110 DERMAL SYSTEM 



walls is increased by impregnation with silica or lime. Measurements 

 of the relative hardness of silicified and calcined epidermal walls have 

 been made by Emma Ott on the lines of the scratching method 

 customary in mineralogy ; it appears that the stems of various species 

 of Equisetum (e.g. E. ramosum, E. litorale, E. silvaticum, E. pratensc, etc.) 

 are capable of scratching calcite, which means that the silicified 

 epidermis has a relative hardness of 3 on the Mohs scale. The more 

 highly silicified epidermal walls of E. hiemale and E. Telmateia scratches 

 iluorite (relative hardness = 4), while the spermoderm of Coix Lacryma 

 actually scratches opal (relative hardness =7). Calcified walls may 

 attain to a relative hardness of 3 ; the seed-coat of Celtis, for example, 

 scratches calcite readily. It is not surprising, therefore, to find that 

 Grasses, Sedges and Horsetails, which all have highly silicified 

 epidermal walls, are to a great extent immune from the assaults of 

 snails. 52 Stahl has remarked that snails will readily devour grass 

 which has been cultivated in the absence of silica, whereas they 

 invariably refuse normal silicified blades, or at most eat them with 

 obvious reluctance. The inclusion of numerous small crystals of 

 calcium oxalate in the outer epidermal wall, a condition exemplified, for 

 instance, by Welwitschia mirabilis, by various species of Mesrmbryan- 

 themum and by many Nyctagineae, probably subserves a similar 

 ecological purpose. 53 



Certain peculiarities of the outer surface of epidermal layers 

 may be mentioned at this stage. Many plants which grow in sunny 

 situations have leaves with smooth and shining upper surfaces. Such 

 polished or varnished leaves are especially common in the tropics, where 

 the reflection of light by foliage constitutes a ubiquitous and charac- 

 teristic physiognomical feature of the vegetation. 54 It is probably 

 permissible to regard this reflective power of the cuticle as a protection 

 against excessive insolation, since it undoubtedly has the effect of 

 preventing a certain proportion of the incident light from penetrating 

 into the leaf. An epidermis which is very smooth is often also easily 

 wetted ; rain falling on such an epidermis runs oft' quickly, or else 

 spreads over the leaf as a thin film, which soon evaporates. Obviously 

 this method of removing water from the leaf-surface is unsuitable 

 where the upper epidermis bears stomata, since these structures would 

 be constantly exposed to the danger of capillary occlusion. The 

 stoma tic epidermis is therefore usually provided with a waxy covering, 

 from which raindrops roll off without wetting the surface at all. 



The velvety leaf-surfaces of many tropical plants (Cyanophyllwm 

 magnificum, spp. of Begonia, various Marantaceae and Orchidaceae, 

 etc.) are also very easily wetted, as Stahl has shown ; 55 here the outer 

 wall of each epidermal cell assumes the form of a prominent papilla, 



