EPIDERMIS. 



75 



from the slit inwards. In the Cactese, it extends from the slit onwards over thd 

 whole wall of the spacious respiratory cavity, and sends tubular branches with open 

 ends into the intercellular spaces of the neighbouring chlorophyll-containing paren- 

 chymal It is wanting as a rule on the inner surface of the epidermis. It is rarely 

 continued from the slits onwards over the whole inner surface of the epidermis, as 

 faras this borders on intercellular spaces, as a lamella, which is interrupted by the 

 surfaces of insertion of the subepidermal cells (v. Mohl, /. c). This is the case on 

 both the stomata-bearing surfaces of the leaf of species of Armeria, especially 

 A. plantaginea, on the under surface of the leaf of Betula alba, Dianthus caryophyllus,- 

 Euphorbia Caput-Medusse, and the stomata-bearing bands of the leaf of Asphodelus 

 luteus. In Helleborus niger and viridis the inner cuticle extends from the stomata- 

 bearing under surface, over the upper side of the leaf, which has no stomata. (On 

 the occurrence "of cuticle in "the deeper-seated inteircellular spaces, comp. Chap. VII ; 

 on the peculiar phenomena in Restio diffusus, see Sect. i8.) 



In the well-established cases, the cuticle cannot by the means of investigation 

 at present in use be separated either mechanically, or optically, into separate parts 

 or segments corresponding to the neighbouring cells. By careful maceration with 

 potash or dilute acids, it may be separated as a continuous skin from large tracts of 

 the underlying cell-membranes. It appears by the action of the above reagents to 

 swell more strongly in the direction of the surface than those membranes. By 

 boiling solution of potash, or Schultze's mixture, it is transformed into a tough shiny 

 mass, and then completely destroyed without leaving a cellulose residue. It is in 

 most cases very thin, especially on submerged parts and roots; on aerial parts, 

 not excepting the punctum vegetaiionis, it is thicker ; only in few cases where it is 

 specially strongly developed (leaf of.Cycas revoluta, Ilex aquifolium), a delicate 

 stratification can be recognised ; as a rule, there is no sign of this. Its thick- 

 ness is usually equal all over one and the same surface ; also on the ridges and 

 warts of the surface so often mentioned the cuticle itself usually runs unthickened 

 over the corresponding outgrowths of the wall (e. g. leaf of Eucomis, Orchis, 

 Helleborus, &c. Comp. also v. Mohl, Verm. Schr. Taf IX. Figs. 7, 8). Projecting 

 thickenings belonging to the cuticle itself are much more rare ; on the hairs of Mono- 

 tropa Hypopitys "^ there is the most exquisite example of this ; the outermost layer 

 of the wall, which is covered with numerous elongated warts, here shows the pro- 

 perties of the cuticle : it is completely dissolved in boiling potash, and leaves the 

 cellulose membrane quite smooth. 



On some very thick epidermal layers, which form large quantities of wax 

 (Acer striatum, Negundo, Sophora japonica), the cuticle follows the increase of 

 thickness of the membranes only a short time, and then breaks up by irregular 

 splits. 



Where the epidermis is delicate the cuticle covers the relatively pure cellulose 

 membrane of the epidermal cells. But where it is thicker, especially when long- 

 lived, the part of the cellulose membrane bordering on the cuticle itself also contains 

 cutin, and consists of layers of cellulose, each of which is permeated by cutin. 



' "Von Mohl, Botan. Zeitg. 1845, p. 3.— Unger, Grundziige (1845), p. 25. 

 ^ Schacht, Lehibuch, I. p. 140. 



