70 CELLULAR TISSUE. 



of the remnants of the cell-contents, and present a lustre which varies according fo 

 their form and position and the character of their surface. They preponderate in 

 very hairy plants.. Thus the dense white felt on the foliage of many Labiatae 

 (Stachys, Teucrium, Salvia, &c.), Compositae (e.g. Gnaphalium), the Verbascums, 

 Banksias, Rubus idseus, &c. ; the silvery white or brown peltate scales of the above- 

 named Elseagnese, Bromeliacese, Croton, Solanese, Olea spec. ; the rustling ' Paleae' 

 of the Ferns ; the white crust consisting of dried capitate hairs in the above-quoted 

 (p. 63) species of Atriplex, and Obione, and other Chenopodiaceae. 



(b) Structure of the walls of the Epidermal Elements. 



Sect. 14. The wall of the epidermal cells is, in very delicate parts, a thin 

 cellulose membrane developed pretty equally all round. In rather more firm parts, 

 in such as are termed herbaceous, and to a greater extent in very tough parts, such 

 as stems and branches of smooth-barked ligneous plants, leathery and fleshy leaves, 

 it is strongly thickened. In rare cases the thickening is almost equal all round, 

 e.g. leaves of Ceratozamia mexicana^, Pinus sylvestris, and its allies'' (Figs. 11, 27; 

 in this case the lumen almost disappears), or is much less on the outer surface thaa 

 on the lateral and inner ones, as is the rule in the Bromeliacese (Fig. 12, p. 37)'. 

 Also in the epidermal cells containing mucilage, which will be described below, 

 the inner wall is -of considerable thickness, often exceeding that of the outer 

 wall. In an epidermis one layer thick and in the outer layer of a many-layered 

 epidermis the outer wall is usually thicker than the lateral or inner walls. In the 

 above-named tough parts, such as leathery and fleshy leaves, old branches of Viscum, 

 Ilex, Laurus, Menisperraum canadense, Palm stems, &c., it is often thickened to such 

 an extent that it occupies the greater part of the whole volume of the cell. The thick 

 outer wall is either sharply marked off" from the thin lateral walls or graduates gently 

 into them. The walls of the inner layers of a many-layered epidermis all resemble 

 in the main the lateral and inner walls of the single-layered epidermis as regards, 

 strength and structure, with the exception of isolated peculiar cases which must be 

 mentioned as being extraordinary. 



The thickened walls have generally the well-known structure of cell-membranes, 

 stratification, striation, and pitting, but never fibrous thickening of the walls. The 

 phenomena connected with special peculiarities of substance — cuticularisation, 

 formation of Cystoliths — will be treated of later. There occurs sometimes on the 

 wavy lateral walls (e.g. under surface of the leaf of Helleborus fcetidus*) at the 

 bottom of a depression a local thickening in form of an excrescence resembling 

 a fold or doubling of the membrane, which protrudes inward, at right angles to 

 the surface. 



Pits of the usual form, corresponding on opposite sides, are very common on 

 the lateral and inner walls. As a rule they do not occur on the thick outer walls, 



' Kraus, Cycadeenfiedem, /. c. 



" Thomas, t.c. p. 25.— Hildebrand, Botan. Zeitg. i860, Taf. IV. 

 ' Von Mohl, Verm. Schriften, Taf. X. 33.— Schacht, Lehrb. I. Taf IV fig 10 

 * Von Mohl, Verm. Schr. Tab. VIII. fig. 21 ; Vegetab. Zelle, p. 14. Compare also Cohn, Nov. 

 Act. Acad. Leopold, vol. XXII. pars ^. 



