EPIDERMIS. 



29 



cells, and these, as the quantity of the secretion increases, become like those reservoirs. 

 For judgment upon these intermediate forms, and the possibility of carrying out the 

 separation of the forms of tissue, the same reflections hold good as for the scleren- 

 chyma. The difficulties of distinguishing them in practice are, besides, much smaller 

 in this case than in that of the sclerenchvma. 



SECTION I. 

 EPIDERMIS. 



Sect. 2, Epidermis, outer skin, is the name given to the layer of cells which is 

 covered by and produces the cuticle. It constitutes the surface of such plants as 

 are several layers of tissue thick, from the beginning of the difterentiation of tissues 

 onwards throughout their life, or till the beginning of the development of cork, which 

 takes its place. 



On the stems and leaves of Angiosperms the Epidermis is sharply marked off, 

 even in the young embryo, while still consisting of few cells ; in this case, as long 

 as it remains in the meristematic condition, it is termed the Dermatogen layer. This 

 grows, as stated on p. 7, with the stem, leaves, and branches, covering them as a 

 * cellular mande, one layer of cells thick. It remains in the large majority of cases 

 throughout its life a simple layer of cells, with exception of hair structures. In 

 relatively few Angiospermous plants divisions of the young epidermal cells appear 

 parallel to the surface, and then in a rather late stage of development ; from a simple 

 layer of cells two or several are thus formed. These assume an essentially identical 

 structure, and are then termed many-layered epidermis. 



Where the differendation of the apical meristem is other than that characteristic 

 for the stem of the Angiosperms, a permanent outer layer of meristem, derived by 

 successive divisions from initial cells common to it and to other layers, assumes the 

 properties of the epidermis. In plants which grow with an apical cell, definite peri- 

 pheral products of division of the segments serve this purpose ; in Gymnospermous 

 roots the transverse portions of the successive layers of periblem from time to dme 

 laid bare by the separadon of the root-cap, &c. Comp. above, p. 14. In these cases 

 we cannot speak of a many-layered epidermis in the same sense as in the stem and 

 leaf of the Angiosperms, since the genetic relations characterisdc of those cases are 

 dilTerent ; that term can at most be conventionally used for single cases which in fact 

 scarcely ever occur. In single special cases also in the Angiosperms, the epidermis 

 originates from other elements than the dermatogen. The perforations (and indeed also 

 the lacinise) in the leaves of many Aroideae originate by the early dying off" of circum- 

 scribed portions of the young leaf, the original epidermis dying off" with them ^. Since 

 the edge of the mature perforations is clothed with epidermis, this must be completed 

 from the inner layers of the young leaf, which point moreover remains still to be more 



* Compare Tiecul, Ann. Sci. Nat. 4 ser. torn. I. p. 37, 



