164 THE ELEMENTS OF VEGETABLE HISTOLOGY 



zone of stems gives rise to a primary epidermis, but 

 does not form tissues corresponding to those of the 

 root cap and root hairs. The primary epidermis is 

 strongly cutinized, may possess stomata and may 

 give origin to plant hairs or trichomes. The periblem 

 .zone of stems gives rise to a hypodermis and an endo- 

 dermis, between which are several layers of parenchyma 

 cells, constituting the primary cortex. The hypo- 

 dermis of stems usually contains chlorophyll or green 

 coloring material. The walls of the primary cortical 

 cells may become thickened through deposition of 

 cellulose and assume collenchymatous forms. The 

 endodermis of stems is not as well developed as that 

 of the root. Fibro-vascular bundles are produced in 

 the plerom zone; but these bundles are complete in 

 form and consist of xylem, cambium and phloem 

 elements even in the earlier stages. The pith paren- 

 chyma, which in roots is gradually replaced by woody 

 tissues, persists in stems even throughout secondary 

 structure. The tissues usually present in stems and 

 rhizomes of primary structure, and the order in which 

 they occur, beginning with the outermost, are as 

 follows : 



1. Epidermis, 



2. Hypodermis, 



3. Cortical parenchyma, 



4. Endodermis, 



5. Phloem elements, 



6. Cambium, 



7. Xylem elements, 



8. Pith parenchyma. 



By reference to Plate 56 it will be noted that each 

 of these tissues presents structural characteristics 

 whereby it may be distinguished. Presence of pri- 



