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^.,|]^ ri^-'^^i^^ '^'- 



NATURE OF PLANTS 



79 



ened on the surfaces receiving the strain and weakened in the 

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

 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 

 very economically distributed at the periphery either as bands 



Fig. 46. Cross-section of stems showing arrangement of strengthening 

 tissues: A, burdock with strands of coUenchyma — 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. 



or strands of collenchyma and stereome fibers and these struc- 

 tures are frequently reinforced by additional mechanical tissue 

 in the phloem or just outside of it (Fig. 46). The vascular 

 bundles themselves frequently furnish excellent examples of the 



