36 NATURE OF THE EPIDERMIS 



ance of the plants. Observe a corn field during a dry period. 

 The interesting feature of all this is that 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 sensitive. 



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- 



FIG. 22. Leaf position of the sensitive plant, Mimosa: A, in light. B, in 

 darkness. The same movements occur as a result of unfavorable temperatures 

 and humidity. 



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. This development of cutin gives the 

 tough leathery aspect to many leaves. Such features are partic- 

 ularly noticeable in desert plants, in long-lived leaves of many 



