38 CELLULAR TISSUE. 



From these examples,' which might easily be multiplied, the conclusion is drawn 

 that the superficial position of the stomata is the rule for herbaceous less thick-skinned 

 parts, and the depressed position for leathery, succulent, and thick-skinned parts ; 

 but that this is by no means the case without exception. Further, that corresponding 

 parts of plants of the same family, otherwise of like nature, such as the firm leaves 

 of the Proteaces and Bromeliacese, may show the most extreme diversity. As an 

 instructive example the tender-skinned leaves of Salvinia natans may here be cited, 

 the small stomata of which are inserted about half way up the epidermal cells, which 

 are eight to nine times their height \ 



It is obvious that when the lateral wall of an epidermal cell abuts on a guard-cell, it 

 must present a difference of form and direction, which is in many cases very slight, 

 from those lateral walls which do not border on stomata. The relations of height of 

 the abutting face follow from what has been said above. The abutting face is in the 

 one series of cases nearly plane, and set perpendicular to the surface, or inclined 

 obliquely to it, in such a way that it converges with the corresponding face on the 

 other side of the stoma, towards the inner face. Both arrangements occur in 

 stomata which are even with the surface, the latter especially in stomata which 

 project outwards. Still cases occur of stomata seated in deep hollows, whi.ch abut 

 on their subsidiary cells with a plane perpendicular face^ In other cases the 

 abutting face is concave towards the stoma, and the guard-cells are fitted into the 

 hollow with their convex side, and are therefore more or less completely enclosed by 

 their neighbours. With this is always connected a depression of the stoma (often 

 only slight) below the outer surface : Iris, Amaryllis formosissima ', Graminese, &c. 

 In deeply depressed stomata (cf the examples given above), also in Iris and sirnilar 

 cases, it often happens that the abutting faces are inclined obliquely towards the 

 outer surface, so that they diverge inwards on both sides of the stomata. In this 

 case it comes about that the guard-cells lie mainly on the inner side of the neigh- 

 bouring cells (compare below. Fig. 24, Equisetum). - 



Irrespective of the faces just described abutting on the stoma, the neigljbouring 

 cells are in many cases of fundamentally similar form to those epidermal cells of the 

 same surface which do not abut on stomata, e.g. Lilium, Orchis *, Hyacinthus, 

 Helleborus, Poeonia, Vicia Faba, Sambucus nigra, many Ferns, Salvinia, and many 

 other plants from the most difierent families ^. But in a large number of epidermal 

 layers, especially of foliage leaves, each stoma is on the other hand bounded by one 

 or two or several epidermal cells, differing in form and size from the rest which do not 

 abut on stomata: these not unfrequently resemble the guard-cells themselves. 

 These peculiar neighbouring cells of the stomata are termed its subsidiary cells, or 

 subsidiary cells of the pore^. 



Their superficial form is generally intermediate between that of the guard-cells 

 and the epidermal cells, or they completely resemble the first. In the latter case the 



' Strasburger, /. c. Taf. XXXVI. figs. 29, 30. 



'^ Restio diffusus,,fasciculatns, Pfitzer in Pringsheim's Jahrb'. VII, Taf.-XXVII. figs. 1-5. 

 ' Von Mohl, Botan. Zeitg. 1856, Taf. XIII. figs. 2, 4. 



' Von Mohl, Botan. Zeitg. 1856. = Compare Strasburger, I.e. 



' Cellula lateraks. H. Krocker, de pi. epidermide. Pfitzer, Pringsheim's Jahib. VII. p. 536.— 

 Compare also Botan. Zeitg. 1871, p. 133 ; Hiilfsporenzellen, Strasburger,. I.e. 



