EPIDERMIS. 



77 



Agave anericana, Epidendron ciliare, Dasylirion, Sanseviera zeylanica, and the 

 phylloclades of Ruscus aculeatus ; or, in so far as it borders on the cuticular layers, 

 it is a very thin layer, which in many cases can only be observed with certainty in 

 very good preparations. This is most frequently the case where the epidermis is 

 thick. Examples : leaf of Hakea ceratophylla and other species, Ilex aquifolium 

 (surface of leaf), Hoya carnosa, Taxus baccata (under surface of leaf) ; one-year-old 

 branches of Viscum album ^, Taxus, Rosa canina, Kerria japonica. Ilex aquifolium, 

 Jasminum ofificinale, Laurus nobilis. Sassafras, Acer striatum ^, &c. 



{c) The whole outer wall is cuticularised, the rest of the wall not. The upper 

 side of one-year-old leaves of Taxus baccata, one-year-old stems of Salix daphnoides, 

 and, according to v. Mohl (Verm. Schr. Tab. IX. 15), epidermis of the stem of 

 Kleinia neriifolia. 



2. The cuticular layers and the non-cuticularised part of the wall are not sharply 

 defined one from the other, but rather — 



{a) either the inmost lamella of each cell-wall is not cuticularised, while the 

 outer layers show the cuticular reaction gradually stronger the further they are from 

 the inmost layer ; e. g. stem of Psilotum triquetrum, young stems of Selaginella 

 inaequalifolia, Martensii, &c. ; 



{b) or the whole wall of the epidermal cells is cuticularised all round : petiole 

 of Arbutus Unedo *, two-year-old branches of Nerium oleander ', leaf of Elymus 

 arenarius '^, stem of Klopstockia cerifera ^, leaf of Pinus, Abies, Cunninghamia 

 lanceolata, older stems of the above-named Selaginellas. Further, the brown-walled 

 epidermis of the stems and petioles of very many Ferns belongs to this category. 



3. The epidermis of the pinnae of the leaf of Cycas revoluta may be cited as 

 an exceptional case ®. The pitted cellulose walls of the epidermis are covered 

 externally by a thick cuticle, which is stratified, but not separable into cuticular 

 layers. From the cuticle there run narrow limiting bands of cuticular substance 

 between the lateral walls of the cells, to the subepidermal tissue. 



Where the cuticular layers border on non-cuticularised membrane the limiting 

 surface is either smooth, e. g. in most epidermal layers of branches cited above 

 under No. i ; or it is rendered uneven by numerous small processes, which 

 penetrate the cellulose layer like little teeth. Very small processes of this sort are for 

 instance to be found on the branches of Taxus ', the leaf of Hoya carnosa ; and larger 

 sharp teeth on the leaves of many species of Aloe (Fig. 25), and the phylloclades 

 of Ruscus aculeatus. Epidendron ciliare has numerous fine teeth both on the 

 surface of the cuticular layers which cover the cellulose wall externally, also on 

 the wedge-shaped ridges, which protrude into the lateral walls, and thirdly on 

 their sharp angular pegs, which protrude further into the lateral angles than the 



' Compare von Mohl, Verm. Schriften, Taf. IX. X. figs. 12, 14, 23, 26. — Vegetab. Zelle, fig. 40. 

 -.Schacht, Lehrbuch, I. Taf. III. figs. 16, 17, 23-25 ; IV. 9, &c. 

 ^ Von Mohl, Botan. Zeitg. 1849, p. 593. 



^ Botan. Zeitg. 1871, p. 596. * Wigand, I.e. p. 78. 



° Petunikow, I.e. pp. 19, 20, fig. 21. ^ Wigand, I.e. p. 105. 



■^ Botan. Zeitg. 187 1, p. 577. 



* Von Mohl, Verm. Schr. /. c. — Schacht, Lehrb. /. c. —Wigand, /. c. fig. 43. 



* Graf zu Solms-Laubach, Botan. Zeitg. 1871, p. 536. 



