EPIDERMIS. 



[ 296 ] 



EPIDER^nS. 



The nature of the cnfide is yet ixncertaiu ; 

 some regard it as a kind of excretion har- 

 dened over the surface, otiiers as the per- 

 sistent original outer wall of the parent-cells 

 of the epidermal cells, metamorphosed che- 

 mically where 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 Confervfe, the cells of Pal- 

 mellaceaj, &c.). This seems borne out to 

 some extent by the change of condition of 

 the consolidated part of the outer walls, 

 coloured yellow by sulplunic acid and iodine ; 

 but it is unlniown wlaether there is here a 

 real chemical change, or merely an infiltra- 

 tion capable of being removed by the action 

 of potash (see Secondaby Deposits), 



Although the cellular plants possess no 

 true epidermal layer, the superficial cells 

 form a kind of cortical struetiu'e in the 

 Lichens and larger Algte ; and in the lower 

 Algae the cells of the filaments «S:c. compo- 

 sing the fronds bear some resemblance to 

 epidermal cells in structure, insomuch that 

 they have laminated walls (partly produced 

 by the persistence of those of the parent 

 cell after cell-division), with the outer layer 

 possessing much of the physical character 

 of the cuticle of the higher plants. As just 

 mentioned, the gelatinous sheaths of the 

 lower Algae must be regarded as a kind of 

 cidicle, 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 Avithin. For 

 further discussion of the nature of the 

 thickening layers of epidermis, see Inter- 



CELLULAR SuBSTANCE. 



The epidermis and its appendages offer a 

 great variety of points of interest to the 

 microscopist. The epidermis of those grow- 

 ing parts of the higher plants which are 

 exposed to the air is not absolutely con- 

 tinuous and without orifices like the epi- 

 dermis of roots, but is perforated with 

 myriads of breathing-pores or stomata 

 (fig. 200, s) as they are called. These con- 

 sist of gaps left by the separation of the 

 superficial epidermal cells at their meeting 

 angles, the interspace between th(>m being 

 guarded and more or less filled up by 

 (usually) a pair of cells, situated just be- 

 neath the outer orifice, and having a slit- 

 like passage between them. 



Hairs, scales, thorns, stinr/s, and the 

 various forms of (/lands of plants, are ap- 

 pendages of the epidermal structure, being 



Fig. 200. 



Epidermis from petal of the balsam, with stomata, S. 



The epidermal cells here have elegantly sinuous 



side walls. 



Magnified 200 diameters. 



produced by the peculiar development of 

 particular cells or groups of cells of this 

 superficial 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 ProteacejB, Cycadaceae, the Holly, Box, 

 itc. (woody), and in the ^Vloes, Cactaceae, 

 Oleander, Ilakea, Ficus, kc. (leathery). In 

 all cases the solid character of foliage de- 

 pends almost exclusively up(m the character 

 of the epidermis by which the leaves are 

 clothed. The epidermis of the outer scales 

 of the winter-buds of trees is remarkably 

 thick. The thickening layers are some- 

 times found on the walls of the stomatal 

 cells and adjacent cells bounding the inter- 

 cellular cavity, forming the pseudo-struc- 

 ture called a cistome (see Stomata). 



The epidermis of petals and similar deli- 

 cate organs never acquires much solidity; 

 but the outer walls often become elevated 

 more or less above the surface, producing a 

 minute papillosity of the epidermis, which 

 gives the peculiar gli.-~teuing appearance. 

 When this elevation goes still further, villi 



