868 A TEXTBOOK OF THEORETICAL BOTANY 



primary stem is chiefly elongation, and the epidermal cells in the young state 

 are consequently elongate and narrow. The radial walls may be somewhat 

 wav}% which is apparently the result of continued expansion of the cells after 

 growth of the stem has ceased. The same factor may result in the outer 

 surfaces being convex, domed or even papillate. The radial walls are not 

 only undulating, when viewed from the exterior, but are thin and have 

 numerous large pits. This implies that lateral movement of water in all 

 directions in the epidermis must be relatively easy, and it is probable that a 

 dangerous loss of water at any one point of the surface may be compensated 

 by the tangential flow of water towards the threatened spot from other parts 

 of the epidermis, which thus acts as a first line of defence, before the radial 

 flow from the xylem can become efi'ective. That epidermal cells do, in fact, 

 part with considerable amounts of water at times, is shown by the great 

 changes of volume they can withstand without injury, especially in leaves. 

 When this occurs the radial walls shrink and the cells become flattened, but 

 they rapidly expand when equilibrium is restored. 



The epidermal cells of some plants {e.g.. Erica and Daphne) have greatly 

 thickened, mucilaginous inner walls, thus providing for increased water 

 storage. The physiological as well as the mechanical cohesion of the epidermal 

 cells is increased by the absence of any spaces at the angles between them. 



The outer surface of the epidermis is covered by the cuticle. This layer, 

 which varies in thickness in general relationship to the moisture conditions 

 in the environment, is extra-cellular and continuous and therefore helps to 

 bind the epidermal cells together. It is tough and elastic, only slightly 

 permeable to either water or gases, and its protective value is very great. 

 Its development begins very early, and it may sometimes be traced even on 

 the tunica in the meristem. 



Priestley has explained it as a non-volatile residue from the surface 

 evaporation of sap, and while this would serve to account for its greater 

 thickness in plants of dry habitats, it is diflicult to accept so simple an explana- 

 tion for cuticles on enclosed surfaces, such as the inner wall of the ovary or 

 on leaves within the winter bud. The cuticle contains no cellulose and is 

 composed chiefly of the insoluble anhydrides of an unknown number of 

 fatty acids, of which two, stearocutic and oleocutic acids, have been described. 

 They probably reach the epidermis in the form of glycerides, i.e., as true 

 fats, and are there decomposed, with the liberation of the free acids. 



The chief difference between cuticle and suberin, apart from their different 

 location in the plant, seems to be the absence from the former of the phellonic 

 acid which is an important constituent of the latter. Their reactions to 

 microchemical test reagents are very similar. 



Continued growth of the epidermis, besides causing the wavy outlines of 

 the cells, also throws the cuticle covering into wrinkles, which often form a 

 minute pattern over the outer surface of each cell. 



The cuticle often extends downwards for some distance along the radial 

 walls, forming distinct wedges between the cells, but it never completely 

 separates them. Below the cuticle proper lies the cutinized layer, which is 



