THE PRIMARY PERMANENT TISSUES 



29 



proof. This occurs in its greatest purity as a thin film at the 

 outer surface known as the cuticle. In some instances scales 

 and rods of wax are deposited at the surface and cause the so- 

 called bloom, as on the plum, grape, stems of some grasses, etc., 

 where they are easily rubbed 

 off on account of their deli- 

 cacy. In sections under the 

 microscope the wax, cuticle, 

 and cutinized parts of the 

 wall are readily distinguish- 

 able by their yellow color 

 when treated with chloro- 

 iodide of zinc, while the 

 cellulose portions of the wall 

 assume at the same time a 

 purplish color. 



The thickening and cutini- 

 zation of the outer wall makes 

 of the epidermis an excellent 

 protective tissue against loss 

 of water, parasitic fungi, and, 

 to a certain extent, mechan- 

 ical injury. How quickly, 

 for example, an apple will 

 begin to dry up and decay 

 when its epidermis is pared 

 away. 



The cells of the epidermis 

 remain living so long as they 



are not cut off from the water supply by the formation of cork 

 tissue. Their protoplasts exist as a very thin and hardly dis- 

 tinguishable film lining the walls. In rare instances, particularly 

 in the Monocotyledons, the epidermal cells contain chloroplasts; 

 and sometimes, as in fruits and foliage plants, a colored cell- 

 sap; but in most cases they are colorless and permit the unim- 

 peded entrance of light. While they live the epidermal cells 



Fig. 12. — I, epidermis of oak leaf; 2, 

 epidermis of Iris leaf, both viewed from the 

 surface; 3, group of cells from petal of Viola 

 tricolor; 4, two epidermal cells in cross section 

 showing thickened outer wall differentiated 

 into three layers, namely, an outer cuticle, 

 cutinized layer (shaded), and an inner 

 cellulose layer; 5 and 6, epidermal outgrowths 

 in the form of scales and hairs. (3 after 

 Strasburger, 4 after Sachs, and s after de 

 Bary.) 



