234 



General Biology 



ym:\ 





hm-r- 



mm&. 





derm of stems is often ruptured and cast off as the inner tissue expands. 

 This does not occur in roots. When such casting off takes place, the 

 primary periderm is replaced by secondary periderm which develops 

 directly from the original phellogen or secondary phellogen layers. The 

 hypodermal and endodermal layers disappear as soon as the phellogen 

 is formed in the primary cortex. The primary fibro-vascular bundles 



become larger by new 

 a xylem and phloem ele- 



ments added by the cam- 

 bium, and the cambium 

 arcs extend until they 

 become a complete ring 

 or circle. 



New fibro-vascular 

 bundles form in the broad 

 medullary rays extending 

 between the original bun- 

 dles, while new woody 

 elements are being added 

 to the xylem. These 

 woody elements, however, 

 never entirely replace the 

 original plerom tissue in 

 the center of the stem. 

 This unchanged central 

 plerom tissue is the pith. 

 As the plerom paren- 

 chyma is entirely replaced 

 by woody tissues in roots, 

 the presence of pith is val- 

 uable in distinguishing 

 stem from root. 



The secondary or per- 

 manent stem tissues are 

 often divided into parenchyma ( ) and prosenchyma. 



Parenchymal cells may be found in all three zones of the embryo. They 

 have thin walls and protoplasmic contents. Prosenchymal cells are 

 formed in the plerom region of the embryo. They have thick walls, and 

 the protoplasmic contents are very inconspicuous or even entirely lack- 

 ing. While these distinctions are by no means absolute, they are of 

 great importance. Further, prosenchymal cells are usually spindle- 

 shaped, while parenchymal cells are more inclined to be spherical or 

 cubical with rounded corners. (Fig. 135.) 



Fig. 135. 



A. Early undifferentiated cells known as Embryonic or 

 Meristem tissue 



B. The secondary (permanent) tissues are divided into 

 parenchyma and prosenchyma. The former have thin wails 

 and protoplasmic contents. They are found in the undiffer- 

 entiated cellular structures of all three zones in the embryo. 

 They are usually spherical in shape, or at least "as broad 

 as they are long." Prosenchyma cells are formed in the 

 Plerom region of the embryo. They have thick walls and 

 little or no cell content. The cells are usually long fiber 

 cells with sharp-pointed ends. 



a. Transverse Section, Triticum Rhizome. 1. Epidermis. 

 2. Hypodermis. 3. Cortical parenchyma. 4. Endodermis. 

 5. Fibers, surrounding sieve and ducts. 6. Sieve. 7. Ducts. 

 8. Concentric fibrovascular bundle. 9. Pith parenchyma. 



b. Powdered Triticum Rhizome. 1. Epidermis. 2. Hypo- 

 dermis. 3. Parenchyma, longitudinal view. 4. Endodermis. 

 5. Fibers. 6. Vessels. (From C. W. Ballard's "Vegetable 

 Histology," Courtesy of John Wiley & Sons.) 



