EPIDERMIS. 



83 



Wax-coverings are extruded on the outer surface of the cuticle in four chief 

 forms, 'vhich may be described as (i) siraia or crusis, (2) rod-like coverings, (3) simple 

 grafiular layers, (4) aggregated coverijigs. 



The strata or crtists are superposed on the cuticle in the form of a continuous 

 membrane. On many epidermal layers they form a clear, smooth, brittle glaze, which, 

 when the epidermis is stripped off, appears cracked and broken into angular pieces. 

 It attains a thickness of about i/x. Thus on the foliage of Thuja orientalis, 

 occidentalis, of Sempervivum tectorum, calcareum, the young stem of fleshy Eu- 

 phorbias (E. Caput Medusae L., E. ornithopus, E, canariensis, piscatoria, balsamifera 

 Ait.'), of Lepismium paradoxum Salm., Kerria japonica. Very delicate, angular, 

 homogeneous scales, like the fragments of a delicate skin of wax, are also found as 

 a rudimentary form of the glaze on many smooth shining epidermal layers : Cereus 

 alatus, species of Opuntia, leaf of Fuchsia globosa, Taxus baccata, leaf and stem of 

 Portulaca oleracea. 



In other cases the wax-layers attain very considerable thickness and then show 

 a more or less complicated structure, stratification parallel to the surface, and a 

 striation or areolation, or both of them together. The wax-crust of Euphorbia 

 canariensis on old branches attains a thickness of 70/x, and shows obvious stratifica- 

 tion : that of Kerria is more than 5/x thick, and is also stratified. The stems of 

 species of Chamaedorea, especially Ch. Schiedeana, Mart., are covered by a stratified 

 glaze of wax up to 14/i thick, which is brittle, and contains silica. The wax 

 covering on the epidermis of the stem of the wax palms of the Andes, Ceroxylon 

 and Klopstockia, are much more massive, attaining a thickness of 5°^™, and show 

 rich stratification and areolation. The young leaves of Corypha (Copernicia) 

 cerifera are covered on both sides by a wax-layer which is also striated perpendicular 

 to the surface, but attains a thickness of only 15-19/x. This having crumbled off 

 from the dried leaf, is collected as Brasilian Carnauba-wax. The stems and leaves of 

 Panicum turgidum, Forsk., are covered by a crust of wax up to 30/ii thick, which is 

 brittle and striated perpendicular to the surface. 



The thin glaze of wax, of the leaves of Sempervivum glaucum, which is in other 

 points like that of S. tectorum, is warty and uneven on its outer surface; that 

 of the leaves of Cotyledon orbiculata, L., is studded with numerous erect converging 

 out-growths in the form of rods lo/i high, and about ifi thick. 



The latter form the transition from the form of wax-covering above described 

 to the rod-like-covering, which covers the epidermis of the under surface of the leaf of 

 Aechmea farinosa, but occurs especially in many Scitamincce and Graminese : 

 e. g. on the under surface of the leaf, and on the petioles of Heliconia farinosa, 

 Strelitzia ovata (Fig. 29), Musa spec, on hypsophyllary leaves of species of Canna, 

 internodes and nodes of Saccharum officinarum (Fig. 28), Eulalia japonica, Trin. ; 

 on the short slightly silicified epidermal cells on the leaf-sheaths and the under sur- 

 face of the lamina of the latter species, further in the same positions, and on the 

 stems of species of Sorghum, Coix lachryma, &c. Rods consisting of wax here 

 stand perpendicularly upon the cuticle, either at relatively wide distances apart 

 (distances equal to or greater than their width), or so near one another as even 



* Schacht, Lehrb. II. p. 559. 

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