MEMBRANE-LENSES 



621 



water (a result which is easily produced in the lens-experiment 

 with the aid of a fine brush), the rounded apices of the cells project 

 above the water like so many islands, and continue to act as con- 

 densing lenses. Velvety leaves are thus able to perceive photic 

 stimuli, even if their surfaces are constantly wet. The conical 

 epidermal papillae of such leaves, therefore, most probably represent 

 an adaptation to the frequent or continual wetting to which the leaves 

 are exposed under natural conditions. 



Where the bulging outer wall is thin, with its two surfaces 

 parallel to one another, it is the cell-sap alone that constitutes the 

 refractive medium. If, on the contrary, 

 the outer wall is very thick, its high 

 refractive index may become of importance, 

 as Sperlich has shown ; the greatest amount 

 of refraction will be obtained if this wall 

 assumes a plano-convex, or even a biconvex 

 shape {e.g. in Faraday a, Paramignya, Al- 

 bertisia). 



In many cases, it is only a circum- 

 scribed area of the outer epidermal walls 

 that protrudes, and this portion is often 

 specially thickened so as to form a minute 

 but powerful condensing lens. In Colocasia 

 antiquorum (Fig. 257 a) and Aquilegia vul- 

 garis (Figs. 257 B, 259), the central pro- 

 tuberance resembles a concavo-convex lens. 

 The remarkably high refractive index of 

 this lens, in the case of Colocasia, is probably due to the presence of 

 pectic compounds. In Vinca major (Fig. 258, c) the very thick outer 

 wall bears at its centre a minute, scarcely perceptible papilla ; beneath 

 this protuberance, the thick cutinised layers are invaded by a plug-like 

 process of the cellulose layers, which are likewise very massive 

 structures. The foliar epidermis of Lonicera fragrantissima is furnished 

 with membrane-lenses of a somewhat flattened form ; here the 

 cutinised layers are separated from the cellulose stratum of the 

 outer wall by an intervening lamella composed of highly refractive 

 material, which expands to form plano-convex or bi-convex lenses 

 beneath the papillae. In this case the high refractive index of the 

 lens is principally due to impregnation of the cell-membrane with wax. 

 The outer walls of the adaxial epidermis of Campanula pcrsicifolia 

 contain strongly silicified bi-convex plugs (the "cell-wall plugs" of 

 Heinricher), which act as condensing lenses. In Petraca volubilis 

 also (Fig. 258 b), practically every cell of the foliar epidermis is 



Pig. 257. 



Local optical specialisation of outer 

 epidermal walls. A. Adaxial foliar 

 epidermal cell of Colocasia antiquorum. 

 B. Adaxial foliar epidermal cell of 

 Aquilegia vulgaris. See text. 



