EPIDERMIS. 



103 



parts, or one part is strongly silicified, another not at all. In the epidermis of many 

 grasses, the upper of the short epidermal cells, which are arranged in pairs one above 

 another, are distinguished from the rest by specially strong silicification of their wall, the 

 others are more slightly silicified, and in many cases not at all (Internodes of Saccharum 

 officinarum). Often the hairs are centres of silicification. They alone may be siUcified 

 (leaf of Campanula cervicaria, Ficus Joannis Boiss., Urtica excelsa, lusitanica, dioica) ; or 

 the process begins in them, and extends around the base of each hair centrifugally over 

 the epidermal surface, and spreads evenly or unevenly over it. In the latter case, even 

 in the mature organ, each hair is seated in the centre of a disc consisting of silicified 

 epidermal cells, which usually turns white after death. This is separated from other 

 similar discs by interspaces with weaker silicification (.f.^. leaf of Humulus, Ulmus cam- 

 pestris, Tectona grandis and other Verbenaceae, Cucurbitaceae, Pulmonaria saccharata, 

 Cerinthe major, Silphium connatum, Helianthus grosseserratus, many Dilleniaceae, 

 Chrysobalaneae, &c.), or by interspaces without silicification (leaf of Cerinthe aspera, 

 minor, Onosma stellulatum, arenarium, Lithosperiimm officinale, Helianthus tuberosus, 

 trachelifolius, &c.) Often {e.g. Ulmus campestris) the cell which is the centre of silici- 

 fication does not grow out to a hair. 



In many leaves, especially of Dicotyledonous plants {e.g. Humulus, Morus alba), the 

 epidermis of the upper side is much more strongly silicified than that of the lower : in 

 the latter the silicification appears often to be absent, while it is present in the former 

 {e.g. Helianthus trachelifolius, Heliopsis laevis, Obeliscaria columnaris). In all these 

 phenomena it is impossible to ignore a certain analogy with the cuticularisation of mem- 

 branes. 



On the intramural deposition of Calcium oxalate, comp. H. Graf zu Solms-Laubach, 

 Bot. Ztg. 1871 ; Pfitzer, in Flora, 1873, p. 97. 



The most remarkable points of deposition of Calcium carbonate in the epidermis 

 are those thickenings of the walls termed by Weddell ^ CysiolUhs. 



The Cystoliths of the Urticacese were discovered by Meyen in Ficus elastica: he 

 described them thoroughly in 1839^: later they, as well as those of allied plants, 

 were investigated by Payen ^ Schacht *, and Weddell (/. c). Schleiden " contributed 

 some observations on them, and brought forward a view of their morphological sig- 

 nificance, which, like some statements of Payen on their structure, have been corrected 

 by investigations recently conducted '. 



On the still folded leaf of Ficus elastica (Fig. 44), surrounded by its stipular 

 sheath, the epidermis of the upper side consists, long before the unfolding, of a single 

 layer of elongated prismatic cells with their longer axis perpendicular to the surface of 

 the leaf {A). These are mostly of equal size and similar shape. They are together 

 covered by the cuticle, and are mostly provided, below this, with a massive cellulose 

 outer wall, which exceeds the lateral walls in thickness. Single cells of this layer now 

 thicken their outer walls four to six or more times as strongly as the rest. The latter 

 then divide by longitudinal and transverse walls, to form the four-layered epidermis. 

 The cells with strongly thickened outer walls remain undivided, their outer wall grows 

 only very little further in the direction of the surface of the leaf, so that it soon appears 

 as a rather stronger band of membrane between laterally adjoining and thickening cell- 



' Ann. Sci. Nat. 4 ser. II. p. 267. ' MuUer's Archiv, 1839, p. 257. 



« Memoires sur le developpement des vegetaux. Mem. presentes de I'Acad. des. Sciences, . 

 torn. IX. 



• Abhandl. der Senckenbergischen Gesellsch. I. p. I33- ' Gnindzuge, 3 Aufl. p. 341. 



= [See also Penzig, Verbreitung d. Cystolithen, Bot. Contralbl, Ed. 8. 1881, p. 393.] 



