EPIDERMIS. 65 



sooner or later thrown off, and replaced by a subjacent protective 

 tissue, — cork. 



218. Except at peculiar openings (stomata, etc.), the epider- 

 mal cells are in close apposition. Upon their exposed surface 

 the}' are cutinized, and thus a continuous hyaline film is formed, 

 known as the Cuticle?- 



219. Sometimes the epidermis may be torn off without much 

 disturbing the underlying tissues. 



220. Besides the cells which compose the proper tissue of the 

 epidermis, there are certain ap- 

 pendages or accessory structures, 

 mainlj' hairs or analogous pro- 

 ductions (together called tri- 

 chomes) , and peculiar cells which 

 constitute the stomata. 



221. Epidermal cells proper are 

 in uninterrupted contact. They 

 are usuallj' of a tabular or pris- 

 matic form. The lines which 

 mark their outlines as viewed 

 from above are sometimes 

 straight, but oftener sinuous, at least on the longer sides of the 

 cell, which here as elsewhere correspond with the direction of 

 growth. Near stomata and trichomes the cells frequently assume 

 very irregular forms. 



222. Their upper or free surface is generally slighth" convex, 

 and often has minute outgrowths, for instance, in velvety petals ; 

 when these arc larger and longer, thej' constitute the simplest 

 form of plant hairs. 



223. Delicate epidermis possesses thin walls ; but in a large 

 number of fleshy and tough plants the walls have considerable 

 thickening, j'et not always on the same part. Thus in the leaves 

 of Cycads the upper wall is the thicker ; in many Bromeliaceae, 

 the lower and side walls. In a few cases the cell-cavit}' is nearly 

 filled by the thickening material. Stratification, striation, and 

 pitting of the cell-wall may also occur, great diversity existing 

 in all these respects. 



224. When tlie epidermis is ver}' delicate, the demonstration 

 of the thiu film of cuticle requires great care in the emploj^ment 



1 By De CandoUe the tei"in cuticle was applied to the layers of epidermal 

 cells, and not restricted to the cutinized film (Physiologie, 1832, p. 109). 



Fig . 45. Stoma of Sambucus nigra surrounded by epidermis. 

 5 



