112 DERMAL SYSTEM 



as the greatly increased breadth of the cells, shows that the epidermis 

 has undergone considerable tangential extension. The narrow radial 

 fissures have widened out into large gaps, while tangential cracks have 

 also made their appearance ; in this way a more or less extensive 

 weathering and crumbling of the outer cutinised layers has gradually 

 taken place. 56 



3. The radial and inner epidermal walls. 



The various properties and functions of the outer epidermal wall 

 having been discussed in detail, attention may now be directed to the 

 other walls. The radial or lateral walls, like the outer wall, exhibit 

 adaptive features which are correlated with more than one physiological 

 function. According to Westermaier, not the least important among 

 the functions of the epidermis is that of water-storage. The detailed 

 consideration of this matter must be postponed until the contents of 

 epidermal cells come to be discussed. For the present it must suffice 

 to note that the lateral walls must be thin, so as to allow water to pass 

 with ease from cell to cell, and also in order that the cells may be able 

 to contract and expand freely in the radial direction. As a matter of 

 fact, it is the rule for epidermal cells to have very thin radial walls ; 

 if the latter are thickened for mechanical reasons, then they are usually 

 provided with numerous circular or oval pits, which facilitate inter- 

 change of materials, and especially of water, between adjacent cells. 

 Burns has even found typical bordered pits which otherwise are 

 confined to the walls of water-conducting elements in the lateral 

 epidermal walls of certain species of Stylidium (especially S. strepto- 

 carpum). 57 



It is, of course, necessary, for mechanical reasons, that every 

 epidermal cell should be firmly attached to its neighbours ; for the 

 epidermis is constantly liable to be subjected to a variety of tensile 

 stresses. In the case of the stem these stresses arise mainly as a 

 result of transverse and longitudinal tissue-tensions, while the bending 

 action of the wind produces tangential stresses in the foliar epidermis. 

 The firm attachment of the epidermal cells to one another is effected by 

 the lateral walls, which are accordingly often thrown into folds in order 

 that the surface of contact between adjacent elements may be increased 

 (Fig. 26 a, b). The individual cells thus acquire a lobed or toothed 

 outline, and their adjacent faces become interlocked in a manner 

 recalling the sutures of the Vertebrate skull. In the case of elongated 

 organs, such as a stem or the leaf of a Grass, where the epidermal cells 

 are likewise elongated, it is only the longitudinal radial walls that are 

 conspicuously folded. The Gramineae in particular which in every 

 respect exhibit a high degree of anatomical specialisation exemplify 



