THE ROOT. 
67 
cortex, and upon the outside, cork. This stage in the 
growth of the root marks the beginning of the develop- 
ment of secondary structures. At the periphery there 
is a disintegration of the cells of the endodermis, cor- 
tex and epidermis of the primary root, these being 
replaced by the secondary structures as shown in the 
illustrations. A transverse section of a root showing 
fully developed secondary structures is also shown. 
The characteristics distinguishing the primary and 
secondary structures of dicotyledonous roots may be 
summarized as follows : 
PRIMARY STRUCTURE. 
Epidermis and root hairs. 
Hypodermis. 
Primary cortex consisting of parenchyma. 
Endodermis. 
Phloem and xylem in alternating radial rows, constituting a 
radial fibrovascular bundle. 
SECONDARY STRUCTURE. 
Cork cells. 
Phellogen. 
Secondary cortex consisting of parenchyma. 
Phloem, Cambium, f arranged in concentric layers, forming 
and Xylem, l concentric fibrovascular bundles. 
Medullary rays separating the fibrovascular bundles. 
Sometimes, as in glycyrrhiza and valerian, a number 
of parenchyma cells are found in the center of the 
root, these constituting the pith 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 phloem, the medullary rays outside 
of the cambium, as well as products of the phellogen. 
