THE EPIDERMAL TISSUE. 83 



parallel to the surface which originate rather late during or after the bud-condition 

 of the organs in question. In such cases the outermost may be distinguished, as the 

 epidermis proper, from those which lie beneath, or the thickening-layers ; these latter 

 generally consist of large thin-walled cells with contents as clear as water, for which 

 reason Pfitzer terms them Aqueous Tissue. Epidermis of this kind, consisting of 

 several layers, occurs in the leaves of most species of Ficus, in the stems and leaves of 

 many Piperacese, and in the leaves of Begonia. In the roots also of some species of 

 Crinum, the epidermis, at first simple, splits into several layers ; but this is much more 

 striking in the aerial roots of Orchids and Aroideae, where these cell-layers afterwards 

 lose their succulent contents and surround the substance of the root as an air-containing 

 root-envelope (velamen). The Hypoderma is distinct in its development from the 

 strengthening-layers which result by division from the originally simple epidermal layer, 

 since it arises from the layers of the fundamental tissue covered by the true and simple 

 epidermis. The cells of the hypoderma may also become developed as aqueous tissue 

 like that mentioned above, and often to an enormous thickness ; this occurs in many 

 Bromeliaceae and some species of Tradescantia. The hypoderma more often exists in 

 the form of layers of very thick-walled often sclerenchymatous cells, whose origin has 

 been proved to be from the fundamental tissue, not from the epidermis, at least in the 

 case of Ephedra and Elegia, and is very probably so in other cases. While this scleren- 

 chymatous hypoderma is especially frequent in Vascular Cryptogams (^. ^. Equisetum and 

 Ferns), and in the leaves of Gymnosperms, a third form, the Collenchyma, occurs very 

 abundantly in the leaf-stalks and succulent stems of Angiosperms, especially of Dicotyle- 

 dons, its usually narrow but long cells being strikingly distinguished by the thickening- 

 masses often deposited to a great extent in internally projecting longitudinal ridges at 

 the angles, and swelling greatly with water or more powerful reagents (Fig. 21, B, p. 24). 

 That the collenchyma originates from the fundamental tissue, and thus not from the 

 epidermis, has been actually observed only in Euonymus latifoUusy Peperomia, Nerium, 

 and Ilex, but is probable also in other cases. 



When in the sequel the term Epidermis is used without further remark, the ordinary 

 simple layer, or the outermost when the epidermal tissue consists of several layers, is 

 always to be understood. 



The cells of the epidermis, as also those of the strengthening-layers and of the hypo- 

 derma, are in close contact on all sides ; intercellular spaces are formed only between the 

 guard-cells of the stomata, through which the intercellular spaces of the fundamental 

 tissue communicate with the surrounding air. This connexion without interstices is 

 sometimes the only distinguishing mark of the epidermis, as in the submerged Hy- 

 drillese, Ceratophyilum, &c. ; in other cases the formation of hairs helps to distinguish 

 it, as in most roots, where the cells of the epidermis are otherwise similar to those of 

 the fundamental tissue in contents and in the nature of their wall. But usually in 

 the stem and foliar organs the epidermis is destitute of chlorophyll, starch, and espe- 

 cially of granular contents, while in Ferns and in the water-plants mentioned above, as 

 well as in other cases, the epidermis-cells contain grains of chlorophyll. Not unfre- 

 quently the otherwise colourless cell-sap is tinged by a red substance. 



The form of the epidermis-cells in organs the development of which is chiefly in 

 length, as roots, long internodes, and leaves of Monocotyledons, is usually elongated 

 longitudinally ; in leaves with a broad surface it is mostly broadly tabular ; in both cases 

 the side-walls are often curved in an undulating manner, so that the adjoining cells 

 project into one another. 



The outermost lamella of the epidermis-cells is always cuticularised, and usually to 

 the extent that cellulose is either not at all, or only with difficulty, to be detected in it. 

 This true cuticle extends uninterruptedly over the boundaries of the cells, and is strongly 

 contrasted with the subjacent layers of the epidermis. With preparations of iodine, 

 with or without addition of sulphuric acid, the cuticle is coloured yellow or yellow- 

 brown ; it is insoluble in concentrated sulphuric acid, but soluble in boiling caustic 



G 2 



