EPIDERMIS. 



3' 



and ribs of slightly elohgaled leaves. The opposite, that is to say, a great 

 transverse extension, occurs but rarely in the case of the epidermal cells in longi- 

 tudinally extended parts, as for instance in the leaves of Cycas, Encephalartos, Tra- 

 descahtia, Crassula, Campelia, Dichorisandra ^ many Bromeliaceas (Pholidophyllum 

 zonatum), and as a peculiarity 'of stems with clearly marked nodes, as Arceutho^ 

 bium, Salicornia. 



The height is as a rale either much snialler than the larger, or than both of 

 the diameters parallel to the surface, the cells are thus of the form of flat plates 

 overlying the surface ; or it is the greatest diameter of all, the cells are then prisms 

 arranged perpendicularly to the surface ; intermediate forms between these two 

 extremes are common enough. The lateral faces are flat, and cut one another with 

 sharp corners, so that the single cell has the form of an angular plate or prism. 

 In other and not less common cases they are wavy and folded, in which case the 

 hollows ami protuberances of neighbouring cells fit exactly into one another^. 



The extent of the waving may vary, or undulated and flat .lateral walls may 

 both occur iii equivalent parts of one amd the same species, according to the 

 adaptation to different surrounding media. Meyen' noticed this relation (which 

 remains to be further followed in other connections), somewhat indefinitely it is true, 

 for ' a large number of species of Gentiana,' in which he found the cells more 

 wavy 'the damper the region of the atmosphere in which the plant was grown.' 

 Conversely' Askenasy* found in Ranunculus aquatilis and divaricatus, on' the 

 submerged form that the epidermal cells of the leaf have flat sides, on the land- 

 form strongly undulated sides.. Also on the amphibious leaves of Marsilea and 

 Sagittaria ^ differences occur in the above-mentioned relation. 



The undulation usually extends equally over the whole height of the lateral wall, 

 but often, e. g. in leaves of grasses and Equisetum *, only over the strip along the 

 outer edge, while the inner part is flat. The outer and inner surfaces of the epi- 

 dermal cells are flat, or to a variable extent convex ; the latter either over the whole 

 extent of the cell, or at one (e.g. leaf of Aloe margaritifolia), or two, or several 

 (Equisetum hiemale) relatively small circumscribed spots. 



Other forms than those possible within the limits laid down are more rare j 

 e. g. spindle-shaped, elongated, oh the leaves of Torreya, Ceratozamia (Kraus, /. c.) ; 

 the often peculiarly formed subsidiary-cells surrounding the stomata, together with 

 the hair structures, must be specially mentioned below. 



One and the same epidermal surface often shows only epidermal cells of nearly 

 similar form: — g. g_ many smooth stems. Much oftener however considerable 

 differences occur on the same surface, (a) In relation to the relief of the surface, 

 and (often connected. with this) in the distribution of stomata and hairs; in angular 

 and ribbed stems in relation to the angles, or ribs on the one hand and the faces 

 or furrows on the other ; in flat leaves and leaf-like organs in relation to the ribs 



' Kraus, BaU d. Cycadeenfiedem. — Pringsteim, Jahrb. II. p. 318. 

 ^ Treviranus, Verm. Schr. IV. p. l(5.^Meyen, Phytotomie, p. 94. 



* Phytotomie, p. 95. 



* Botan. Zeitg. 1870^ No. 13. 

 ^ Hildebrand, ibid. No. i. 



° Mohl, Verm. Schriften,- p. 262.' — Mettenius, Hymenophyllaceen, p. 444. 



