70 STRENGTHENING DEVICES 



ery either as bands or strands of collenchyma and stereome 

 fibers and tflfese structures are frequently reinforced by addi- 

 tional mechanical tissue in the phloem or just outside of it (Fig. 

 45) . The vascular bundles themselves frequently furnish excellent 

 examples of the girder, masses of stereome being found on the 

 outside of the phloem and on the inside of the xylem (Fig. 

 45, D). The resistance of these tissues to cutting and breaking 



FIG. 45. Cross-section of stems showing arrangement of strengthening 

 tissues: A, burdock with strands of collenchyma st, at periphery; v, vas- 

 cular bundles. B, sweet clover, showing a similar arrangement. C, moon- 

 seed showing cylinder of stereome fibers outside of vascular bundles. 

 D, rush with strands of stereome fibers at periphery of stem and also on 

 inner and outer sides of the bundles ; v, vascular bundles ; st, stereome. 



affords abundant evidence of their hardness and rigidity. It 

 may be surprising to note that the sustaining strength of the 

 stereome fibers is equal to that of the best wrought iron or 



