MORPHOLOGY OF HIGHER PLANTS. 



203 



opment of the secondary structure. There is a disintegration of 

 the cells of the endodermis (EN), cortex (C) and epidermis (E) 

 of the primary root, these being replaced by the secondary struc- 

 ture as shown in Fig. 112. 



The characteristics distinguishing the primary and secondary 

 structures of dicotyledonous roots may be summarized as follows : 

 Primary structure: Epidermis and root hairs. Hypoder- 

 mis. Primary cortex consisting of parenchyma. Endodermis. 

 Xylem arranged in radial rays which alternate with phloem plates, 

 constituting a radial fibrovascular.-bundle (Fig. no). 



Secondary structure: Corlj cells. Phellogen. Secondary 

 cortex consisting of parenchynaa.':?! Phloem, cambium and xylem, 

 arranged in radial groups, forming open collateral fibrovascular 

 bundles. Medullary rays separating the fibrovascular bundles. 

 Sometimes, as in glycyrrhiza and valerian, a number of paren- 

 chyma cells are found ip the, -center of the root, these constituting 

 the PITH (Fig. 115) or medulla; but they are usually wanting in 

 dicotyledonous roots. / 



Wood and bark are terms used to distinguish those portions 

 of the root or stem separated by the cambium ; all that portion 

 inside of the cambiuiu, including xylem, medullary rays and pith, 

 being known asithe wood. The bark includes the phloem, the 

 medullary rays outside of the cambium, and the tissue formed by 

 the phellogen. .' ,<;!/ 



The followiiie; di|^ram of the secondary- structure of a dicoty- 

 ledonous root rn^y be of assistance in understanding the origin 

 and relation of thg'jtissues comprising it.: 



' Pith, which may he wanting. 



Wood made up of . 



Cambium produces 



Bark made up of . 



Xylem . 



Phloem . . 



Composed of ducts, wood parenchyma 

 and wood fibers ; . or tracheids may 

 replace these cells, or be associated 

 with them. These are arranged in 

 groups forming radial rows which are 

 separated by medullary rays. 



■ Consisting of sieve and bast parenchy- 

 ma ; bast fibers may also be present. 

 These are arranged in collateral 

 groups and form radial rows which 

 ' are separated by medullary rays. 



Phellogen, producing parenchyma and cork. 



