THE PLANT CELL 81 



primary plerom elements. The primary root epider- 

 mis is replaced by bark structures which originate 

 in the periblem zone. Certain of the primary cortical 

 cells become meristematic and constitute the bark 

 cambium or phellogen. The phellogen cells rapidly 

 subdivide and the new tissue formed on their outer 

 side forms the periderm or bork, while the tissues 

 produced on their inner surface form the phellogen 

 or true bark. The phellogen retains its meristematic 

 power throughout the life of the individual and can 

 thus provide tissues to keep pace with internal growth. 

 The primary fibro-vascular bundles consists of either 

 xylem or phloem, and, in the change to secondary 

 structure, a meristematic tissue termed cambium is 

 developed in connection with these. The cambium 

 occurs on the outer face of each xylem bundle and on 

 the inner face of each phloem bundle. The cambium 

 arc on each xylem bundle produces xylem on its inner 

 face and phloem elements on its outer side. Similarly 

 the cambium arc on each phloem bundle produces 

 xylem on the inner side and phloem upon the outer. 

 Each fibro-vascular bundle now consists of xylem 

 and phloem elements separated from each other by 

 a strip of cambium. The bundles of secondary struc- 

 ture which have been completed by the cambium are 

 termed complete fibro-vascular bundles in contradis- 

 tinction to the incomplete fibro-vascular bundles of 

 primary structure. Growth through activity of the 

 cambium is continuous, and finally the parenchymatic 

 tissues of the plerom are almost entirely replaced by 

 xylem. Formation of new fibro-vascular bundles 

 takes place in the broad primary medullary rays, 

 resulting in multiplication of these structures which 



