CHAPTER XL 



TEGQMENTARY SYSTEM EPIDERMIS. 



Persistence and thickness of epidermis Cuticle Functional im- 

 portance Mucilage, wax, resin, &c. Structure of epidermis 

 Cell-contents Outgrowths of epidermis Hairs Glands 

 Emergences Prickles, &c. 



WITH the exception of some cases reserved for discus- 

 sion later, the whole of the shoot-system of a higher 

 plant, at any rate while young, is covered by a layer of 

 cells known as the epidermis. In the typical case, as 

 exemplified by herbaceous stems, leaves, shoots of trees 

 and shrubs, and all the parts of flowers, this epidermis 

 originates in the same way and is one cell thick ; but in 

 the cases departing from the type, after-changes bring it 

 about that the epidermis is composed of more than a 

 single layer of cells, or the layer which acts physio- 

 logically as an epidermal covering has a different origin 

 from the true epidermis, and is therefore something mor- 

 phologically different. 



These cases, again, have to be distinguished from 

 those where the true epidermis is eventually cast off, 

 and other coverings provide the protection hitherto ful- 

 filled by that layer, as occurs sooner or later in most twigs 

 and branches and other perennial organs of higher plants. 



w. i. 6 



