EPIDERMIS. 



[ 237 ] 



EPIDERMIS. 



some authors in the sense in which cuticle is 

 used by others, and vice versa. Oui' limits 

 prevent us from entering far upon the dis- 

 cussions, and om* object here, therefore, will 

 be to state as briefly as possible the most 

 remarkable facts, and the explanations which 

 are received by the best authorities. 



If we gently scrape up the surface of the 

 leaf of a hyacinth, or other soft-leaved 

 bulbous plant, and seize a little piece of the 

 ragged edge with a pair of fine forceps, we 

 ma}^ strip off large pieces of what appears to 

 the naked eye to be a thin homogeneous 

 pellicle. When this is placed under the micro- 

 scope^ it is found to be composed of a layer 

 of cells united firmly together by their sides, 

 like stones in a pavement, but loosely con- 

 nected with the subjacent tissue, which 

 adheres here and there to the detached strip 

 in ragged patches. The firm continuous 

 layer of cells is what botanists call the ej)i- 

 dermis of plants. Such a layer of cells 

 clothes the entire surface of the higher 

 plants, from the Flowering plants down to 

 those in which the organs, such as the leaves, 

 are reduced to mere layers of cells like the 

 epidermis itself, as in the Mosses. In a very 

 3-oung and delicate state, such as we find it 

 clothing the surface of organs still concealed 

 in buds, or of yoimg ovules in the ovary, it 

 has been called epiblema (Schleiden). A 

 rather more solid form, but still soft and 

 devoid of thickenino; layers, such as exists 

 on the sm*face of the growing parts of root- 

 lets, &c., is called epithelium (Schleiden) ; 

 both these terms appear useless, and only 

 calculated to confuse the student still more 

 than the use of the words epidermis and 

 cuticle, which akeady endanger misconcep- 

 tion from the very difi^erent characters of the 

 structures called by those names in animal 

 organs. 



When a layer of epidermis is macerated in 

 nitric acid, a thin pellicle, destitute of 

 cellular structm-e, becomes detached in sheets 



Fig. 203. 



Cuticle of a cabbage-leaf, removed by the action of nitric 

 p, hairs ; /, orifices corresponding to stomates. 

 Magnified 250 diameters. 



acid 



from the outer surface of the plate of epi- 

 dermal cells ; this is the cuticle (fig. 203), of 

 botanical anatomists, concerning which much 

 misconception has prevailed. As epidermis 

 advances in age it becomes considerably soli- 

 dified, especially on evergreen leaves, and on 

 shoots of shrubs, &c. which remain green 

 for a lengthened period, such as Aucuha and 

 Viscum. In most cases, however, the epider- 

 mis of structures belonging to the stem dis- 

 appears about the same time as the leaves 

 fall off", and is replaced by the suberous 

 layer of the bark structure, which change is 

 evident externally by the surface assuming a 

 brown colour, the subjacent tissue containing 

 chlorophyll being hidden. The green colour 

 of parts clothed with epidermis depends 

 upon the subjacent tissue showing through 

 the transparent epidermis, the cells of which 

 are always colourless, and filled with watery 

 contents. 



When sections are made perpendicularly to 

 the surface of any fully developed leaf, but 

 above all of those of leathery texture, the 

 walls of the cells next the external surface 

 are found much thicker than the rest, this 

 thickening extending more or less down over 

 the contiguous side-walls. When such sections 

 are treated with sulphuric acid and iodine, the 

 greater part of the thickness, from without 

 inward, of this outer wall, is stained yellow, 

 while the rest of the walls assume the blue 

 colour ordinarily taken by cellulose with 

 these reagents. Some authors suppose that 

 the whole of this yellow part corresponds to 

 the cuticle above mentioned, but such is not 

 the case; if such a section is boiled or 

 macerated for a long time in solution of 

 caustic potash, then washed well with water 

 and treated with tincture of iodine, the thick 

 upper wall also assumes the blue tint, and, 

 moreover, a laminated structure becomes 

 evident in it, showing that it is produced by 

 the de})osition of secondary layers inside the 

 cell. The true layer of cuticle (which is 

 dissolved ofi^ by the continued action of 

 potash) is really extremely thin in 

 almost all cases. The true nature of 

 this thickening of the outer walls is 

 well illustrated by the epidermis of 

 Viscum (Mistletoe), which remains 

 upon the shoots for many years ; here 

 several layers of cells subjacent to the 

 original superficial stratum become 

 involved in the process of solidifica- 

 tion, and theii- cavities completely filled 

 up by the secondary deposits. The 

 true structure of the enormously thick 



