8 4 



STRENGTHENING DEVICES 



the strain falls especially on the convex and concave surfaces. 

 The cells will be compressed on the concave side and stretched 

 on the convex side, while the tissues in the center will be subject 

 to the least disturbance. For this reason girders are strength- 

 ened on the surfaces receiving the strain and weakened in the 

 center (Fig. 45). So also given a certain amount of material 



FIG. 46. 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 stereomefibers at periphery^of stem and also on inner 

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



this is more effectively distributed in the form of a hollow column 

 than in a solid column since less strain falls upon the center of 

 the column. Therefore in stems we find the strengthening tissue 



