48 



ORGANOGRAPHY. 



walls of the epidermal cells may be especially observed in leaves 

 of a leathery or hardened texture, as in those of the Oleander 

 {fig. 97), Aloes, Hoya {fig. 98), Box, Holly, &c., and in the stems 

 of Cactacese {fig. 99). These thickening layers upon the inner 



Fig. 98. 



Fig. 99. 



Fig. 98. The epidermis of Hoya carnn.m treated with caustic, alkali, a. 

 The cuticle separatiuf?. b. The swollen, laminated, cutlcular layers of 



the epidermal cells. After Mohl. Fig. 99. Thickened epidermal cells 



of a Cactus. After Schleideu. 



walls of the upper or external surface of the epidermal cells 

 were formerly confounded with the Cuticle or thin pellicle which 

 is situated on the outside of the epidermis {fig. 98, a). Mohl, to 

 whom belongs the merit of first establishing this point, has pro- 

 posed the name of cutioidar layers of the epidermis {fig. 98, b), 

 for these secondary deposits in the epidermal cells. 



The cells of the epidermis are always filled with colourless 

 fluids ; the green or other colours which leaves and other organs 

 assume is due therefore to colouring matters of various kinds 

 which are situated in the subjacent parenchymatous cells, and 

 which show through the transparent epidermal cells. In the 

 epidermal cells of many plants, waxy matter is contained ; in 

 those of Chara, carbonate of lime ; and in those of Equiseta and 

 the Grasses, silica is met with in such abundance that, if the 

 organic matter be removed by the agency of heat or acids, a 

 perfect skeleton of the structure will be obtained. These sub- 

 stances are probably deposited in the walls of the cells. 



The epidermis covers all parts of plants upon which it is found 

 that are directly exposed to the air except the stigma, and it is 

 in all cases absent from those which live under water. In the 

 Fungi, Algse, and Lichens, it is altogether wanting. The young 

 branches of trees are always covered with epidermis, which 

 is replaced at a subsequent period by epiphlceum. The roots of 

 plants are invested by a modified epidermal tissue to which the 

 term Ejnhlcma has been given by Sehleiden. This consists of 

 cells with thin walls, without stomata, but possessing cellular 

 prolongations externally, called Hairs, which will be described 

 hereafter. The inner surface of the ovary, the canal of the 



