180 PLANT ANATOMY. 
and aqueous vapor, and which for the previously described reasons 
is primarily adapted to fulfil the function of the epidermis 
namely, protection against too strong evaporation. 
The cuticle, which is often directly visible upon transverse 
sections (Figs. 160,161, 155), or is easily rendered visible by 
dilute chromic acid, sulphuric acid, iodine, or potassa, covers all 
the organs of the plant which are exposed to the air, and, in the 
case of favorable objects, can often be removed asa coherent 
film. 
If the outer wall is thin, as for instance in the leaves of the 
indigenous foliage trees and all the officinal leaves, then the 
cuticle is in direct connection with the cellulose layer (Fig. 155); 
if, however, the outer wall is very strongly thickened, the inter- 
vening layers of the outer membrane of the epidermal cells are 
for the most part cuticularized (cuticular layers, Fig. 161 
cs, 63), that is, they have become more similar to the cuticle 
AI ISe 
Fie, 89.—Transverse section through Fig. 88 A, more highly magnified. 
itself by the deposition of cutin. These cuticular layers often 
project in a somewhat cone-like form towards the interior (Figs. 
161, 63). 
The outer wall is frequently somewhat arched in an outward 
direction (Fig. 155). These outward arches may become defi- 
nitely shaped processes, whereby the exterior surface acquires 
a pitted appearance. The same appearance may, however, also 
be produced by the prominent development of thick lateral walls 
(testa of the seed of Hyoscyamus, Fig. 88, and the leaves of 
Gentiana cruciata). In the latter case, the outer wall is always 
very thin, and indented (Figs, 88 and 89). Occasionally the sur- 
face delineation is also produced by the projection of sharply 
circumscribed groups of epidermal cells. 
The outer wall (like the thick lateral walls, Figs. 88, 89) 
shows, as a rule, a distinct stratification. 
