84 THE ELEMENTS OF VEGETABLE HISTOLOGY 



perennials provision must be made for a more durable 

 covering tissue and the extension of the plerom struc- 

 tures. The primary epidermis is replaced by periderm 

 tissues produced by a phellogen developed in the pri- 

 mary cortex. The periderm of stems may differ from 

 that of roots in that it is often cast off or ruptured 

 because of expansion of the inner tissues through 

 growth. Primary periderm thus destroyed is replaced 

 by a secondary periderm arising directly from the 

 original phellogen or secondary phellogen layers. 

 The hypodermal and endodermal layers disappear 

 with the formation of phellogen within the primary 

 cortex. The primary fibro-vascular bundles increase 

 in size through the addition of xylem and phloem 

 elements by the cambium. The short arcs of cam- 

 bium are extended laterally so as to form a complete 

 cambium ring or circle. New fibro-vascular bundles 

 are formed in the broad medullary rays which extend 

 between the original bundles. Although new woody 

 elements are continuously added to the original xylem 

 bundles, the growth of wood never entirely replaces 

 the original plerom tissue in the center of the stem. 

 This unchanged plerom parenchyma in the center of 

 stems is termed the pith. In roots, the woody tissues 

 entirely replace the plerom parenchyma; therefore 

 presence of a pith region is characteristic of stem 

 structure. 



Although the primary tissues serve well enough 

 in the earlier stages of growth, full development 

 necessitates the production of secondary or permanent 

 tissues, and these may be roughly divided into two 

 groups, parenchyma and prosenchyma. Representa- 

 tive parenchyma cells are found in the undifferentiated 



