INTRODUCTION TO PLANT HISTOLOGY 457 



A true epidermis is essentially a protective layer, securing the inner 

 tissues against desiccation. This function is associated with the deposit on 

 the outer wall of an impervious layer of cuticle, which varies in thickness 

 with the degree of exposure of the plant to a drying climate. In roots and m 

 water plants it is non-existent, but it is found on the exposed parts of all 

 land plants. It is especially thick in evergreens, while in a few desert plants 

 it may reach such a thickness that it can be stripped off. Fruits, such as 

 Apples, often have a heavy cuticle, and their keeping qualities largely depend 

 on the retention of their moisture by this covering. The cuticle is very 

 impervious to water, and if even a small portion be removed from an apple 

 the rapid loss of weight which follows is striking evidence of its protective 

 power. The cuticle is also important in resisting the penetration of parasitic 

 Fungi or the sucking organs of insects, and the immunity of certain cultivated 

 varieties to such attacks may sometimes be traced to the thickness of their 



cuticle. . 



The cuticle forms a continuous layer over the epidermis, closely moulded 

 to the cell surfaces, so that, when isolated, it shows the pattern of the epidermal 

 cells. It often bears fine ridges or papillae on the outer surface, which give 

 it a velvety appearance. In plants where the epidermis persists for several 

 years the cuticle is formed in successive layers, the outer being cracked and 

 shed as new layers are formed beneath. 



Cuticle is a complex mixture of fatty acids, partly free and partly 

 combined with alcohols. They are mostly in a condensed condition and are 

 in part saponified. It stains readily with fat-soluble dyes such as Sudan III. 

 These fatty substances are apparently excreted through the outer walls of 

 the epidermal cells, though the process is little understood. It also contains 

 wax, and this may accumulate on the surface as a layer of minute granules 

 or rods, which are the cause of the greyish " bloom " on many fruits and 

 leaves, which are described botanically as glaucous. The Wax Palni, 

 Copernicia cerifera, of Brazil produces such quantities on its leaves that it is 

 collected commercially as Carnauba Wax. 



Normally the cuticle is quite distinct from the epidermal cell walls, but 

 in some cases these may become infihrated with cutin, the name given to 

 the cuticle substance, and are then said to be cutinized (Fig. 445). The 

 cutinized cell wall may be much thicker than the cuticle itself. Walls which 

 are cutinized may occur even in internal tissues, as in the coats of some seeds. 



The epidermis is sometimes reinforced beneath by a second layer of cells, 

 which may be cutinized or lignified. This is called the hypodermis. 



The Endodermis 



The endodermis is a single layer of cells surrounding the vascular tissues. 

 It is formed by modification of the innermost layer of the cortical parenchyma. 

 Typical endodermis is found in both stems, leaves and roots of Pteridophyta 

 and in the roots of Spermatophyta, but in the stems of Spermatophyta it is 

 often missing or is represented by a layer of cells with large starch grains. 



