MORPHOLOGY OF HIGHER PLANTS. 



201 



with leptome or sieve strands, constituting a radial fibrovascular 

 bundle (Fig. no). 



Secondary structure : Cork cells. Phellogen. Secondary cor- 

 tex consisting of parenchyma. Leptome, cambium and hadrome, 

 arranged in radial groups, forming open collateral fibrovascular 

 bundles. Medullary rays separating the fibrovascular bundles. 



CA---H 



5P - 



Fig. hi. Section in the older part, higher up on the root of pea (Pisum) showing in 

 addition to what has been observed in Fig. no, the beginning of the change from primary to 

 secondary structure: CA, the development of a cambium; SX, secondary hadrome (or 

 vessels) and SP, secondary leptome (or sieve). 



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. 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 cambium, including hadrome, medullary rays and 

 pith, being known as the wood. The bark includes the leptome, 



