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 grai^e. 



Eoot-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 gi^'e 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, Pigs. 17, 15 and 3). The hardier A'arie- 

 ties of apple, as the Oldenburgh (Duchess), ha\'e 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 aboiit 150,000 stomata to the square 

 inch on its lower surface. 



