STEMS 



703 



v e c 



FIG. 1030. A diagrammatic cross 

 section of the rhizome of Juncus balticus 

 littorahs, note the symmetrically ar- 

 ranged air chambers (a) in the cortex 

 (c)\ e, endodermis; v, vascular tract, 

 composed of a matrix of mechanical 

 cells (w) surrounding groups of con- 

 ductive vessels (/). 



Tensile strength. Resistance to tension depends not on the position 

 but on the size of the mechanical strand. However, since tension al- 

 most always is unequal, hence involving flexion, a massing of strands 

 into a central, solid cylinder is most 

 advantageous, since the danger of 

 rupture through differential strain 

 is reduced to a minimum. Roots are 

 subject to considerable tension, espe- 

 cially when the stems sway in the 

 wind, and they differ from erect stems 

 somewhat generally in that the cen- 

 tral portion is occupied by thick- 

 walled wood cells of considerable 

 strength instead of by a core of pith. 

 Rhizomes (as in Lathyrus and Juncus, 

 fig. 1030) are subject to the same 

 tensions as are roots, and like them 

 differ from erect stems in the absence 

 or weak development of central pith. 

 The relatively central vascular tract usually has thick-walled wood 

 elements, and there may be a mechanical cylinder or scattered 

 mechanical strands outside the wood. 



Lianas are subject to rather extraordinary strains, the combination of tension, 

 flexion, and pressure in a woody twiner, owing to the growth of the supporting tree, 

 being especially severe; in such stems there is a large development of bast and of 

 thick-walled wood. In tendril climbers much of the strain is borne by the relatively 

 slender tendrils, which often have central mechanical tissues that may become 

 strengthened on attachment. In flattened and spirally twisted tendrils (as in 

 Echinocystis) the strains are similar to those sustained by twining stems. 



Submersed stems, especially in running streams, may be subject to considerable 

 tension, but the conditions are such as to oppose the development of mechanical 

 tissue; however, the vascular tract usually is in a central position, as in roots and 

 rhizomes, and therefore situated where the strain is least (fig. 791). The absence 

 of rigid mechanical tissues is distinctly beneficial in such plants as Nereocystis 

 and the water lilies, since the floating synthetic organs thus are enabled to rise and 

 fall with the water. Furthermore, the absence of mechanical tissues and the conse- 

 quent easy rupture of the stems favor vegetative propagation. 



Pendulous organs, such as fruit stalks, commonly have central mechanical 

 strands. Thus the most diverse of organs, namely roots, rhizomes, tendrils, sub- 

 mersed stems, and pendulous stalks, organs which agree only in frequent or con- 

 stant exposure to tension, have central mechanical tissues, contrasting with the 

 peripheral mechanical tissues of erect herbaceous stems. 



