TISSUE SYSTEMS 355 



position of both protoxylem and protophloem in these is always 

 external {fig. 740), so that the differentiation of the primary 

 wood is centripetal instead of centrifugal, as in the collateral 

 bundles. 



This difference of direction of formation of the primary 

 wood is one of the most important anatomical differences 

 between the stem and the root of a phanerogamic plant. 



