THE PLANT CELL 83 



are derived from the original plerom parenchyma. 

 The arrangement of the various secondary root struc- 

 tures is diagrammatically represented in Plate 31. 



Primary Stem Tissues. In sections of young 

 stems the dermatogen, periblem and plerom zones 

 are apparent. The dermatogen zone in the stem 

 gives rise to an epidermis consisting either of one 

 layer or of several layers which may show slight 

 variations in structure. Tissues corresponding to 

 those of the root cap and root hairs are not developed, 

 although the stem epidermis may be clothed with 

 various types of hairs. The primary stem epidermis 

 may possess stomata or breathing pores, structures 

 which are never present in the primary epidermal 

 layer of roots. The periblem zone in stems gives rise 

 to hypodermal and endodermal tissues similar to those 

 of the root. The hypodermal cells may contain 

 chlorophyl or green coloring material, which is never 

 present in the corresponding tissue of roots. In most 

 instances, the endodermal cells of primary stems are 

 not as well developed as those of roots. The plerom 

 zone of primary stems is quite different from that of 

 primary roots, as regards arrangement and develop- 

 ment of tissues. The fibre-vascular bundles developed 

 in the plerom region of the primary stem are complete 

 bundles, showing phloem, xylem and cambium elements 

 even in the earlier stages of growth (Plate 31). There- 

 fore the primary fibro-vascular bundles of the stem 

 show structures found only in the secondary bundles 

 of the root. 



Secondary Stem Tissues. In annual plants the 

 primary stem structures heretofore described persist 

 throughout the short life of the individual; but in 



