394 THE STRUCTURE OF ECONOMIC PLANTS 



readily distinguished in the young stem by their dense cytoplasmic 

 contents. 



The pericycle at first consists of a single layer of thin-walled 

 cells, but later forms a multiseriate zone between the endodermis 



Fig. 7.01. A, transection of sector of mature flax stem; B, transection of comparable sec- 

 tor of young stem ; ca, cambium ; chl, chlorenchyma ; con, connective tissue ; en, endodermis ; 

 ep, epidermis ; hd, hypodermis ; pel, pericyclic fibers ; ph i, primary phloem ; ph 2., secondary 

 phloem; pi, pith; xy i, primary xylem; xy 2., secondary xylem. 



and the phloem, and in this region groups of fibers develop which 

 constitute the fiber bundles of commerce. The development of the 

 pericyclic fibers is centripetal, the first ones differentiated being 

 adjacent to the endodermis, while the later formed ones abut the 

 phloem. (Fig. xoi.) 



The vascular bundles form a dissected siphonostele in which 



