THE GROUPS OF TISSUES, OR TISSUE-SYSTEMS. 59 



fundamental system are so disposed that the periphery is 

 harder and firmer than the usually soft interior, although 

 there are many exceptions. This general structure has 

 given rise to the term Hypoderma for those portions of the 

 fundamental system which lie immediately beneath or near 

 to the epidermis. Hypoderma is not a distinctly limited 



Fto. 38. — Transverse section of one-year-old stem of Ailanthus. c, epider 

 mis ; k, cork-cells ; r, inner green cells ; between k and r a layer of cells 

 filled with protoplasm, called the phellogen, or pork-cambium. Magnified 

 350 times. 



portion — in fact, it is often difficult to say how far it does 

 extend ; however, it usually includes several, or even many, 

 layers of cells, or the whole of each of the tissue-masses 

 (e.g., thick-angled, stony, and fibrous tissues, etc.) which 

 immediately underlie the epidermis. 



104. Cork.^ — ^ Within the zone which the hypoderma in- 

 cludes there frequently takes place a peculiar development 

 of the young parenchyma, giving rise to layers of dead 

 cells, whose cavities are filled with air only. The walls in 



