THE EPIDERMAL TISSUE. 



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circumference, there arises a stratified cork-envelope, the Periderm, replacing the 

 epidermis which is in the meantime generally destroyed, and surpassing it in effici- 

 ency as a means of protection. But not unfrequently the formation of cork pene- 

 trates much deeper ; lamellce of cork arise deep within the stem as it increases in 

 thickness ; parts of the fundamental tissue and of the fibro -vascular bundles, or of the 

 masses of tissue which afterwards proceed from them, become, as it were, cut out 

 by lamellae of cork. Since everything which lies outside such a structure dies and 

 dries up, a peripheral layer of dried masses of tissue collects, which are very dif- 

 ferent in form and origin ; this structure, abundant in pines and in many dicotyle- 

 donous trees, is the Bark, the most complicated epidermal structure in the vegetable 

 kingdom. 



(a) The Formation of the Epidermis of Thallophytes is chiefly confined to this, — that 

 the cells of the fundamental tissue become smaller and firmer the nearer they are to 

 the surface ; the cell-walls very generally become darker, as in the outer layers of 



Fig. 69.— Transverse section of the stem of Bryum 

 roseuvi (X90): w root-hairs formed by the develop- 

 ment of single cells of the outermost cell-layer. 



Fig. 68.— Receptacle of Boletus flavidiis in longitudinal section slightly magnified ; st stipes : hu pUeus. hy hymenium ; -v velum ; 

 h cavity beneath the hymenium ; y prolongation of the hymenial layer on the stipes; Aif the separable yellow skin of thepileus. 



the cortical tissue of many Lichens, the outer layers of the peridia in Gastromycetes 

 and Pyrenomycetes ; in the pileus of many Hymenomycetes, the epidermal layer may 

 be detached in large pieces (Fig. 68). From the small development of the difference 

 between cortex and pith in these Thallophytes, it may appear doubtful whether the 

 outer layer should be termed bark or epidermis ; when the cortical tissue is thicker, 

 the epidermis 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 Muscines (Hepatic^, Sphagnum, Mosses) exhibit a great variety with reference 

 to the formation of epidermis. While in many other Hepaticae we have scarcely any 

 indications of one, in the group of March an tieae (Fig. 65) an epidermis perfectly developed 

 with stomata suddenly makes its appearance. In the Mosses the formation of epidermis 

 on the leafy stem is limited to this, that the cells towards the surface become narrower 

 and thicker-walled, while their walls assume a deeper red colour ; the outermost layer 

 often produces numerous long root-hairs (Fig. 69). In the Bog-mosses (Sphagnum), on the 



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