THE EPIDERMAL TISSUE, 



97 



of the inner tissue; the walls sometimes show slender thickening-bands running in 

 a spiral manner, and open externally by large orifices, being also in communication 

 with one another by similar ones (/). In the mature state they contain nothing 

 but air or water, which rises in them by capillarity. Within this epidermal tissue the 

 stem is similar to that of Mosses; the cells become towards the surface gradually 

 narrower, thicker-walled, and of a darker colour. A similar epidermal layer, and with 

 similar hygroscopic properties, occurs in the aerial roots of Orchids and of some 

 Aroidege. 



Like the other forms of tissue, the epidermis also attains a greater perfection in the 

 sporogonia of Mosses ; the variously differentiated internal tissue of the sporangium 



Fig. 81.— Transverse section of the stem of Sphagw 

 cymbifolium (X 900) ; x inner cells with colourless soft walls ; 

 r cortical cells, becoming gradually narrower and thicker- 

 walled towards the surface ; e e the epidermal layer ; 

 / orifices through which adjacent cells communicate with 

 one another. 



I 



Fig. 82. — Part of a radial longitudinal section through the sporangium of Funaria hygrotnetrica (X 300) ; e epidermis ; the thick 

 black line on the outside is the cuticle. (For further explanation of the Fig. see Book II.) 



is surrounded by a highly developed true epidermis, sometimes provided with stomata 

 (Fig. 82). 



(b) The Epidermis^. In Vascular Plants the epidermal tissue consists usually only 

 of a single superficial layer of cells, the true Epidermis. In its origin it always consists of 

 a single layer ; but this sometimes splits into two or more by divisions parallel to the 



^ H. von Mohl, Vermischte Schriften hot. Inhalts. Tubingen 1845, p. 260— F. Cohn, De 

 . Cuticula. Vratislaviae 1850.— Leitgeb, Denkschriften der Wiener Akad. 1865, vol. XXIV. p. 253. — 



Nicolai, Schriften der phys.-okonom. Gesells. Konigsberg, 1865, p. 73. — Thomas, Jahrb. fiir wiss. 

 ' Bot. vol. IV. p. 33.— Kraus, ibid., vol. IV. p. 305, and vol. V. p. 83. — Pfitzer, ibid., vol. VII. p. 561, 



and vol. VIII. p. 17.— De Bary, Bot. Zeitg. 1871, nos. 9-11 and 34-37. 



