^6 CELLULAR TISSUE. 



According as this is the case, the membrane shows the characteristic reactions of 

 cuticle. Treatment with the reagents, which dissolve cutin, removes it successively 

 from the persistent cellulose membranes, which retain their original form and struc- 

 ture, though necessarily with considerable loss of substance (comp. Fig. 25). These 

 layers containing cutin are called cuHcularised, or cuticular layers *. 



The cuticularisation may extend over all the elements of the epidermis, in- 

 cluding the epidermal cells, the hair-structures, and the cells of the stomata also. 

 On the latter the cuticular layers are it is true often thinner, in correspondence 

 with the smaller size of the cells, but often, especially in the ridges of entry, they are 

 strongly developed, and are continuous with those of the neighbouring cells, e. g. 

 leaves of Clivia nobilis", Dasylirion^, Epidendron ciliare and other tough-leaved 

 Orchidese, Ficus elastica, &c. 



The cuticularisation is evenly continuous over the epidermal cells of one 

 surface, and in the above-mentioned cases over the stomatal cells also, so that 

 one cell resembles another : thus the cuticular layers, in a simple case, appear in a 

 transverse section as an even broad band surrounding the whole epidermis. The 

 cells of the epidermis may be separated from the cuticle, which covers them, both 

 optically and (by help of the above-named ■ reagents) mechanically. The latter fact 

 is certain, though it is not always easily done. The form, relative thickness, and 

 extension of the cuticle over the cell-walls connected with it is no less various and 

 characteristic in special cases than the other relations of form and structure above 

 described. The following typical forms may accordingly be distinguished : — 



I. The cuticular layers form in the great majority of cases a covering on 

 the outer side of the epidermal cells, which is sharply marked off internally from the 

 non-cuticularised membrane. This may be — 



(a) A layer of almost universally equal thickness, which follows the surface, and 

 does not attain a thickness equal to that of the outer wall ; e. g. leaf of Dianlhus 

 plumarius, Caryophyllus, Helleborus foetidus, Vanilla, Galanthus nivalis *, &c. 



Or (h) a thick layer, which follows the outer surface, and projects inwards in the 

 shape of a ridge, of a conical form, in the middle of each lateral wall, and where 

 several cells join. The projections are usually sharply wedge-shaped towards the 

 inside, and do not reach as far as the inner wall (Fig. 25). Or they reach as far as 

 the latter, and are continued into the layer (' intercellular substance ') which marks the 

 limit towards the subepidermal layer, and which is also cuticularised ; e. g. branches 

 of Jasminum oflBcinale, Ephedra distachya, leaf of Phormium tenax ^ Ilex (Fig. 26), 

 and Pinus (Fig. 27). 



The non-cuticularised layer (coloured blue with Schultze's solution), which 

 in all these cases surrounds the cell-cavity, is either relatively thick, and consists of 

 many layers, as in the leaves of Pinus, Ilex (leaf-nerves), many species of Aloe, 



' Von Mohl, Botan. Zeitg. 1847, p. 502. = Von Mohl, Botan. Zeite. i8«;6, I.e. 



= Schacht, Lehrbuch, I. Taf. IV. fig. 9. 



' Von Mohl, Venn. Schriften, p. 260 ff.— Wigand, Intercellularsubstanz 11. Cuticula (1850), fig. 

 96, &c.— Petunikow, Recherches sur la Cuticula, p. 191, figs. 1, 32 (Bulletin Soc. Imp. de Moscou, 

 1866). ^ 



* Von Mohl, Verm. Schriften, Taf. X. fig. 28, 27. 



