32 NATURE OF THE EPIDERMIS 



these reactions and adjustments are purposive but not intelligent. 

 The movements are not called forth by consciousness but by 

 stimuli to which the irritable living substance is attuned or sensi- 

 tive. 



1 6. The Significance of Certain External Leaf Structures. 

 Many features connected with the structure of the leaf furnish 

 admirable illustrations of the fitness of the leaf for the perform- 

 ance of its work. The stimulus of light and moisture have a 

 marked influence upon the external structure and form of leaves. 

 The epidermis is strikingly modified by such forces. In shade 

 plants the epidermis consists of a rather delicate layer of cells 

 with very thin cuticle. This gives sufficient protection to such 

 plants, but in leaves exposed to intense -sunlight and a hot and 

 dry air, the epidermal cells become greatly thickened and often 

 of two or more rows, while the cuticle may often form the larger 

 part of the outer cell wall or even extend in between the cells 

 (Fig. 23, c). Cell walls that are filled with cutin in this manner 

 are said to be cutinized. Such features are particularly notice- 

 able in desert plants, in long-lived leaves of many evergreens, 

 as laurels and rhododendrons, in the needles of cone-bearing 

 trees, and in all plants that are exposed to the hot, dry air of 

 summer or the drying winds of winter. Such leaves may be 

 further protected and strengthened by thick walled elongated 

 cells, stereome (Fig. 23, st). Coatings of wax, mucilage and 



FIG. 23. Cross-section of the outer cells of a leaf of pine, showing the 

 firm character of the outer cells of the tough leaf : s, stoma ; e, epidermis ; 

 c, cuticle; st, stereome; m, mesophyll cells. 



lime are also frequently developed upon the cuticle to further 

 re-inforce the impermeability of the epidermis. 



