102 DERMAL SYSTEM 



and in fact all glandular organs that are ontogenetically or phylo- 

 genetically epidermal in origin, must be relegated to the secretory 

 system. Epidermal sense-organs again do not belong to the dermal 

 system. Lastly, specialised strengthening cells, even if superficial in 

 position and derived from the protoderm, must unquestionably be 

 referred to the mechanical system. 



To sum up the epidermis, in the anatomieo-physiological sense, 

 comprises only those superficial cells or cell-layers, the histological 

 features of which clearly indicate that their principal function is that 

 of a primary tegumentary or dermal tissue. 



A. THE SIMPLE EPIDERMIS. 47 



1 . The shape of epidermal cells. 



Epidermal cells are usually more or less tabular in shape : in 

 the case of leaves they are not infrequently lenticular (biconvex). 

 They are always in uninterrupted contact with one another laterally. 

 In typical cases their radial diameter is small, though it may become 

 considerable where the water-storing capacity of the epidermis is pro-' 

 nounced. In organs like the majority of Dicotyledonous leaves, 

 which grow slowly and which are not conspicuously elongated in any 

 one direction, the epidermal cells are approximately isodiametric. In 

 various Monocotyledonous leaves, on the contrary, in most petioles and 

 stems and, in fact, in all distinctly elongated organs, they are as a rule 

 obviously elongated in the same sense as the whole organ. Excep- 

 tionally (as in Cycads, Bromeliaceae, Tradescantia, Crassula, Silene 

 fruticosa, etc.) they are elongated at right angles to the long axis of 

 the leaf. Not uncommonly the epidermis is made up of cells of more 

 than one kind ; in most Grasses, for instance, curious " dwarf cells " of 

 unknown significance are interpolated at regular intervals in the 

 longitudinal rows of typical elongated epidermal elements [Fig. 26]. 



2. The outer epidermal wall. Wax. 



From the physiological point of view the outer wall must be 

 considered the most important part of an epidermal cell. This wall 

 usually differs from the others, first, in its greater thickness, and, 

 secondly, in certain physical and chemical peculiarities which are due 

 to the presence of fatty substances collectively termed cut'm ; it may 

 in addition be impregnated with compounds of a ' waxy nature. 

 Typically, the outer wall consists of three distinct zones. The inner- 

 most of these, bordering upon the cell-cavity, comprises the cellulose 

 layers (Fig. 23 A. &.), which, as their name implies, consist of unaltered 

 cellulose. Next follow the cutinised layers (Fig. 2'3a,cs), which 



