ii6 



LECTURE VIII. 



this as the proper function of the epidermis ; though it is by no means gain- 

 said that the most various other functions are required of it according to. circum- 

 stances. 



Let us- first consider the arrangements of the epidermis serving for exclusion'. 

 It consists usually of a single layer of tabular, or more rarely of columnar cells, the 

 lateral walls of which are in close contact on all sides ; so that the layer usually 

 constitutes a very thin but, in relation to its thickness, extremely firm membrane 

 formed of cells, which on leaves and young shoot-axes can in many cases be 

 stripped off' over wide areas. The external wall of the epidermis cells is usually 

 of considerable thickness, and is occasionally so thickened that the cavity is 

 relatively insignificant. In rarer cases it is the wall of the inner side which 

 presents the strong thickening. The contents of the epidermis cells are usually 

 devoid of chlorophyll grains, even on leaves and shoot-axes abounding in chloro- 

 phyll ; yet not unfrequently, especially in Ferns 

 and some Phanerogams, and particularly those 

 growing in the shade, the protoplasmic lining 

 of the wall also contains chlorophyll grains; 

 and in submerged water-plants the chloro- 

 phyll contents of the external cell-layer (repre- 

 senting the epidermis) may be even particu- 

 larly abundant. The shutting off, already more 

 or less completely ensured by the thickening 

 of the, outer walls, is however considerably 

 promoted by means of a pellicle, usually 

 very thin but sometimes thicker, which runs 

 continuously over the outer surface of all the 

 epidermis cells, and is distinguished as the 

 Cuticle. The substance of this thin pellicle, 

 the properties of which agree essentially 

 with those of cork, differs from cellulose: 

 it is dissolved by alkalies, but withstands 

 concentrated sulphuric acid (which liquifies 

 cellulose), and its most important property consists in that it is permeated only 

 with difficulty by water, thus hindering the entrance and exit of that liquid. 

 Hence the cuticle is especially thick in leaves and shoot-axes which have need 

 of considerable protection against the evaporation of the water of the cells, as on 

 the leaves of ever-green plants and of succulents (e.g. Agave, Aloe, Firs, and 

 species of Cactus). Very frequently the protection afforded by the cuticle is 

 enhanced by the cutin or cuticular substance being also deposited in the outer 

 walls of the epidermis cells themselves, and forming in these the so-called 

 cuticularised layers of the walls of the epidermis cells. As a further step towards 

 the shutting off from the outer world, and especially against the entrance of 

 rain-water and dew, we may regard the formation of wax, partly in the sub- 



Fig. 115.— Epidermis of the midrib of the leaf of I /ex 

 agm/olium. A transverse section; B surface view from 

 without ; a, b the very thick cuticle, with its processes b* ; 

 t wall of the cells of the epidermis. 



^ The most thorough description of the epidermis is found in De Bary's classical work, 'Ven 

 gleickende Anatomie der Vegetations-organe der Phanerogamcn und Farne^ Leipzig, 1877. 



