THE GROWING PLANT 



41 



64. The epidermis. — The plant is covered by thin, 

 translucent skin that extends over the entire surface of 

 the leaves, stem and root, called the epidermis (Fig. 17 

 Ep.). This skin is formed of comparatively thick-walled 

 cells and serves to protect the more delicate parts within. 

 It may be readily withdrawn in some plants, as from the 

 leaf of the liveforever {Sedum telephuim), the cotyledon of 

 echeveria, and young stem of the plum. The ex- 

 posed surface of the epider- 

 mis of the leaves, fruit and 

 young stems of many plants 

 is transformed into a layer 

 that is more or less imper- 

 vious to water, called the 

 cuticle, which serves to re- 

 strict evaporation (74) . To 

 protect further the parts, a 

 layer of wax (bloom) is 

 sometimes secreted upon the 

 outside of the cuticle, as in 



the fruit of many varieties Fig- 17. — Section through the 



J. ,, , 1 leaf of Oldenburg apple. Ep. 



of the plum and grape. epidermis ; Pal. palisade cells ; I 



Root-hairs (100) and the intercellular spaces. Highly 



, . 1 1 • J.1 J.1 magnified. See also Figs. 13 and 



hairs and bristles on the 20. 



stems and leaves of- many 



plants are cells of the epidermis elongated outward. 



The epidermis must not be confounded with the bark. 



It is replaced by bark in the older stems of woody 



perennial plants. 



On the upper surface of the leaf, the first two or three 

 tiers of cells beneath the epidermis are usually placed end- 

 wise (palisade cells, Figs, 17, 15 and 3). This arrange- 

 ment favors the entrance and absorption of light. Certain 



