96 
MORPHOLOGY OF TISSUES. 
lies outside such a structure dies and dries up, a peripheral layer of dried tissue 
collects, which is very various in its form and origin. This structure, abundant in 
Coniferae and in many dicotyledonous trees, is the Bark, the most complicated 
epidermal structure in the vegetable kingdom. 
(a) The Epidermal Formation of Ihallophytes is chiefly confined to the cells of the 
fundamental tissue becoming smaller and firmer the nearer they he to the surface; 
the cell-walls very generally become darker, as in the outer layers of the cortical 
tissue of many Lichens, and the outer layers of the peridia in Gasteromycetes and 
Pyrenomycetes ; in the pileus of many Hymenomycetes the epidermal layer may 
be detached in large pieces (Fig. 79). From the small difference between cortex 
and fundamental tissue in these Thallophytes, it may appear doubtful whether the 
r 
Fig. 79.— Fructification of Boletus flavidus in longitudinal section slightly magnified ; st stipes ; h7i pileus, hy hynienium ; v velum ; 
Ji cavity beneath the hymenium ; f prolongation of the liymenial layer on the stipes ; ht the separable yellow epidermal layer of the 
pileus. 
outer layer should be termed cortex or epidermis; when the cortical tissue is 
moderately thick, this layer can usually be distinguished from it. With Thallophytes, as 
with higher plants, the outermost layer of cells displays a tendency to the formation 
of hairs. 
The Muscinese (Hepaticse, Sphagnacege, Musci) exhibit a great variety with reference 
to the epidermal formation. While in many other Hepaticse we have scarcely any 
indications of one, in the family of Marchantieae (Fig. 65, p. 78) an epidermis perfectly 
developed and provided with stomata suddenly makes its appearance. In Mosses the 
epidermal formation on the leafy stem is limited to this, — that the cells towards 
the surface become narrower, their walls becoming thicker and assuming a deeper 
red colour ; the outermost layer often produces numerous long rhizoids (Fig. 80). 
In Bog-mosses (Sphagnaceae), on the other hand, a single outermost layer of cells 
of the stem, or from two to four such, assume an entirely different character. These 
cells (Fig. 81, have thin colourless walls; their cavities are much larger than those 
