Exercise V 



THE EPIDERMIS 



I. Introduction. — From a purely topographical viewpoint, the 

 term epidermis may be applied to tiie superficial layer of cells 

 in young- stems and roots, and in foliar structures. Since the 

 epidermis represents, in this sense, the point of clireet eontaet 

 between the plant and its external environment, it is not surpris- 

 ing that this "tissue" exhibits considerable diversity in its 

 structure and functions. Haberlandt (p. 102) has proposed a 

 restricted physiological definition of the epidermis which would 

 include only "those superficial cells or cell-layers, the histologi- 

 cal features of which clearly indicate that their principal function 

 is that of a primary tegumentary or dermal system." Accord- 

 ing to this viewpoint, absorbing hairs and stomata would be 

 excluded on physiological grounds from the epidermis. But as 

 Linsbauer (1930, pp. 4-5) has clearly pointed out, it seems hardly 

 justifiable to place the chief emphasis on the function of "pro- 

 tection" in the definition of the term epidermis. On the con- 

 trary, the cells which are morphologically a part of the epidermis 

 may perform varied functions, important among which are me- 

 chanical protection, restriction of transpiration, water storage, 

 aeration, storage of various metabolic products, absorption and 

 photosynthesis. To subdivide such a "continuous" layer as the 

 epidermis into various "anatomico-physiological systems" is 

 more likely to result in confusion than is the retention of the 

 broader topographical -morphological concept expressed above. 

 The ontogenetic development of the epidermis likewise justifies its 

 interpretation as a "morphological unit," since its origin is 

 traceable to an external embryonic layer or "protoderm." In 

 lower vascular plants and in many gymnosperms, the protoderm 

 of the shoot appears some distance from the summit of the apex 

 as a superficial layer derived from the periclinal division of the 

 segments of the apical initial or initials. But in many angio- 



4.5 



V 



