/ 



46 THE EPIDERMIS 



sperms the protoderm is directly continuous with the outermost 

 tunica layer or "dermatogen" of the shoot apex (for further 

 tletails, ef. Exercise III). In roots, the ])rotoderm likewise is 

 demarcated from the internal meristems in the vicinity of the 

 apex. Hay ward (pp. 45-47) has recently summarized the more 

 important "types" of root apices from the standpoint of the way 

 in which the protoderm ("dermatogen") is related to the devel- 

 opment of root cap, cortex and stele. 



In the majority of seed plants, the epidermis is a uniseriate 

 layer of cells which clothes the "primary body." Aside from 

 the epidermal appendages or trichom.es, which will be studied 

 later in the exercise, the common cell types composing this layer 

 are epidermal cells and the guard cells. The epidermal cells, 

 although exhibiting considerable variation in size, shape and 

 arrangement, are usually closely joined with one another, thus 

 forming a sheet of cells which is pierced only by the intercellular 

 spaces or pores found between the guard cells. An exceptional 

 type of epidermis occurs in petals, however, since here inter- 

 cellular spaces are found between ordinary epidermal cells. 

 According to Eames and MacDaniels (pp. 284-28")) these "spaces 

 do not open to the outer air, however, since they are covered in 

 all cases by the cuticle." Epidermal cells are roughly "tabular" 

 in form and especially in the laminae of dicotyledonous foliage 

 leaves, have a characteristic undulate contour when seen in sur- 

 face view. A protoplast is normally retained and a great variety 

 of ergastic substances such as anthocyanin pigments, tannins 

 and oils occur in the cells. Epidermal cells exhibit to a notable 

 extent an a])ility for regressive diffen idiaiion. This is shown 

 not merely by the origin in certain plants of the jihellogen in this 

 layer (Eames and IMacDaniels, p. 210) but es])ecially by the 

 important role of epidermal cells in the ])roducti()n of adventi- 

 tious bud-primordia (cf. Crooks, 1933, Naylor and Johnson. 

 19.37, McVeigh, 1938, and Naylor, 1940). 



With the exception of roots and the submersed portions of 

 aquatic plants, the outer walls of the epidermis are covered by a 

 sheet of waxy material which is termed the cuticle. This waxy 

 layer is continuous, except for (lie stomatal openings, and serves 

 to restrict the loss of water fi-om plant organs. The thickness of 



