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 lie 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 



Fig. 80.— Transverse section of the stem of Pryum 

 rosenni (X90): iv rhizoids developed from single 

 cells of the outermost layer. 



Fig. 79.— Fructification of Boletus flavidus in longitudinal section slightly magnified ; st stipes ; htt pileus, hy hymenium ; v velum ; 

 h cavity beneath the hym'enium ; /"prolongation of the hymenial 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 Muscineae (Hepaticae, Sphagnaceae, Musci) exhibit a great variety with reference 

 to the epidermal formation. While in many other Hepaticae 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 



