USES OF THE EPIDERMIS 



117 



140. Uses of the epidermis. 1 The epidermis, by its tough- 

 ness, tends to prevent mechanical injuries to the leaf. After 

 the change of the outer portions of its cell walls into a corky 

 substance it greatly diminishes evaporation from the general 

 surface. This process of becoming filled with cork material, 

 suberin (or a substance of similar properties known as cutin), is 

 essential to the safety of leaves or of young stems which have 

 to withstand heat and dryness. The corky or cutinized cell 

 wall is waterproof, while ordinary cellulose allows water to 

 soak through it with ease. 

 Merely examining sec- 

 tions of the various kinds 

 of epidermis will not give 

 nearly as good an idea of 

 their properties as can be 

 obtained by studying 

 during severe droughts 

 the behavior of plants 

 which have strongly cuti- 

 nized surfaces and of 

 those which have not, or 

 by exposing thin-leaved 

 plants and thick leath- 

 ery-leaved ones to a very dry atmosphere without watering. 

 Fig. 121, however, may convey some notion of the difference 

 between the two kinds of structure. 



In A the shaded part is all cutinized; it consists of the 

 thick cuticle proper and, beneath this, cutinized layers of cell 

 wall, under which is a heavy layer of cellulose. In B the cuticle 

 is thin, and the outer portion of the cell walls consists wholly 

 of cellulose. 



In most cases, as in the india-rubber tree, the external 

 epidermal cells (and often two or three layers of cells beneath 

 these) are filled with water, and thus serve as reservoirs from 

 1 See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 273-362. 



FIG. 121. Unequal development of cuticle 

 by epidermis cells 



A, epidermis of butcher's broom (Ruscus) ; J5, 

 epidermis of sunflower ; c, cuticle ; e, epider- 

 mis cells. After Frank and Tschirch 



