EPIDERMIS. 



[ 238 ] 



EPIDERMIS. 



epidermal layer of old shoots, as brought out 

 by the action of potash, is seen in the 

 example, fig. 26 of PL 38. The true cuticle 

 is sometimes of considerable thickness, as in 

 the leaves of Cycas (PI. 38. fig. 28). The 

 thickening layers of the epidermal cells are 

 true Secondary Deposits ; the nature of 

 the cuticle is yet uncertain ; some regard it 

 as a kind of excretion hardened over the 

 surface, others as the persistent original 

 outer wall of the parent cells of the epi- 

 dermal cells, metamorphosed chemically 

 when exposed directly to the action of the 

 air (in a manner analogous to that in which 

 the parent-cell membranes become converted 

 into a gelatinous investment of the filaments 

 of Conferva?, the cells of Palmellaceae, &c.). 

 This seems borne out to some extent by the 

 change of condition of the consolidated part 

 of the outer walls, coloured yellow by sul- 

 phuric acid and iodine ; but it is unknown 

 whether there is here a real chemical change, 

 or merely an infiltration capable of being 

 removed by the action of potash (see Secon- 

 dary Deposits). 



Although the cellular plants possess no 

 true epidermal layer, the superficial cells 

 form a kind of cortical structure in the 

 Lichens and larger Algae, and in the lower 

 Algae the cells of the filaments, &c. compo- 

 sing the fronds, bear some resemblance to 

 epidermal cells in structure, insomuch that 

 they have a laminated wall (partly produced 

 by the persistence of those of the parent-cell 

 after cell- division), with the outer layer pos- 

 sessing much of the physical characters of 

 the cuticle of the higher plants. As just 

 mentioned, the gelatinous sheaths of the 

 lower Algae must be regarded as a kind of 

 cuticle, and as produced by gradual disor- 

 ganization of the outer layers of membrane 

 while cell-development and the formation of 

 new layers is going on within. For further 

 discussion of the nature of the thickening 

 layers of epidermis, see Intercellular 

 Substance. 



The epidermis and its appendages offer a 

 great variety of points of interest to the 

 microscopist. The epidermis of those 

 growing parts of the higher plants which are 

 exposed to the air is not absolutely continu- 

 ous and without orifices like the epidermis of 

 roots, but is perforated with myriads of 

 breathing pores or stomates (fig. 204 S) as 

 they are called. These consist of gaps left 

 by the separation of the superficial epidermal 

 cells at their meeting angles, the interspace 

 between them being guarded and more or 



less filled up by a pair of cells, situated just 

 beneath the outer orifice, and having a slit- 

 like passage between them. 



Hairs, scales, thorns, stings, and the 

 various forms of glands of plants, are appen- 

 dages of the epidermal structure, being pro- 

 duced by the peculiar development of parti- 

 cular cells or groups of cells of this superfi- 

 cial layer. 



We have already alluded to the different 

 conditions of the epidermis in different parts 

 of plants. The delicate layer covering young 

 organs in buds becomes very variously deve- 

 loped as these attain the complete conditions. 

 On the leaves and shoots the epidermis be- 

 comes consolidated by secondary deposits, 

 and this in greatest proportion on leathery 

 or woody leaves, &c., such as those of ever- 

 greens, shrubs, and trees. Remarkable ex- 

 amples of this may be found in the leaves of 

 the Proteaceae, Cycadaceae, the Holly, Box, 

 &c. (woody), and in the Aloes, Cactaceae, 

 Oleander, Hakea, Ficus, &c. (leathery). In 

 all cases the solid character of foliage de- 

 pends almost exclusively upon the character 

 of the epidermis by which the leaves are 

 clothed. The epidermis of the outer scales 

 of winter-buds of trees is remarkably thick. 

 The thickening layers are sometimes found 

 on the walls of the stomatal cells and adja- 

 cent cells bounding the intercellular cavity, 

 forming the pseudo-structure called a cistome 

 (see Stomates). 



The epidermis of petals and similar deli- 

 cate organs never acquires much solidity, 

 but the outer walls often become elevated 

 more or less abovethe surface, producinga mi- 

 nute papillosity of the epidermis, which gives 

 the peculiar glistening appearance. When 

 this elevation goes still further, villi 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.; butnotunfre- 

 quently 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- 

 selae, but the component cells are fitted to- 

 gether so as to present lines, which, when 

 regular, might be described by the heraldic 

 terms, scalloped, wavy, indented, &c. (PI. 28. 

 fig. 15) ; and when less regular, resemble 

 roughly the lines of joint in the old-fashioned 

 puzzle-maps of children (fig. 204). Such 

 forms of the epidermis are found on petals 



