EPIDERMIS AS WATERPROOF COVERING 65 



As might be expected, the amount of cutinization, as well as 

 thickness of the outer wall, depends upon the severity of the 

 demands made by the environment. In submerged water plants 

 the outer wall is usually thin and little if at all cutinized, while 

 parts rising above the surface of the water show greater thickness 

 of wall and more cutinization. In land plants the waterproofing 

 characters become more pronounced and reach their fullest 

 development in desert regions, or in alpine and arctic regions, 

 and bogs and salt marshes, where the water, although present 

 in abundance, is difficult of absorption on account of its low 

 temperature or the inimical nature of the substances dissolved 

 in it. Plants of the same kind grown in dry and in moist atmos- 



FIG. 32.- A, portion of cross section of leaf of Avicennia growing in salty soil; outer 

 wall of epidermis very thick. B, cross section through skin of apple. C, cross section 

 through upper half of petal of Japan quince. D, upper, and E, lower epidermis of leaf of 

 Hibiscus moscheutos. F, epidermis of leaf of Lactuca scariola in the sun; and G, in the 

 shade. 



spheres are apt to differ decidedly in their epidermal defenses; 

 and even the different parts of the same plant or the same mem- 

 bers of a plant are apt to differ in this respect according to the 

 demands made upon them. Thus, the upper epidermis of a 

 leaf has, as a rule, a thicker and more highly cutinized outer wall 

 than the lower epidermis (Fig. 32), and the epidermis of the petals 

 of a flower, which are to endure for so short a time, is insignifi- 

 5 



