94 BOTANT. 



Caryophyllus), etc. ; (4) coatings of minute needles or grains 

 irregularly covering the surface with several layers — e.g., on 

 the leaves of Eucalyptus globulus, rye, etc. 



126. — The protoplasm of the epidermal . cells generally 

 disappears in those cases where there is much thickening of 

 the walls ; it is always present in young plants and parts of 

 plants ; it is also frequently present in older portions, which 

 are not so much exposed to the drying action of the atmos- 

 phere, as in roots, and the leaves and shoots of aquatic plants, 

 and of those growing in humid places. In few cases, how- 

 ever, are granular protoplasmic bodies {e.g., chlorophyll) pres- 

 ent in epidermal cells.* 



127. — While the epidermis always consists at first of but 

 one layer of cells, it may become split into two or more lay- 

 ers by subsequent divisions parallel to its surface. These 

 layers may resemble the outer one and have their walls 

 thickened, as in the leaves of the Oleander, or they may con- 

 sist of thin-walled cells with watery contents (constituting 

 the so-called Aqueous Tissue), as in the leaves of Picus and 

 Begonia. 



{a) Epidermis may be studied with comparatively little difficulty. 

 In many cases it may be stripped off in thin sheets and mounted in 

 tlie usual way ; such preparations, with thin cross-sections (which are 

 readily made by placing a piece of leaf between pieces of elder pith), 

 are sufficient, in most cases, to give a good knowledge of the structure. 

 The leaves of m&ny LUiacecB (hyacinths, lilies, etc.) and Qraminem may 

 be examined for regular cells, and those of many Dicotyledons, as bal- 

 sams, primroses, and fuchsias, for irregular ones. 



(6) Thickened epidermal walls may be found in leaves of a hard tex- 

 ture, as those of the pines, holly, oleander, mistletoe, many Oompositm, 

 and in the stems of many Cactacece. The stratification of the thickened 

 walls may be brought out in the cross-sections by heating in a solution 

 of potash. 



(c) A series of specimens of the epidermis, taken from leaves of all 

 ages, from their youngest and smallest rudiments in the bud up to full- 

 grown ones, is instructive. 



* In the leaves of Primuln, sinensis, grown in the green-house, the 

 epidermal cells contain many chlorophyll-bodies ; the leaves of Fuchsias, 

 under similar conditions, possess a few chlorophyll-bodies in the epider-, 

 mal layer. 



