42 BOTANY. 



which extends uninterruptedly over the cells, and may be 

 readily distinguished from the other portions of the outer 

 epidermal walls. It is insoluble in concentrated sulphuric 

 acid, but may be dissolved in boiling caustic potash. 

 Treated with iodine it turns a yellow or yellowish-brown 

 color. A waxy or resinous matter is frequently developed 

 upon the surface of the cuticle, constituting what is called 

 the bloom of some leaves and fruits. 



74. The protoplasm of the epidermal cells generally dis- 

 appears in those cases where there is much thickening 6f 

 the walls; it is always present in young plants and parts 

 of plants; it is also frequently present in older portions, 

 which are not so much exposed to the drying action of the 

 atmosphere, as in roots, and the leaves and shoots of aquatic 

 plants and of those growing in humid places. In few 

 cases, however, are granular protoplasmic bodies (e.g., 

 chloroplasts) present in epidermal cells. 



75. While the epidermis always consists at first of but 

 one Ikyer of cells, it may become split into two or more 

 layers by subsequent divisions parallel to its surface, as in 

 the Oleander and Cactus. 



76. The Hairs of the epidermis originate mostly from the 

 growth of single epidermal cells, and on their first appear- 

 ance consist of slightly enlarged and protruding cells (Fig. 

 26, e, f, c). These may elongate and form single-celled 

 hairs, which may be simple or variously branched. The 

 most important of these hairs are those which clothe so 

 abundantly the young roots of most of the higher plants, 

 and to which the name of Eoot-hairs has been applied 

 (Pig. 37). These are composed of single cells, which have 

 very thin and delicate walls, and are the active agents in 

 the absorption of nutritive matters for the plant. Some- 



