EPIDERMIS 



47 



special functions. Most of those found on the leaf and stem, 

 however, must be classed with protective structures, protecting 

 the parts they cover from too intense sunlight, too rapid trans- 

 piration, attacks of animals, wetting and frosts, etc. 



101. The epidermis is elastic and stretches to a remarkable 

 degree as the parts covered by it expand with growth. But 

 on the roots and stems of perennials 



the limit of elasticity is reached after 



a few years and then the epidermis 



gives way, breaking in various ways 



and exposing the tissues beneath. Its 



place as a protective structure is taken 



by the underlying layers of the bark 



which have then become modified into 



cork. This serves more efficiently the 



function of protection than did the 



epidermis, though at the expense of 



depriving the tissues beneath of the 



sunlight which had before been trans- 



mitted by the transparent epidermis. FlG . ._,, Ho oked hair 



The corky layers are thick and opaque, from the stem of Phaseolus 



i , . i multiflorus: 2, climbing hair 



though at the same time extremely on stem of Humulus Lupu- 

 impervious, extremely poor conductors lus > 3, rod-like wax coating 



r ' r . on stem of Saccharum offic- 



Of heat, not readily yielding to the inarum; 4, climbing hair 



claw or tooth of beast or the beak of 

 bird, almost valueless as food for 

 animals and offering an excellent pro- 

 tection against the attacks of fungous parasites. The outer 

 layers of the cork are dead tissue, which usually splits into 

 ridges as the stem, expands, and later the outer layers even 

 scale off and drop away, while new layers are constantly 

 forming beneath from the cork cambium. 



102. The most delicate tissues of the plant are those in which 

 growth is taking place by the multiplication of cells. The three 



of Losa hispida; 5, stinging 

 hair of Urtica ureus. (From 

 Stevens, after deBary and 

 Haberlandt.) 



