THE AXGIOSPERMAE : STEMS 915 



embedded. These tissues are, however, to the sclerenchyma what the 

 concrete is to the embedded iron framework. The turgid cells of the ground 

 tissue support the framework of sclerenchyma and take up part of the strain, 

 so that the stress in the mechanical strands is greatly lessened. The similarity 

 between a reinforced concrete column and a monocotyledonous stem in this 





B 



Fig. 896. — Comparison of the section of a reinforced concrete bridge 

 (A) and a transverse section of the leaf of Phormium tenax (B). 

 {After Rasdorsky.) 



respect is shown by the comparison of the stress diagrams in Fig. 897, which 

 show that the stresses in the individual sclerenchyma strands are not those 

 typical of girders. 



The ground tissue has advantages over concrete, in its own inherent 

 flexibility and elasticitv, as well as in its perfect adhesion to the sclerenchyma, 

 which permit a considerable degree of bending of the organ without that 

 risk of rupture which would occur in a similar concrete structure. 



The close similaritv of plan in the natural and the artificial constructions 

 is well illustrated by the comparison in Fig. 898. 



In the lamina of a monocotyledonous leaf, especially if flat, the girder 

 model is more closely approached, each vascular bundle with the sclerenchyma 

 above and below it forming one girder. 



Roots, which have to withstand only compression and extension strains, 

 have the mechanical tissues, vascular tissues and sclerenchyma combined, 

 forming a single, composite, axile strand, which, like a cable, provides the 

 maximum degree of resistance to extension. This type of arrangement is 

 seen at its best in roots and rhizomes of JNIonocotyledons. In Dicotyledons 

 there is as a rule onlv a small amount of sclerenchyma and resistance may 

 be afforded by the xylem alone. 



From the foregoing account it will be realized that although lignified 

 tissues have the highest mechanical value, yet the mechanical requirements 

 of the plant's architecture are shared to some degree by all its tissues, and the 

 sclerenchyma does not deserve the name of the " mechanical system " which 



