98 



MORPHOLOGY OF TISSUES. 



surface, during or after the bud-condition of the organ in question. In such cases 

 the outermost may be distinguished as the Epidermis proper from those which lie 

 beneath, or the Strengthening-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 Ficuj, in the stems and leaves of many Piperaceae, and in the leaves 

 of Begonia. In the roots also of some species oi Crinum the epidermis, at first simple, 

 splits into several layers; but this is much more striking in the aerial roots of Orchids 

 and Aroidea!, where these cell-layers afterwards lose their succulent contents and sur- 

 round the substance of the root as an air-containing envelope to the root {Felamen). 



The Eypoderma is genetically distinct from the strengthening-layers which result 

 by division from the originally simple epidermal layer, since it arises from the layers of 

 the fundamental tissue which are 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 Tradescontia. 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 E/egia, and is very probably so in other cases. While this scleren- 

 chymatous hypoderma is especially frequent in Vascular Cryptogams (e.g. Equisetaceae 

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

 very abundantly in the petioles and succulent stems of Angiosperms, especially of 

 Dicotyledons; its usually narrow but long cells are strikingly distinguished by the 

 thickening-masses often forming longitudinal ridges at the angles projecting internally, 

 and swelling greatly with water or more powerful reagents (Fig. 2r, B, p. 24). That 

 the collenchyma originates from the fundamental tissue, and thus not from the epi- 

 dermis, has been actually observed only in Euonymus latifolius^ Peperomia, Nerium, and 

 Ilex, but is probable also in other cases. 



When the term Epidermis is hereafter 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 

 hypoderma, are in close contact on all sides ; the only intercellular spaces are those 

 between the guard-cells of the stomata, through which the large cavities in the funda- 

 mental tissue communicate with the external air. This close approximation of its 

 cells is sometimes the only distinguishing mark of the epidermis, as in the submerged 

 Hydrilleae, Ceratophyllum, &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 

 granular contents generally, while in Ferns and in the water-plants mentioned above, 

 as well as in other cases, the epidermal cells contain chlorophyll-granules. Not unfre- 

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



In organs which grow chiefly in length, as roots, long internodes, and the leaves 

 of Monocotyledons, the epidermal cells are usually elongated longitudinally; in leaves 

 with a broad surface they are mostly broadly tabular; in both cases the side-walls 

 are often undulated, so that the adjoining cells interlock with one another. 



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

 such an extent that cellulose cannot be detected in it, or only with difficulty. This 

 true Cuticle extends uninterruptedly over the surface, and is strongly contrasted 

 with the subjacent layers of the cell-wall. 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 potash. 

 In submerged organs and roots it is very thin, difficult to be seen immediately, but 



