158 THE ELEMENTS OF VEGETABLE HISTOLOGY 

 SECONDARY ROOT STRUCTURE 



Secondary root structures are present in mature 

 dicotyledonous plants. They represent the perma- 

 nent tissues of this class of plants and are replacements 

 of, or enlargements upon, the primary tissues pre- 

 viously described. The changes occurring in the 

 transition from primary to secondary structure may 

 be summarized as follows: 



1. The primary epidermis and hypodermis are replaced by 



tissues originating from a phellogen or bark cambium 

 which develops within the primary cortex. 



2. The endodermis is gradually replaced by tissues developed 



from the cambium. 



3. The incomplete primary bundles are completed by the 



addition of xylem tissues on the inner face of the phloem 

 bundles and phloem tissues on the outer face of each 

 xylem bundle. 



4. Appearance of distinct medullary rays between the fibro- 



vascular bundles. 



5. Replacement of the pith parenchyma by xylem tissues. 



As shown in Plate 55, the tissues usually present 

 in roots of secondary structure and the order in which 

 they occur, beginning at the outermost layer, are as 

 follows: 



1. Cork, 



2. Phellogen, 



3. Cortical parenchyma, 



4. Phloem tissues, 



5. Cambium, 



6. Xylem tissues, 



7. Medullary rays. 



CHARACTERS OF THE SECONDARY ROOT TISSUES 



Cork. — The corky tissues which replace the pri- 

 mary epidermis usually consist of several layers of 

 dark-colored cells. The individual cells are thin- 

 walled, and may contain tannin deposits. In trans- 



