160 



ACADEMIC BOTANY. 



393. Leaf-epidermis. — The 



cells of the leaf-epidermis 

 (Fig. 233) are flattened, 

 coherent by their edges, 

 and variously shaped. 

 Here and there between 

 the walls of two adja- 

 cent cells small open- 

 ings appear (Fig. 233, 

 s, s); these are the 

 storaata. 



The stomata communioate 

 with the air-chambers and 

 lacunes [1) in the paren- 

 chyma, thus making direct 

 connection between the 

 plant and the open air and 

 establishing thorough cir- 

 culation from the topmost 

 leaf of the tallest tree to 

 the fibrils of its deepest 

 root. We have already 

 There are no stomata in 



Fig. 233.— Epidermi" of If. of Jf)ower-de luce (TrU 

 gprnumica) ; s, s, stomata ', p,p, cuticle, or pellicle ; 

 /, opening in the cuticle, corresponding to the 

 stomata of the epidermis, which has been removed. 



seen them in the Acrogens (Fig. 29). 

 Thallogens. 



394. The plant respires through the stomata as animals 

 do through the pores of the skin, exhaling certain elements 

 and inhaling others. 



Each stoma consists of two oblong cells, which have been compared 

 to lips ; they open or shut the orifice, thus controlling respiration. 

 These tiny door-keepers do their work with exemplary fidelity ; open- 

 ing wide when the air is moist, that inhalation may. be promoted, but 

 closing promptly when it is dry, lest the precious juices should be con- 

 sumed by drought. The stomata, like the air-chambers, are most 

 abundant on the under surface of the leaf. The vine has none on its 

 upper face, and 13,000 to the square inch on its lower one. The lilac 

 has few on its upper face, and 160,000 to the square inch on the lower. 

 The mistletoe has nearly an equal number on each face, — 200 to the 

 square inch. 



395. The Outicle, or Pellicle (L. pellicula, little skin), is 

 a thin membrane covering the epidermis (Fig. 233, p, p). 

 It has no cells, but is a mere expansion of the cell-wall, 

 and separable from it. It is pierced with little openings 

 (/) corresponding to the stomata. 



