140 GENERAL STRUCTURE OF ROOT AND STEM 



The Ilypodermis. — Its outermost portion, usually of one layer of 

 cells, presents a different appearance from the subjacent layers, and 

 is the Hypodermis (Fig. 422, h) in the case of the root becoming the 

 Exodermis. The hypoderm lies against the inner face of the epiderm 

 (a), while that persists, l)ecoming afterward the superficial layer, and 

 persists for a longer or shorter period. Its characteristics are of great 

 importance in histological determinations. 



The Endodermis. — The innermost layer of the primary cortex is even 

 more distinct in appearance than the hypoderm, and is the Endodermis 

 (c). It lies in contact with the outer surface of the structure developed 

 from the plerom. 



Disappearance of the Primary Cortex. — The production of primary 

 cortex is quickly completed. If then the growth inside of it continues 

 indefinitely it, in most plants, involves the destruction and disappearance 

 of the primary cortex, which must be replaced by some other covering. 



Promsion by Phellogen for a New 'Covering. — A new meristematic 

 region must then be established for the purpose of manufacturing such 

 a covering. This almost always arises in some part, and it may be in any 

 part, of the primary cortex. It is the Phellogen (Fig. 422, d). The 

 phellogen may be in the form of a continuous circle or the usual form 

 in that of blades or plates (d), variously placed and directed. 



Periderm and PheUoderm. — Upon its outer surface the phellogen 

 develops corky tissue, the Periderm, and upon its inner a secondary 

 cortex, the PheUoderm. Occasionally it will produce only periderm or 

 only phelloderm. 



Secondary Periderm. — As the periderm becomes impervious to the 

 nourishing fluids, it and all the tissues exterior to it must die, and may 

 be cast off, a new phellogen then appearing farther toward the interior 

 to form a new periderm, so that we may have successive periderms — 

 the primary, secondary, and so on. This process is comparatively' rare 

 in the case of the root, very common in that of the stem. 



The Bork. — In such cases, the corky layers which become successively 

 superficial, observed in the scales of bark which peel off from tree-trunks, 

 constitute the Bork or Rhytidoma. Bork is called Ring-bork when it 

 forms a cylinder, Scale-bork when it occurs in detached plates. It must 

 be noted that the origin of the bork, and, as will be shown later, its 

 structural nature dependent thereon, will depend upon the depth at 

 which the phellogen develops. The same feature will also determine the 

 amount and character of the tissue, if any, existing between it and the 

 structure developed from the plerom. 



