ROOT STRUCTURE. 



63 



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g'-O'^n^aCl), 





afterward the superficial layer, and per- 

 sists for a longer or shorter period. Its 

 characteristics are of great importance in 

 histological determinations. The inner 

 most layer of the primary cortex is even 

 more distinct in appearance than the hy- 

 poderm, and is the Endodermis (c). It 

 lies in contact with the outer surface of 

 the structure dereloped from the plerom. 

 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 cov- 

 ering. A new meristematic region must 

 thee be established for the purpose of 

 manufacturing such a covering. This al- 

 most always arises in some part, and it 

 may be in any part, of the primary cor- 

 tex. It is the Phellogen. The phellogen 

 may be in the form of a continuous circle velopment in any part. 



or in that of blades or plates (d), vari- 

 ously placed and directed. Upon its outer 

 surface the phellogen develops corky tis- 

 sue, the Periderm, and upon its inner a 

 secondary cortex, the Phelloderm. Occa- 

 casionaJIy it will produce only periderm 

 or only phelloderm. As the periderm be- 

 comes impervious to the nourishing fluids 

 it, and all tissue exterior to it. must die, 

 and may be cast off, a new phellogen then 

 appearing further 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. In such 

 case the corky layers which become suc- 

 cessively superficial 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 de- 

 velops- The same feature will also de- 

 termine the amount and character of the 

 tissue, if any, existing between it and the 

 structure developed from the plerom. No 

 tissue developed directly or indirectly 

 from the periblem is in the form of dis- 

 tinct and regular bundles of vessels, 

 though irregular and isolated or anastomo- 

 sing tubes are frequently developed by it. 



The essential characteristic of the body 

 developed from the plerom of the root is 

 that it is invested by the endodermis and 

 is free from any other endodermal de- 



It therefore con- 



