EPIDERMIS. 



39 



arrangement is such that the whole convex side of each guard-cell is bordered by one 

 subsidiary cell ; the stoma thus appears to be surrounded by two pairs of cells, one 

 pair bounding the slit, and one peripheral to these (e.g. Graminese, Proteaceae, and 

 the other examples of two lateral subsidiary cells to be cited below) ; often even by 

 three pairs, since the first pair of subsidiary cells is often surrounded by a second 

 similar pair (Hakea ceratophylla, saligna \ &c.). 



- If there be a difference of height between the guard-cells and the epidermis, 

 the subsidiary cells often hold an intermediate position also in this respect ; where 

 the difference in height is great, they are of equal height to the guard-cells, or a litde 

 higher, and with them are fitted either in the outer surface, or at the bottom of the 

 depression. Rarely the subsidiary cells are much higher than the epidermal cells ; 

 this is the case in the Scitaminese (Strelitzia ovata, Heliconia farinosa, cf. Bot.- Ztg. 

 187 1, Taf. I. and our Fig. 28 B), where they connect the stoma with the Epidermis 

 and Hypoderma. 



The arrangement of the subsidiary cells may be most intelligibly described in connec- 

 tion with the history of their development, and that of the stoma ; this shall therefore 

 here be given. 



The stoma itself makes its appearance by the bisection of a cell of the epidermis, 

 which may be called its Mother-cell '^. The two products of division are the guard-cells. 

 When they separate from one another, as will be described below, a chink appears 

 between them. 



The development of the stomata takes place in the epidermis at the close of its meri- 

 stematic (dermatogen-) stage and not quite simultaneously in neighbouring parts, so that 

 one may find the most different stages of development close side by side. 



The origination of the stomata begins thus : The hitherto almost similar polyhedral 

 cells of the meristematic dermatogen are arranged in longitudinal rows, or irregularly: 

 either all, or the majority or only single ones of these divide into two dissimilar daughter- 

 cells. One of these becomes the Initial cell of the stoma, the other an Epidermal cell. 

 Where the dermatogen-cells form rows, it is as arule'' always the apical, or peripheral 

 part of the cell, which becomes the Initial cell. Exceptions to this are only known 

 among those abnormalities or deformities which will be described below as twin sto- 

 mata. Where the serial arrangement of the dermatogen-cells is absent, the relative 

 position of the initial cells is also indefinite. 



The wall which cuts off the initial cell is perpendicular to the Epidermis, or originally 

 only slightly oblique ; it either stretches as a plane (transverse-) wall between two lateral 

 walls of the developing dermatogen-cells ; or it is curved in surface view to a U-forra, 

 and then with its two ends it is attached either to one or two lateral faces of neigh- 

 bouring epidermal cells, or (as a rule in Aneimia) it has the form of a closed ring, which 

 touches no lateral wall. In the last case the initial cell is surrounded laterally by a ring- 

 shaped cell, in the preceding case by a cell of more or less horseshoe shape. 



In the further growth the three following chief cases occur :— 



I. The Initial cell is the direct Mother-cell of the stoma, and the epidermal cells 

 undergo no further division. This is the case in Iris, Hyacinthus, Orchis, Sambucus 

 nigra, Ruta graveolens, Salvinia natans, Selaginella denticulata, Asplenium furcatum; 

 Silene infiata, Chrysodium vulgare, the two last have a U-formed wall, the others a plane 

 division-wall*: further Aneimia has as a rule an annular wall. 



» Von Mohl, SpaltoETn. d. Proteaceen, I.e.; Strasburger, 

 = Specialrautterzelle, Strasburger, /. c, 

 ' Strasburger, /. c. ; Pfitzer, /. c. 

 * Compare Strasburger, I.e. 



