S'z 



MORPHOLOGY OF TISSUES. 



other hand, one outermost layer of cells of the stem, or 2-4 such, assume an entirely 

 different character. These cells (Fig. 70, e) have thin coloured walls, they are much 

 broader than those of the inner tissue ; the walls sometimes show thin thickening-bands 

 running in a spiral manner, and open externally by large orifices, being also in com- 

 munication with one another by similar orifices (/), In the fully developed condition 

 they contain only air or water, which rises in them as in an actual capillary apparatus. 

 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 Aroideae. 



FIG. 70.— Transverse section of the stem of Sj>hagnu77i 

 cymbifoliunt (X900) ; x inner cells with colourless soft wallsj 

 / cortical cells, becoming gradually narrower and thicker- 

 walled towards the surface ; e e the epidermal layer ; / orifices 

 through which the opposite cells communicate with one 

 another. 



Fig. 71.— Piece of a radial transverse section through the sporangium oi Fitnaria hygrotnetrica (X300) ; e epidermis; the thick 

 black streak at its circumference is the cuticle. (For further explanation of the fig. see Book II.) 



As in Mosses the formation of tissue attains especially a greater perfection in the 

 sporangia, this is also the case in reference to the formation of epidermis ; the variously 

 differentiated internal tissue of the capsule is surrounded by a highly developed true 

 epidermis (sometimes provided with stomata) (Fig. 71). 



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

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

 a single layer ; but it sometimes becomes split into two or more layers by divisions 



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

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

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

 p. 33.— Kraus, ditto, IV, p. 305 and V, p. 83.— Pfitzer, ditto, VII, p. 561 and VIII, p. 17.— De Baiy, 

 Bot. Zeitg. nos. 9- 11 and 34-37, 1871. 



