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 Ficus, in the stems and leaves of many Piperaceae, 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 sur- 
round the substance of the root as an air-containing envelope to the root {Velamen). 
The Hypoderma 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 Trade scantia. 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 {e.g. Equisetaceae 
and Ferns) and in the leaves of Gymnosperms, a third form, the Collenchyma, 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. 21, 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- r 
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 
