n6 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



SECTION 21. Diosmotic Properties of Cuticle and Cork. 



While it is necessary that wherever rapid transference is of importance, 

 permeable cell-walls must be retained, it is, on the other hand, in the highest 

 degree essential that plants should be able to produce on their exterior 

 cell-walls, which permit water and other substances to pass through them 

 only with difficulty, or even not at all. It is by such means, as illustrated 

 by the formation of cuticle and cork, that exchanges with the external 

 world, such as those involved in transpiration and absorption of water, 

 are limited both in locality and extent to actual requirements; while 

 in case of need, internal tissues may be isolated by special impermeable 

 investing layers. Details of these adaptations will be given in connexion 

 with the functions to which they are subservient (Sects. 30, 38), but their 

 importance depends mainly upon the diosmotic properties of the cork and 

 cuticle, of which a short account must be given here. It is clear that, as 

 the cell-wall becomes more and more impermeable, the existence of the 

 enclosed protoplast will be more and more difficult, and will ultimately 

 become impossible. Hence, as is well known, cork-cells are always dead, 

 if the walls are completely suberized ; and those tissues die, which are 

 isolated by the development of a cork layer beneath them. Epidermal 

 cells, however, can remain alive, because communication with the tissue 

 beneath remains undisturbed ; although by the cuticularization of the 

 outer wall the transpiration and absorption of the protoplasts are markedly 

 diminished. 



The physical properties of cork are indicated by its technical uses, and 

 it is these same properties which give cork its biological importance. The 

 function of the cuticle covering aerial parts is similar in character. When 

 well developed it opposes a marked hindrance to the passage of water or 

 water vapour, but all grades of transition may be shown to the readily 

 permeable cuticle of submerged aquatic plants, which, as is well known, 

 rapidly wither when exposed to the air. The permeability of cork also 

 varies, as is shown by the widely different powers of imbibition possessed by 

 various kinds of cork L . 



That cork is not completely impermeable to water is at once shown by 

 the fact that it swells somewhat in water and shrinks again on drying. 

 Indeed in every case the impermeability to water gradually increases as 

 development progresses, until under favourable conditions the cuticle may 

 be so thick as to allow only the merest trace of water to pass through it ; 

 while cork may form a series of layers, through which practically no water 

 is either exhaled or absorbed (Sects. 30, 38). 



Tissue paper acquires similar properties when it is impregnated more 

 or less thoroughly with wax, fat, or resinous materials, and leaves covered 



1 See \Viesner und Molisch, Sitzungsb. d. Wien. Akad., 1889, Bd. XCVIH, Abth. i, p. 707. 



