The Inner Structure of the Plantlet. 49 



formed into a layer that is more or less impervious to 

 water, called the cuticle (cu'-ti-cle), which serves to re- 

 strict evaporation (75). To further protect the parts, a 

 layer of wax (bloom) is sometimes secreted upon the 

 outside of the cuticle, as in the fruit of many varieties 

 of the plum and grape. 



Root-hairs (101) and the hairs and bristles on the 

 stems and leaves of many plants are cells of the epider- 

 mis elongated outward. The epidermis must not be con- 

 founded with the bark. It is replaced by bark in the 

 older stems of woody perennial plants. 



To give further strength to the upper surface of the 

 leaf, the first two or three tiers of cells beneath the epi- 

 dermis on the upper side are usually placed endwise, 

 (palisade cells, Figs. 17, 15 and 3). The hardier varie- 

 ties of apple, as the Oldenburgh (Duchess), have more 

 numerous and more crowded palisade cells than less 

 hardy varieties. Compare the palisade cells of a leaf 

 of the Oldenburgh apple (Fig. 17), with those of Fig. 

 3, which shows a section from a leaf of a tender variety 

 of apple. 



66. Stomata (stom'-a-ta). Minute openings through 

 the epidermis occur in the leaves and young stems of 

 land plants, connecting intercellular spaces (I, Fig. 17) 

 with the external air. These openings are each bounded 

 by a pair of crescent- shaped guard-cells, called stomata, 

 (singular, stoma, (sto'-ma), (Figs. 18 and 19, St). They 

 are chiefly found on the lower side of leaves, and are ex- 

 tremely numerous, but are too small to be seen without 

 the microscope. An average apple leaf has been com- 

 puted to contain about 150,000 stomata to the square 

 inch on its lower surface. 



