EPIDERMIS. 



[ 297 ] 



EPIDERMIS. 



or short hairs are produced, rendering the 

 surface velvety (see HAIRS). 



The side walls of epidermal cells are 

 sometimes flat faces of tolerably regular 

 geometrical figures, such as cubes, parallelo- 

 pipeds, hexagonal prisms, &c. ; but not un- 

 frequently they are very sinuous, and then, 

 when the epidermis is seen from above, it 

 does not look like ordinary parenchyma, 

 with square, rectangular, or hexagonal tes- 

 sellse, but the component cells are fitted 

 together so as to present lines, which, when 

 regular, might be described by the heraldic 

 terms scallopped. wavy, indented,c. (PL 47. 

 fig. 15), and when less regular resemble 

 ro ughly the lines of j oint in the old-fashioned 

 puzzle-maps of children (fig. 200). Such 

 forms of the epidermis are frequently found 

 on petals, on the leaves of Ferns, on those 

 of Hellebore, &c., and constitute very pleas- 

 ing microscopic objects; especially as, in 

 addition to the lines, the stomata at the 

 angles add to the elegance of the pattern. 



The cuticle on many petals, as those of 

 the Daffodil, and on leaves, as those of the 

 genus Helleborus, Dianthus, &c., when the 

 epidermis is viewed from above, exhibits 

 elevated strise running in various ways over 

 the surface, sometimes converging in the 

 centre of each cell, in other cases running 

 in tortuous lines over the surface, con- 

 tinuous beyond the boundaries of the indi- 

 vidual cells. A similar condition of the 

 cuticle occurs upon the HAIRS of many 

 plants, especially of Cruciferte, Ranuncu- 

 laceae, Boragineee, &c. This condition is 

 evidently analogous to the equally myste- 

 rious states of the outer membrane of POL- 

 LEN-GRAINS and SPORES, where points, 

 ridges, reticulations, &c. of the same kind 

 constantly occur. 



The stomata are found on both surfaces of 

 many leaves of delicate structure, but most 

 abundantly on the lower surface ; in other 

 plants they occur exclusively on the lower 

 face ; in floating leaves they exist only on 

 the upper face ; while on submerged leaves 

 none at all occur, and the epidermis here 

 has no very distinct difference from that of 

 young roots. The characters of STOMATA 

 are spoken of more at length under that 

 head, as also those of HAIRS, SCALES, 

 STINGS, THORNS, GLANDS. 



The cuticular layers of the epidermis 

 often contain deposits of wax upon or with- 

 in the tissues. De Bary distinguishes four 

 kinds : 1, heaps of granules in several 

 layers, or of delicate needles, as in Euca- 



lyptus, Acacia, grasses, &c.; 2, a simple 

 layer of -grains, in Allium cepa, Brassica 

 okracea, &c. ; 3, a layer of rods perpendi- 

 cular to the surface, as in Musacece, Saccha- 

 rum, &c. ; and 4, a membranous wax-layer 

 or crust, as in Sempervivum, Thuja, and 

 Taxus. Some of the wax-grains are not 

 recognizable until the tissues are heated to 

 212, when they form drops. 



The epidermis of the Equisetacese and 

 the Grasses is remarkable for the deposition 

 of silica, apparently in the walls of the cells 

 of the epidermis, to such an extent and so 

 equably, that the whole of the organic 

 matter may be removed by heat or acids, 

 and a perfect skeleton of the structure be 

 obtained, composed exclusively of silex, 

 exhibiting the boundary lines of the epi- 

 dermal cells and the stomata (the dentate 

 side walls, with the stomata arranged in 

 linear series, are described in most micro- 

 scopic books in a very curious manner, from 

 an old paper by Sir D. Brewster). Prepa- 

 rations 01 this structure are obtained by 

 treating little pieces of the wall of the fistu- 

 lar stem with strong nitric acid, to remove 

 alkalies, and then burning them until quite 

 white on a slip of platinum or very thin 

 glass. These should be mounted in Canada 

 balsam. 



In the Equiseta, the siliceous films thus 

 obtained are covered with minute spines, 

 presenting somewhat the dotted appearance 

 of the valves of the Diatomacese. 



The seeds of many plants are clothed 

 with an epidermis of remarkable character, 

 the cells containing spiral fibres ; this oc- 

 curs in the ACANTHACE^:, in COLLOMIA, 

 SALVIA, &c., and is further treated under 

 those heads and under HAIRS and SPIRAL 

 STRUCTURES. 



BIBL. Mohl, Vegetable Cell, 1852, Linncea, 

 xvi. 401, Verm. Schrift. 260, Ann. Sc. Nat. 

 2 ser. xix. 201, ibid. 3 ser. iii. 158, Sot. Zeit. 

 v. 497 (1847); ibid. vii. 593, 1849; Schlei- 

 den, Wiss. Bot. 3 ed. 335 (Principles, 70) ; 

 Brongniart, Ann. Sc. Nat. xviii. 427, 2 ser. 

 i. 65; Link, Elem. Phil. Bot. i. 83; Wigand, 

 Inter cellular- Substanz u. Cuticula, 1850 j 

 Karsten, Bot. Zeit. vi. 729, 1848; Cohn, 

 Linncea, xxiii. 337, 1850 ; Hartig, Entwickl. 

 der Pft. 1843, Ann. Sc. Nat. 3 s<r. i. 352 ; 

 linger, Bot. Zeit. v. 289, 1847; Garreau, 

 Ann. Sc. Nat. 3 s6r. xiii. 304 ; Mulder and 

 Harting, Mulder's Phys. Chem.\ Goldmann, 

 Bot. Zeit. vi. 857, 1848; Schacht, Pflan- 

 zenzelle, 89, 1852 ; Wiesner, Techn. Mikr. 

 1867 ; Leitgeb, Denkschr. Wien. Ak. 1865, 



