THE ANGIOSPERMAE : STEMS 



867 



The continuity of the cork layer is interrupted at numerous points by 

 clusters of rounded cells, corky in nature but so loosely packed that air can 

 readily penetrate between them. These are the lenticels, which allow access 

 of air to the living tissues within (Fig. 854) (see also p. 878). 



Complementary 

 tissue 



Phellogen 





Fig. 854. — Satubucus nigra. Transverse section of a 

 woodv stem with lenticel. 



Anatomy of the ^Iature Stem 



I. The Epidermis. The essential character of an epidermis is that it is a 

 protective layer, or, in other words, that it is a barrier between the internal 

 organization of the plant and the environment. This protection is not 

 exclusively concerned with water loss, though that is a very important factor, 

 and the surface layer of some submerged aquatics may be truly classed as an 

 epidermis even though it is not a protection against desiccation. But the 

 emphasis on the barrier function excludes from this category secretory 

 surfaces such as those of glands, and absorptive surfaces such as those of 

 the young root. Furthermore, a true epidermis is a primary layer, developed 

 from the apical meristem, and the name does not apply to the exodermis of 

 roots or the periderm of older stems, which are both secondary. 



On the other hand, certain specialized cells of the surface layer, such as 

 hair cells and sclereids, which may not be primarily protective in function, 

 are none the less morphologically part of the epidermis and we shall treat 

 them as such. 



Epidermal cells are usually described as tabular in form, which means 

 somewhat flattened radially, but when viewed from the outer surface they 

 are seen to be nearly always vertically elongated, and they are by no means 

 alwavs radially flattened. In outline they vary greatly, but the variation is 

 much less in the epidermis of the stem than in the leaf. Growth of the 



