EPIDERMIS. 67 



to yield twenty-five pounds of wax to each tree. Bayberiy wax 

 is a more familiar example. 



226. To such waxy coatings is due the glaucous appearance 

 of the lea\'es and fruits ol' many plants. The coatings are chiefl3' 

 of the following kinds (de Bary ') : — 



1. Coherent laj'ers or incrustations upon the epidefmis. 2. 

 Crowded vertical rods of considerable length, as, for instance, 

 those on tlie internodes of Saccharum offlcinarum, from ten to 

 fifteen hundredths of a millimeter in height. 3. Very short 

 rods or rounded grains. These, on the leaves of Tropoeolum, 

 are not very near together, but on those of the cabbage, tulip, 

 etc., are more crowded. 4. When the grains are more minute, 

 and have the shape of needles irregularly massed together, they 

 constitute the peculiar bloom of the leaves of Eucalyptus, 

 Eicinus, etc. 



227. Between the above kinds there are many intermediate 

 ones, Agave Americana, for instance, furnishing forms between 

 the two last named. 



228. Epidermal cells proper have a delicate lining of proto- 

 plasm and a distinct nucleus. The cell-sap is generally colorless 

 and transparent, allowing light to pass with very little obstruc- 

 tion to the layers beneath the epidermis ; but in some cases 

 it is so colored as to impart a conspicuous hue to the plant. 

 In manj' water-plants there is no well-marked distinction be- 

 tween epidermis and the subjacent tissue, even the cells of 

 the upper layer containing chlorophyll, but epidermal cells are 

 mostly free from either chlorophyll or starch. Brongniart has 

 shown that some amphibious plants have chlorophyll in the 

 epidermal cells of the aquatic but not of the terrestrial form. 

 That the rule is not universal is shown by Callitriche, which, 

 according to Hegelmaier, has epidermis without chlorophyll in 

 both forms. 



229. Epidermis usually consists of only one stratum of cells, 

 but it may be made up of two, three, or even more layers. 

 Division of the original epidermal cells by one or more partitions 

 parallel to the surface of the leaf gives rise to superposed cells ; 

 and thus multiple epidermis results, as in the upper surface of 



cuticle, which is soluble in caustic potash, and in most cases forms but a very 

 thin coating over the epidermal cells" (Veg. Cell, Henfrey's trans., p. 35). 

 Good examples for study of the different kinds of cuticular infiltrations are 

 afforded by the following, — leaves of Dianthuscaryophyllus, Galanthus nivalis, 

 Ilex, Finns, Hoya, Sassafras, and Taxus, and twigs of Viscura and of Oleander. 

 1 Botanische Zeitung, 1871. 



