SECONDARY EPIDERMAL LAYERS 149 



are covered by a thin cuticle. The underlying cutinised layer is 

 distinctly stratified, and provided with a reticulate system of flanges 

 which project into the lateral walls. Finally, there is a thin innermost 

 layer of cellulose, so that the outer walls have a perfectly typical 

 structure (Fig. 46). Even the relative thicknesses of the different 

 layers correspond exactly in the primary and in the secondary cells of 

 the margin. The total thickness of the outer wall is about 6^, which is a 

 little more than the thickness of the outer wall of an epidermal cell of 

 the leaf-surface. 



If young leaves suffer mechanical injury at an early stage of 

 development, the margin of the wound may be occluded by a secondary 

 epidermal layer, which again arises from the fundamental meristem. 

 A typical case has been described in detail by Pfitzer. If a young 

 leaf of Peperoruia iKreskiacfolia is injured by insect agency or other- 

 wise, it first of all proceeds to form ordinary " wound-cork " ; the outer- 

 most layer, however, that remains alive beneath the corky covering 

 acquires thickened outer walls, resembling those of a typical epidermis. 

 This process may properly be termed regeneration of the epidermis. 

 More recently Vochting has observed a similar phenomenon in the 

 case of wounded tubers of Kohl Eabi (Brassica oleracea, var. (jongylodes), 

 where the regenerated epidermis reproduces all the essential features of 

 the normal layer ; it is a remarkable fact that, in this case, even the 

 stomata are regenerated. 



