CUTICULAR EPITHELIUM. DOUBLE EPIDERMIS 123 



Fig. 32. 



dermal tissue consisting essentially of numerous superimposed cutinised 

 layers, which here and there enclose the remains of dead protoplasts. 

 In old stems of Viscum the total thickness of this " euticular epithelium," 

 as Damm calls it, may amount to more than half a millimetre. As the 

 branches grow in thickness the surface of the epithelium becomes 

 traversed by radial fissures (Fig. 31); these gradually enlarge to such 

 an extent that the outermost layers of the cutinised mass ultimately 

 crumble away or detach themselves in the form of scales. The 

 Viscoideae form no periderm, so that here the multiple epidermis is 

 the only dermal tissue produced. Some Menispermaceae, on the other 

 hand, develop a transient euticular 

 epithelium, which after a certain 

 number of years is replaced by 

 cork. 



In Pinus sylvestris, and in certain 

 other species of Pinus, the walls of 

 the foliar epidermal cells are so 

 greatly thickened on all sides that 

 the cavities are reduced to narrow 

 clefts, from which fine canals or 

 pores radiate in various directions, Double epidermi t s a c [ .^ .Jf of Pinus mon ' 

 but especially towards the corners 



of the cells (Fig. 32); in this way the mechanical strength of the 

 epidermis is considerably increased. The actual cutinised layers are 

 only moderately thick ; but all the other regions of the epidermal cell- 

 membrane are also either cutinised or lignified. This superficial cell- 

 layer therefore, which corresponds morphologically to the epidermis, 

 evidently functionates as a waterproof covering and as a hard exo- 

 skeleton. The hypodermis, as seen in transverse section, resembles a 

 normal epidermis in the shape of its cells, in the colourless nature of 

 the cell contents, and in the moderate thickness of its lateral and inner 

 walls ; this layer has apparently undertaken that partial function of 

 the ordinary epidermis which is associated with these histological 

 characteristics, that is to say, it represents a rather feebly developed 

 peripheral water-tissue. The foliar dermal system of Pinus therefore 

 displays a relatively high degree of division of labour, inasmuch as the 

 partial functions performed by an ordinary epidermis are, in this 

 case, distributed between two cell-layers, 



In the majority of Bromeliaceae the leaf is likewise furnished with 

 a double epidermis. The cells of the outer layer have slightly thickened 

 but highly cutinised external walls ; their inner walls are, however, as 

 a rule so thick that the cell-cavities are reduced to narrow clefts. In 

 certain species {Tillandsia aloifolia, T. goyacensis, T. violacea, etc.), 



