COETICAL SYSTEM. CUTICLE. 



521 



Ficr. 570. 



air, become more or less. chemically changed, and at the same time fused, 

 as it were, into a continuous layer all over the surface of the organ ; and 

 by maceration, or applying 

 nitric acid, we may separate 

 this outer stratum as a con- 

 tinuous sheet or pellicle. This 

 layer, which strongly resists 

 decomposition, is called the 

 cuticle (figs. 569 & 571, a), 

 and is constituted by the 

 altered outer walls of the 

 cells. It is usually blended 

 with waxy material, which 

 sometimes exudes in the form 

 of "bloom," as on the sur- 

 face of some fruits and leaves. 

 Unlike cellulose, cuticular 

 structures are dissolved in 

 caustic potash, but not by 

 sulphuric acid. Iodine and 

 sulphuric acid stain them 

 yellow. These characteris- 

 tics are similar to those pos- 

 sessed by cork. Anilin and 

 sulphuric acid, according to 

 Wiesner, do not stain them 

 as they do vasculose. 



In Cycas the inner laminae 

 of the secondary deposits 

 exhibit pits like those found 

 on the walls of wood-cells ; 

 but this is a very rare phe- 

 nomenon. 



The aerial roots of Orchi- 

 daceae exhibit a curious struc- 

 ture, the growing extremities 

 being clothed by a whitish 

 cellular tissue composed of 



several layers of cells with a delicate spiral fibrous deposit on their walls. 

 - layer forms a kind of coat over the real epidermis of the root, and 



Vertical section of epidermal cells of old stem of 

 Viscum album. Magn. 400 diam. 



Fig. 571. 



Vertical section of epidermal cells of ILelleborus 

 foetidus: a, cuticle. Magn. 440 diam. 



This 



is known by the name of the velamen radicum. 



Hypoderm. In some cases, beneath the layers of epidermal cells 

 are layers of wood-cells (sclerencJiyma} or of elongated thick-walled 

 cells, like liber-cells. These serve to strengthen the epidermis, 

 and are called hypodennal cells. They originate from the 

 plerome. 



Cork. The young shoots of Dicotyledonous trees and shrubs 

 are clothed with epidermis like herbaceous plants ; but before the 

 close of the first season of growth, in most cases, the green colour 

 gives place to brown, which is owing to the formation of a layer 



