114 



DERMAL SYSTEM 



depressions in the inner epidermal walls (Fig. 26 c). In this way the 

 cells of the epidermis and the hypodermal fibres are firmly dove-tailed 



together. 



Like the lateral walls, the inner 

 walls of epidermal cells are generally 

 thinner and softer than the outer walls, 

 so that diosmotic interchange with the 

 underlying tissues can proceed unhin- 

 dered. In a number of plants (Betula 

 alba, Quercus peduneidata, Erica carnea, 

 spp. of Salix, Daphne, Primus and 

 Gytisus, certain Sapindaceae and Dios- 

 meae, etc.) certain cells of the foliar 

 epidermis are provided, according to 

 Eadlkofer, 59 with greatly thickened, 

 mucilaginous inner walls, a condition 

 which probably represents a water- 

 storing device (Fig. 27); in these cases 

 the mucilaginous membrane undertakes 

 a function which in the typical epidermis is assigned to the cell-sap. 



Fig. '27. 



Epidermal cells with thick mucilaginous 

 inner walls. A. From the adaxial foliar 

 epidermis of Theobroma Cacao : B. From 

 the adaxial foliar epidermis of Pterocarpus 

 saxatilis. 



4. Cell-contents. 



Every typical epidermal cell is provided with a living protoplast in 

 the shape of a thin peripheral layer of cytoplasm, containing a nucleus, 

 and enclosing a sap-cavity which is filled with transparent and usually 

 colourless cell-sap. Very frequently leucoplasts (especially in Mono- 

 cotyledons) or chloroplasts are also present. Chloroplasts are, however, 

 not conspicuously developed in the case of typical brightly illuminated 

 epidermal layers. 60 Nevertheless, the comparatively small chlorophyll- 

 content of the epidermis must not lie regarded as a consequence of the 

 destructive action of intense light upon chloroplasts ; the presence of a 

 well-developed chlorophyll-apparatus in the guard-cells of stomata 

 proves beyond doubt that the sharp contrast between colourless 

 epidermis and green palisade-tissue cannot be altogether attributed to 

 differences in the intensity of illumination. The feeble development 

 or complete absence of chloroplasts should rather be looked upon as 

 the result of a division of labour, whereby the epidermis is entrusted 

 with the task of storing water, and in return is relieved of photo- 

 synthetic duties. 



Westermaier has, in fact, shown that the epidermal layers of 

 vegetative organs, and especially those of foliage-leaves, play the part 

 of superficial water-jackets, which protect the underlying tissues against 

 damage through loss of water. 01 A whole series of anatomical and 



