56 CELLULAR TISSUE. 



closely investigated. Similar relations, which however also require further observation, 

 may hold for the margins of the leaf-segments in the Palms, since these segments 

 originate by the splitting of the continuous young lamina ^ In one or few-layered 

 parts, like the leaf-surfaces of the Hymenophyllacese - and Hydrillese ^, either there 

 is no diflerentiation of the epidermis from the parenchyma, or it has been obliterated. 

 One can in this case speak of epidermis only on the ground of the cuticular covering, 

 which is present, or as in the two-layered leaf lamina of the Hydrillese, on the ground 

 of genetic relations. In the many-layered parts also of submerged water-plants the 

 diflerentiation of epidermis and parenchyma often becomes less plain, as will later be 

 described. 



In the overwhelming majority of cases the epidermis is sharply distinguished 

 from the cells which it surrounds. 



1. COMPOSITION OF THE EPIDERMIS. 



Sect. 3. The following kinds of cells or cell-groups are to be distinguished as 

 parts of the Epidermis : — ■ 



( 1 ) Epidermal cells. 



(2) Slomaial, guard-cells, pairs of which enclose a slit-shaped intercellular space, 

 and together with this form the stoma. 



(3) Hair-siructtires (Trichomes). 



Sect. 4. Epidermal cells in the strict sense is the name given to those cells 

 of the epidermis whose lateral walls are in uninterrupted connection with one 

 another and with stomatal cells. Exceptions to this occur only in the slightly 

 differentiated epidermis of the base of the leaf of the Osmundaceae and Isoetes 

 (comp. Sect. 9). The term lateral walls is here used for all those which are at 

 right angles to the surface. With reference to the longitudinal axis of growth of the 

 member of highest rank to v;hich they belong, we may therefore speak of superior 

 or inferior lateral walls, and of side or flanking walls ; and, in obvious contrast 

 to these, of outer and inner walls. The direction at right angles to the surface, 

 i. e. that of the lateral walls, may be called the height of the cell ; length and breadth 

 will be used in the same sense as for the whole organ of highest rank, to w'hich 

 they belong. 



Form of the Epidermal cells (comp. Figs. 10-20, which follow below). 



a. Epidermis one layer of cells thick. 



The general form of the epidermal cells is endlessly various according to 

 the special cases. A.s a rule the diameters in the two directions parallel to 

 the surface are equal, or but slightly diff"erent, in parts which grow slowly and 

 equally in two or three dimensions, e. g. lamina of many leaves : but tha longitudinal 

 diameter is greatly developed in longitudinally extended organs, as most stems, 

 roots, narrow linear leaves, especially of the Monocotyledons, also on the nerves 



' Compare Mohl, Verm. .Schriften, p. 177. 



* Mettenius, Ueber d. Hymenophyllaceen, in Abhandl. d. sachs. Gesellsch. d. Wissensch. IX, 

 p. 403. 



^ Caspary, in Pringsheim's Jahrh. I. p. 49. 



