MORPHOLOGY OF HIGHER PLANTS. 



317 



structures of dicotyledonous roots may be summarized as follows : 

 PRIMARY STRUCTURE: Epidermis and root-hairs. Hypoder- 

 mis. Primary cortex consisting of parenchyma. Endodermis, 

 pericambium, xylem arranged in radial rays which alternate 

 with phloem or sieve strands, constituting a radial nbrovascular 

 bundle (Figs. 169-174). 



SECONDARY STRUCTURE: Cork cells, phellogen, secondary cor- 

 tex consisting of parenchyma. Phloem, cambium, and xylem 

 arranged in radial groups, forming open collateral nbrovascular 

 bundles. Medullary rays separating the fibrovascular bundles 



(Figs. I75- 1 /;)- 



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

 chyma cells are found in the center of the root, these constituting 

 the PITH (Fig. 176) 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 cambium, including xylem, medullary rays, and 

 pith, being known as the WOOD. The BARK includes the hadrome, 

 the medullary rays outside of the cambium, and the tissues formed 

 by the phellogen, viz., secondary cortical tissue and cork. 



The following diagram of the secondary structure of a dicoty- 

 ledonous root may be of assistance in understanding the origin 

 and relation of the tissues comprising it : 



Wood made up of 



Cambium produces 



Bark made up of. 



Pith, which may be wanting. 



['Composed of vessels, wood parenchy- 

 i ma and wood fibers ; or tracheids may 

 Xylem </ replace these cells, or be associated 

 with them. These are arranged in 

 groups forming radial rows which 

 are separated by medullary rays. 



Consisting of leptome and companion 

 cells; bast fibers may also be present. 

 Phloem.. 1 These are arranged in collateral 

 groups and form radial rows which 

 are separated by medullary rays. 



Meristem of pericambium producing pericambial- 

 cork and parenchyma. Phellogen later forming 

 periderm in stems several years old, and bork 

 in the trunk of large shrubs and trees. 



