completely 

 impermeable to water. Similarly a cuticle, through which feeble tran- 

 spiration is possible, may not allow the traces of CO 2 present in the 

 air to diffuse through with sufficient rapidity to permit a formation of 

 starch in the assimilating cells immediately beneath (Sect. 57), but may 

 permit sufficiently rapid gaseous exchanges for the continuance of respiration 

 (Sects. 29, 30), for by means of the bacterium method it may be readily 

 observed that oxygen passes with comparative ease through the thin cuticle 

 of an assimilating epidermal hair of Cucurbita? . Changes of permeability 

 may be produced by impregnation with other substances 3 or by alterations 

 in the quality of the membrane (see Sect. 14). Thus by the death of the 

 non-suberized layers of the bark, an increased protection against transpiration 

 is afforded, while in producing that diminution of permeability which old 

 wood fibres undergo, the formation of gummy substances appears to be of 

 considerable importance. It is, however, impossible at present to determine 

 precisely why functionally active tracheae only allow air to diffuse through 

 them with difficulty (Sect. 32). 



The rate of diffusion is inversely proportional to the thickness of the 

 membrane, and hence the thick gelatinous or slimy coverings of many algae 

 and certain roots may exercise a distinct protective influence, although they 

 allow some substances to pass through them with comparative ease, as is 

 shown by the readiness with which plasmolysis may be induced 4 . Enclosing 

 membranes of this character interpose a marked hindrance to transpira- 

 tion as they dry. Gelatinous membranes have a special tendency to retain 



1 Kohl, Transpiration, 1886, p. 113; Tittmann, Jahrb. f. wiss. Dot., 1896, Bd. xxx, p. 116. 

 See Sect. 38. 



2 [Ewart, Journ. Linn. Soc. Bot., Vol. xxxi, p. 366.] 



3 On the influence of silicification, see Kohl, Kalksalze u. Kieselsaure i. d. Pflanze, 1889, p. 228. 



4 See Schilling, Flora, 1894, p. 351 ; Walliczek, Jahrb. f. wiss. Bot., 1893, Bd. XXV, p. 273. 



